update for split-screen mode

This includes splitting GameRunner (the main game instance) from Game1 (now a per-screen game state), adding a PerScreen<T> utility to simplify per-screen values, adding separate per-screen input handling and events, adding new Context fields for split-screen, and logging the screen ID in split-screen mode to distinguish log entries.

15 files changed, 619 insertions, 254 deletions

diff --git a/src/SMAPI.Web/Framework/LogParsing/LogMessageBuilder.cs b/src/SMAPI.Web/Framework/LogParsing/LogMessageBuilder.cs
index 42e283a9..992876ef 100644
--- a/src/SMAPI.Web/Framework/LogParsing/LogMessageBuilder.cs
+++ b/src/SMAPI.Web/Framework/LogParsing/LogMessageBuilder.cs
@@ -16,6 +16,9 @@ namespace StardewModdingAPI.Web.Framework.LogParsing
         /// <summary>The log level for the next log message.</summary>
         public LogLevel Level { get; set; }
 
+        /// <summary>The screen ID in split-screen mode.</summary>
+        public int ScreenId { get; set; }
+
         /// <summary>The mod name for the next log message.</summary>
         public string Mod { get; set; }
 
@@ -36,10 +39,11 @@ namespace StardewModdingAPI.Web.Framework.LogParsing
         /// <summary>Start accumulating values for a new log message.</summary>
         /// <param name="time">The local time when the log was posted.</param>
         /// <param name="level">The log level.</param>
+        /// <param name="screenId">The screen ID in split-screen mode.</param>
         /// <param name="mod">The mod name.</param>
         /// <param name="text">The initial log text.</param>
         /// <exception cref="InvalidOperationException">A log message is already started; call <see cref="Clear"/> before starting a new message.</exception>
-        public void Start(string time, LogLevel level, string mod, string text)
+        public void Start(string time, LogLevel level, int screenId, string mod, string text)
         {
             if (this.Started)
                 throw new InvalidOperationException("Can't start new message, previous log message isn't done yet.");
@@ -48,6 +52,7 @@ namespace StardewModdingAPI.Web.Framework.LogParsing
 
             this.Time = time;
             this.Level = level;
+            this.ScreenId = screenId;
             this.Mod = mod;
             this.Text.Append(text);
         }
@@ -74,6 +79,7 @@ namespace StardewModdingAPI.Web.Framework.LogParsing
             {
                 Time = this.Time,
                 Level = this.Level,
+                ScreenId = this.ScreenId,
                 Mod = this.Mod,
                 Text = this.Text.ToString()
             };
diff --git a/src/SMAPI.Web/Framework/LogParsing/LogParser.cs b/src/SMAPI.Web/Framework/LogParsing/LogParser.cs
index 227dcd89..f69d4b6f 100644
--- a/src/SMAPI.Web/Framework/LogParsing/LogParser.cs
+++ b/src/SMAPI.Web/Framework/LogParsing/LogParser.cs
@@ -14,7 +14,7 @@ namespace StardewModdingAPI.Web.Framework.LogParsing
         ** Fields
         *********/
         /// <summary>A regex pattern matching the start of a SMAPI message.</summary>
-        private readonly Regex MessageHeaderPattern = new Regex(@"^\[(?<time>\d\d[:\.]\d\d[:\.]\d\d) (?<level>[a-z]+) +(?<modName>[^\]]+)\] ", RegexOptions.Compiled | RegexOptions.IgnoreCase);
+        private readonly Regex MessageHeaderPattern = new Regex(@"^\[(?<time>\d\d[:\.]\d\d[:\.]\d\d) (?<level>[a-z]+)(?: +screen_(?<screen>\d+))? +(?<modName>[^\]]+)\] ", RegexOptions.Compiled | RegexOptions.IgnoreCase);
 
         /// <summary>A regex pattern matching SMAPI's initial platform info message.</summary>
         private readonly Regex InfoLinePattern = new Regex(@"^SMAPI (?<apiVersion>.+) with Stardew Valley (?<gameVersion>.+) on (?<os>.+)", RegexOptions.Compiled | RegexOptions.IgnoreCase);
@@ -304,9 +304,11 @@ namespace StardewModdingAPI.Web.Framework.LogParsing
                         builder.Clear();
                     }
 
+                    var screenGroup = header.Groups["screen"];
                     builder.Start(
                         time: header.Groups["time"].Value,
                         level: Enum.Parse<LogLevel>(header.Groups["level"].Value, ignoreCase: true),
+                        screenId: screenGroup.Success ? int.Parse(screenGroup.Value) : 0, // main player is always screen ID 0
                         mod: header.Groups["modName"].Value,
                         text: line.Substring(header.Length)
                     );
diff --git a/src/SMAPI.Web/Framework/LogParsing/Models/LogMessage.cs b/src/SMAPI.Web/Framework/LogParsing/Models/LogMessage.cs
index f7c99d02..1e08be78 100644
--- a/src/SMAPI.Web/Framework/LogParsing/Models/LogMessage.cs
+++ b/src/SMAPI.Web/Framework/LogParsing/Models/LogMessage.cs
@@ -12,6 +12,9 @@ namespace StardewModdingAPI.Web.Framework.LogParsing.Models
         /// <summary>The log level.</summary>
         public LogLevel Level { get; set; }
 
+        /// <summary>The screen ID in split-screen mode.</summary>
+        public int ScreenId { get; set; }
+
         /// <summary>The mod name.</summary>
         public string Mod { get; set; }
 
diff --git a/src/SMAPI.Web/Views/LogParser/Index.cshtml b/src/SMAPI.Web/Views/LogParser/Index.cshtml
index d4ff4f10..fd472673 100644
--- a/src/SMAPI.Web/Views/LogParser/Index.cshtml
+++ b/src/SMAPI.Web/Views/LogParser/Index.cshtml
@@ -13,6 +13,8 @@
         .ToDictionary(level => level.ToString().ToLower(), level => level != LogLevel.Trace);
 
     string curPageUrl = this.Url.PlainAction("Index", "LogParser", new { id = Model.PasteID }, absoluteUrl: true);
+
+    ISet<int> screenIds = new HashSet<int>(Model.ParsedLog?.Messages?.Select(p => p.ScreenId) ?? new int[0]);
 }
 
 @section Head {
@@ -35,7 +37,8 @@
                 showMods: @this.ForJson(Model.ParsedLog?.Mods?.Select(p => Model.GetSlug(p.Name)).Distinct().ToDictionary(slug => slug, slug => true)),
                 showSections: @this.ForJson(Enum.GetNames(typeof(LogSection)).ToDictionary(section => section, section => false)),
                 showLevels: @this.ForJson(defaultFilters),
-                enableFilters: @this.ForJson(!Model.ShowRaw)
+                enableFilters: @this.ForJson(!Model.ShowRaw),
+                screenIds: @this.ForJson(screenIds)
             }, '@this.Url.PlainAction("Index", "LogParser", values: null)');
         });
     </script>
@@ -305,6 +308,10 @@ else if (Model.ParsedLog?.IsValid == true)
                         @if (message.IsStartOfSection) { <text> v-on:click="toggleSection('@message.Section')" </text> }
                         v-show="filtersAllow('@Model.GetSlug(message.Mod)', '@levelStr') @sectionFilter">
                         <td v-pre>@message.Time</td>
+                        @if (screenIds.Count > 1)
+                        {
+                            <td v-pre>screen_@message.ScreenId</td>
+                        }
                         <td v-pre>@message.Level.ToString().ToUpper()</td>
                         <td v-pre data-title="@message.Mod">@message.Mod</td>
                         <td>
@@ -325,7 +332,7 @@ else if (Model.ParsedLog?.IsValid == true)
                     if (message.Repeated > 0)
                     {
                         <tr class="@levelStr mod mod-repeat" v-show="filtersAllow('@Model.GetSlug(message.Mod)', '@levelStr') @sectionFilter">
-                            <td colspan="3"></td>
+                            <td colspan="4"></td>
                             <td v-pre><i>repeats [@message.Repeated] times.</i></td>
                         </tr>
                     }
diff --git a/src/SMAPI/Constants.cs b/src/SMAPI/Constants.cs
index 5ff324b0..d0f34cee 100644
--- a/src/SMAPI/Constants.cs
+++ b/src/SMAPI/Constants.cs
@@ -39,6 +39,9 @@ namespace StardewModdingAPI
 
         /// <summary>The game's assembly name.</summary>
         internal static string GameAssemblyName => EarlyConstants.Platform == GamePlatform.Windows ? "Stardew Valley" : "StardewValley";
+
+        /// <summary>The <see cref="Context.ScreenId"/> value which should appear in the SMAPI log, if any.</summary>
+        internal static int? LogScreenId { get; set; }
     }
 
     /// <summary>Contains SMAPI's constants and assumptions.</summary>
diff --git a/src/SMAPI/Context.cs b/src/SMAPI/Context.cs
index a7238b32..b1b33cd6 100644
--- a/src/SMAPI/Context.cs
+++ b/src/SMAPI/Context.cs
@@ -1,5 +1,7 @@
+using System.Collections.Generic;
 using StardewModdingAPI.Enums;
 using StardewModdingAPI.Events;
+using StardewModdingAPI.Utilities;
 using StardewValley;
 using StardewValley.Menus;
 
@@ -9,16 +11,49 @@ namespace StardewModdingAPI
     public static class Context
     {
         /*********
+        ** Fields
+        *********/
+        /// <summary>Whether the player has loaded a save and the world has finished initializing.</summary>
+        private static readonly PerScreen<bool> IsWorldReadyForScreen = new PerScreen<bool>();
+
+        /// <summary>The current stage in the game's loading process.</summary>
+        private static readonly PerScreen<LoadStage> LoadStageForScreen = new PerScreen<LoadStage>();
+
+        /// <summary>Whether a player save has been loaded.</summary>
+        internal static bool IsSaveLoaded => Game1.hasLoadedGame && !(Game1.activeClickableMenu is TitleMenu);
+
+        /// <summary>Whether the game is currently writing to the save file.</summary>
+        internal static bool IsSaving => Game1.activeClickableMenu is SaveGameMenu || Game1.activeClickableMenu is ShippingMenu; // saving is performed by SaveGameMenu, but it's wrapped by ShippingMenu on days when the player shipping something
+
+        /// <summary>The active split-screen instance IDs.</summary>
+        internal static readonly ISet<int> ActiveScreenIds = new HashSet<int>();
+
+        /// <summary>The last screen ID that was removed from the game, used to synchronize <see cref="PerScreen{T}"/>.</summary>
+        internal static int LastRemovedScreenId = -1;
+
+        /// <summary>The current stage in the game's loading process.</summary>
+        internal static LoadStage LoadStage
+        {
+            get => Context.LoadStageForScreen.Value;
+            set => Context.LoadStageForScreen.Value = value;
+        }
+
+
+        /*********
         ** Accessors
         *********/
         /****
-        ** Public
+        ** Game/player state
         ****/
         /// <summary>Whether the game has performed core initialization. This becomes true right before the first update tick.</summary>
         public static bool IsGameLaunched { get; internal set; }
 
         /// <summary>Whether the player has loaded a save and the world has finished initializing.</summary>
-        public static bool IsWorldReady { get; internal set; }
+        public static bool IsWorldReady
+        {
+            get => Context.IsWorldReadyForScreen.Value;
+            set => Context.IsWorldReadyForScreen.Value = value;
+        }
 
         /// <summary>Whether <see cref="IsWorldReady"/> is true and the player is free to act in the world (no menu is displayed, no cutscene is in progress, etc).</summary>
         public static bool IsPlayerFree => Context.IsWorldReady && Game1.currentLocation != null && Game1.activeClickableMenu == null && !Game1.dialogueUp && (!Game1.eventUp || Game1.isFestival());
@@ -29,22 +64,36 @@ namespace StardewModdingAPI
         /// <summary>Whether the game is currently running the draw loop. This isn't relevant to most mods, since you should use <see cref="IDisplayEvents"/> events to draw to the screen.</summary>
         public static bool IsInDrawLoop { get; internal set; }
 
-        /// <summary>Whether <see cref="IsWorldReady"/> and the player loaded the save in multiplayer mode (regardless of whether any other players are connected).</summary>
-        public static bool IsMultiplayer => Context.IsWorldReady && Game1.multiplayerMode != Game1.singlePlayer;
-
-        /// <summary>Whether <see cref="IsWorldReady"/> and the current player is the main player. This is always true in single-player, and true when hosting in multiplayer.</summary>
-        public static bool IsMainPlayer => Context.IsWorldReady && Game1.IsMasterGame;
-
         /****
-        ** Internal
+        ** Multiplayer
         ****/
-        /// <summary>Whether a player save has been loaded.</summary>
-        internal static bool IsSaveLoaded => Game1.hasLoadedGame && !(Game1.activeClickableMenu is TitleMenu);
+        /// <summary>The unique ID of the current screen in split-screen mode. A screen is always assigned a new ID when it's opened (so a player who quits and rejoins has a new screen ID).</summary>
+        public static int ScreenId => Game1.game1?.instanceId ?? 0;
 
-        /// <summary>Whether the game is currently writing to the save file.</summary>
-        internal static bool IsSaving => Game1.activeClickableMenu is SaveGameMenu || Game1.activeClickableMenu is ShippingMenu; // saving is performed by SaveGameMenu, but it's wrapped by ShippingMenu on days when the player shipping something
+        /// <summary>Whether the game is running in multiplayer or split-screen mode (regardless of whether any other players are connected). See <see cref="IsSplitScreen"/> and <see cref="HasRemotePlayers"/> for more specific checks.</summary>
+        public static bool IsMultiplayer => Context.IsSplitScreen || (Context.IsWorldReady && Game1.multiplayerMode != Game1.singlePlayer);
 
-        /// <summary>The current stage in the game's loading process.</summary>
-        internal static LoadStage LoadStage { get; set; }
+        /// <summary>Whether this player is running on the main player's computer. This is true for both the main player and split-screen players.</summary>
+        public static bool IsOnHostComputer => Context.IsMainPlayer || Context.IsSplitScreen;
+
+        /// <summary>Whether the current player is playing in a split-screen. This is only applicable when <see cref="IsOnHostComputer"/> is true, since split-screen players on another computer are just regular remote players.</summary>
+        public static bool IsSplitScreen => LocalMultiplayer.IsLocalMultiplayer();
+
+        /// <summary>Whether there are players connected over the network.</summary>
+        public static bool HasRemotePlayers => Context.IsMultiplayer && !Game1.hasLocalClientsOnly;
+
+        /// <summary>Whether the current player is the main player. This is always true in single-player, and true when hosting in multiplayer.</summary>
+        public static bool IsMainPlayer => Game1.IsMasterGame && !(TitleMenu.subMenu is FarmhandMenu);
+
+
+        /*********
+        ** Public methods
+        *********/
+        /// <summary>Get whether a screen ID is still active.</summary>
+        /// <param name="id">The screen ID.</param>
+        public static bool HasScreenId(int id)
+        {
+            return Context.ActiveScreenIds.Contains(id);
+        }
     }
 }
diff --git a/src/SMAPI/Framework/Logging/LogManager.cs b/src/SMAPI/Framework/Logging/LogManager.cs
index 1e484709..a2c0125e 100644
--- a/src/SMAPI/Framework/Logging/LogManager.cs
+++ b/src/SMAPI/Framework/Logging/LogManager.cs
@@ -84,10 +84,11 @@ namespace StardewModdingAPI.Framework.Logging
         /// <param name="writeToConsole">Whether to output log messages to the console.</param>
         /// <param name="isVerbose">Whether verbose logging is enabled. This enables more detailed diagnostic messages than are normally needed.</param>
         /// <param name="isDeveloperMode">Whether to enable full console output for developers.</param>
-        public LogManager(string logPath, ColorSchemeConfig colorConfig, bool writeToConsole, bool isVerbose, bool isDeveloperMode)
+        /// <param name="getScreenIdForLog">Get the screen ID that should be logged to distinguish between players in split-screen mode, if any.</param>
+        public LogManager(string logPath, ColorSchemeConfig colorConfig, bool writeToConsole, bool isVerbose, bool isDeveloperMode, Func<int?> getScreenIdForLog)
         {
             // init construction logic
-            this.GetMonitorImpl = name => new Monitor(name, this.IgnoreChar, this.LogFile, colorConfig, isVerbose)
+            this.GetMonitorImpl = name => new Monitor(name, this.IgnoreChar, this.LogFile, colorConfig, isVerbose, getScreenIdForLog)
             {
                 WriteToConsole = writeToConsole,
                 ShowTraceInConsole = isDeveloperMode,
diff --git a/src/SMAPI/Framework/ModHelpers/DataHelper.cs b/src/SMAPI/Framework/ModHelpers/DataHelper.cs
index 41612387..0fe3209f 100644
--- a/src/SMAPI/Framework/ModHelpers/DataHelper.cs
+++ b/src/SMAPI/Framework/ModHelpers/DataHelper.cs
@@ -69,8 +69,8 @@ namespace StardewModdingAPI.Framework.ModHelpers
         {
             if (Context.LoadStage == LoadStage.None)
                 throw new InvalidOperationException($"Can't use {nameof(IMod.Helper)}.{nameof(IModHelper.Data)}.{nameof(this.ReadSaveData)} when a save file isn't loaded.");
-            if (!Game1.IsMasterGame)
-                throw new InvalidOperationException($"Can't use {nameof(IMod.Helper)}.{nameof(IModHelper.Data)}.{nameof(this.ReadSaveData)} because this isn't the main player. (Save files are stored on the main player's computer.)");
+            if (!Context.IsOnHostComputer)
+                throw new InvalidOperationException($"Can't use {nameof(IMod.Helper)}.{nameof(IModHelper.Data)}.{nameof(this.ReadSaveData)} when connected to a remote host. (Save files are stored on the main player's computer.)");
 
 
             string internalKey = this.GetSaveFileKey(key);
@@ -87,8 +87,8 @@ namespace StardewModdingAPI.Framework.ModHelpers
         {
             if (Context.LoadStage == LoadStage.None)
                 throw new InvalidOperationException($"Can't use {nameof(IMod.Helper)}.{nameof(IModHelper.Data)}.{nameof(this.WriteSaveData)} when a save file isn't loaded.");
-            if (!Game1.IsMasterGame)
-                throw new InvalidOperationException($"Can't use {nameof(IMod.Helper)}.{nameof(IModHelper.Data)}.{nameof(this.WriteSaveData)} because this isn't the main player. (Save files are stored on the main player's computer.)");
+            if (!Context.IsOnHostComputer)
+                throw new InvalidOperationException($"Can't use {nameof(IMod.Helper)}.{nameof(IModHelper.Data)}.{nameof(this.WriteSaveData)} when connected to a remote host. (Save files are stored on the main player's computer.)");
 
             string internalKey = this.GetSaveFileKey(key);
             string data = model != null
diff --git a/src/SMAPI/Framework/ModHelpers/InputHelper.cs b/src/SMAPI/Framework/ModHelpers/InputHelper.cs
index 09ce3c65..e1317544 100644
--- a/src/SMAPI/Framework/ModHelpers/InputHelper.cs
+++ b/src/SMAPI/Framework/ModHelpers/InputHelper.cs
@@ -1,3 +1,4 @@
+using System;
 using StardewModdingAPI.Framework.Input;
 
 namespace StardewModdingAPI.Framework.ModHelpers
@@ -8,8 +9,8 @@ namespace StardewModdingAPI.Framework.ModHelpers
         /*********
         ** Accessors
         *********/
-        /// <summary>Manages the game's input state.</summary>
-        private readonly SInputState InputState;
+        /// <summary>Manages the game's input state for the current player instance. That may not be the main player in split-screen mode.</summary>
+        private readonly Func<SInputState> CurrentInputState;
 
 
         /*********
@@ -17,41 +18,41 @@ namespace StardewModdingAPI.Framework.ModHelpers
         *********/
         /// <summary>Construct an instance.</summary>
         /// <param name="modID">The unique ID of the relevant mod.</param>
-        /// <param name="inputState">Manages the game's input state.</param>
-        public InputHelper(string modID, SInputState inputState)
+        /// <param name="currentInputState">Manages the game's input state for the current player instance. That may not be the main player in split-screen mode.</param>
+        public InputHelper(string modID, Func<SInputState> currentInputState)
             : base(modID)
         {
-            this.InputState = inputState;
+            this.CurrentInputState = currentInputState;
         }
 
         /// <inheritdoc />
         public ICursorPosition GetCursorPosition()
         {
-            return this.InputState.CursorPosition;
+            return this.CurrentInputState().CursorPosition;
         }
 
         /// <inheritdoc />
         public bool IsDown(SButton button)
         {
-            return this.InputState.IsDown(button);
+            return this.CurrentInputState().IsDown(button);
         }
 
         /// <inheritdoc />
         public bool IsSuppressed(SButton button)
         {
-            return this.InputState.IsSuppressed(button);
+            return this.CurrentInputState().IsSuppressed(button);
         }
 
         /// <inheritdoc />
         public void Suppress(SButton button)
         {
-            this.InputState.OverrideButton(button, setDown: false);
+            this.CurrentInputState().OverrideButton(button, setDown: false);
         }
 
         /// <inheritdoc />
         public SButtonState GetState(SButton button)
         {
-            return this.InputState.GetState(button);
+            return this.CurrentInputState().GetState(button);
         }
     }
 }
diff --git a/src/SMAPI/Framework/ModHelpers/ModHelper.cs b/src/SMAPI/Framework/ModHelpers/ModHelper.cs
index d9fc8621..058bff83 100644
--- a/src/SMAPI/Framework/ModHelpers/ModHelper.cs
+++ b/src/SMAPI/Framework/ModHelpers/ModHelper.cs
@@ -51,7 +51,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
         /// <summary>Construct an instance.</summary>
         /// <param name="modID">The mod's unique ID.</param>
         /// <param name="modDirectory">The full path to the mod's folder.</param>
-        /// <param name="inputState">Manages the game's input state.</param>
+        /// <param name="currentInputState">Manages the game's input state for the current player instance. That may not be the main player in split-screen mode.</param>
         /// <param name="events">Manages access to events raised by SMAPI.</param>
         /// <param name="contentHelper">An API for loading content assets.</param>
         /// <param name="contentPackHelper">An API for managing content packs.</param>
@@ -63,7 +63,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
         /// <param name="translationHelper">An API for reading translations stored in the mod's <c>i18n</c> folder.</param>
         /// <exception cref="ArgumentNullException">An argument is null or empty.</exception>
         /// <exception cref="InvalidOperationException">The <paramref name="modDirectory"/> path does not exist on disk.</exception>
-        public ModHelper(string modID, string modDirectory, SInputState inputState, IModEvents events, IContentHelper contentHelper, IContentPackHelper contentPackHelper, ICommandHelper commandHelper, IDataHelper dataHelper, IModRegistry modRegistry, IReflectionHelper reflectionHelper, IMultiplayerHelper multiplayer, ITranslationHelper translationHelper)
+        public ModHelper(string modID, string modDirectory, Func<SInputState> currentInputState, IModEvents events, IContentHelper contentHelper, IContentPackHelper contentPackHelper, ICommandHelper commandHelper, IDataHelper dataHelper, IModRegistry modRegistry, IReflectionHelper reflectionHelper, IMultiplayerHelper multiplayer, ITranslationHelper translationHelper)
             : base(modID)
         {
             // validate directory
@@ -77,7 +77,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
             this.Content = contentHelper ?? throw new ArgumentNullException(nameof(contentHelper));
             this.ContentPacks = contentPackHelper ?? throw new ArgumentNullException(nameof(contentPackHelper));
             this.Data = dataHelper ?? throw new ArgumentNullException(nameof(dataHelper));
-            this.Input = new InputHelper(modID, inputState);
+            this.Input = new InputHelper(modID, currentInputState);
             this.ModRegistry = modRegistry ?? throw new ArgumentNullException(nameof(modRegistry));
             this.ConsoleCommands = commandHelper ?? throw new ArgumentNullException(nameof(commandHelper));
             this.Reflection = reflectionHelper ?? throw new ArgumentNullException(nameof(reflectionHelper));
diff --git a/src/SMAPI/Framework/Monitor.cs b/src/SMAPI/Framework/Monitor.cs
index 533420a5..04e67d68 100644
--- a/src/SMAPI/Framework/Monitor.cs
+++ b/src/SMAPI/Framework/Monitor.cs
@@ -30,6 +30,9 @@ namespace StardewModdingAPI.Framework
         /// <summary>A cache of messages that should only be logged once.</summary>
         private readonly HashSet<string> LogOnceCache = new HashSet<string>();
 
+        /// <summary>Get the screen ID that should be logged to distinguish between players in split-screen mode, if any.</summary>
+        private readonly Func<int?> GetScreenIdForLog;
+
 
         /*********
         ** Accessors
@@ -56,7 +59,8 @@ namespace StardewModdingAPI.Framework
         /// <param name="logFile">The log file to which to write messages.</param>
         /// <param name="colorConfig">The colors to use for text written to the SMAPI console.</param>
         /// <param name="isVerbose">Whether verbose logging is enabled. This enables more detailed diagnostic messages than are normally needed.</param>
-        public Monitor(string source, char ignoreChar, LogFileManager logFile, ColorSchemeConfig colorConfig, bool isVerbose)
+        /// <param name="getScreenIdForLog">Get the screen ID that should be logged to distinguish between players in split-screen mode, if any.</param>
+        public Monitor(string source, char ignoreChar, LogFileManager logFile, ColorSchemeConfig colorConfig, bool isVerbose, Func<int?> getScreenIdForLog)
         {
             // validate
             if (string.IsNullOrWhiteSpace(source))
@@ -68,6 +72,7 @@ namespace StardewModdingAPI.Framework
             this.ConsoleWriter = new ColorfulConsoleWriter(Constants.Platform, colorConfig);
             this.IgnoreChar = ignoreChar;
             this.IsVerbose = isVerbose;
+            this.GetScreenIdForLog = getScreenIdForLog;
         }
 
         /// <inheritdoc />
@@ -143,7 +148,9 @@ namespace StardewModdingAPI.Framework
         private string GenerateMessagePrefix(string source, ConsoleLogLevel level)
         {
             string levelStr = level.ToString().ToUpper().PadRight(Monitor.MaxLevelLength);
-            return $"[{DateTime.Now:HH:mm:ss} {levelStr} {source}]";
+            int? playerIndex = this.GetScreenIdForLog();
+
+            return $"[{DateTime.Now:HH:mm:ss} {levelStr}{(playerIndex != null ? $" screen_{playerIndex}" : "")} {source}]";
         }
     }
 }
diff --git a/src/SMAPI/Framework/SCore.cs b/src/SMAPI/Framework/SCore.cs
index 1b4c32bb..a7f8fbed 100644
--- a/src/SMAPI/Framework/SCore.cs
+++ b/src/SMAPI/Framework/SCore.cs
@@ -85,17 +85,14 @@ namespace StardewModdingAPI.Framework
         private readonly CommandManager CommandManager = new CommandManager();
 
         /// <summary>The underlying game instance.</summary>
-        private SGame Game;
-
-        /// <summary>Manages input visible to the game.</summary>
-        private SInputState Input => SGame.Input;
-
-        /// <summary>The game's core multiplayer utility.</summary>
-        private SMultiplayer Multiplayer => SGame.Multiplayer;
+        private SGameRunner Game;
 
         /// <summary>SMAPI's content manager.</summary>
         private ContentCoordinator ContentCore;
 
+        /// <summary>The game's core multiplayer utility for the main player.</summary>
+        private SMultiplayer Multiplayer;
+
         /// <summary>Tracks the installed mods.</summary>
         /// <remarks>This is initialized after the game starts.</remarks>
         private readonly ModRegistry ModRegistry = new ModRegistry();
@@ -103,11 +100,6 @@ namespace StardewModdingAPI.Framework
         /// <summary>Manages SMAPI events for mods.</summary>
         private readonly EventManager EventManager;
 
-        /// <summary>Monitors the entire game state for changes.</summary>
-        private WatcherCore Watchers;
-
-        /// <summary>A snapshot of the current <see cref="Watchers"/> state.</summary>
-        private readonly WatcherSnapshot WatcherSnapshot = new WatcherSnapshot();
 
         /****
         ** State
@@ -127,25 +119,15 @@ namespace StardewModdingAPI.Framework
         /// <summary>Whether post-game-startup initialization has been performed.</summary>
         private bool IsInitialized;
 
+        /// <summary>Whether the player just returned to the title screen.</summary>
+        public bool JustReturnedToTitle { get; set; }
+
         /// <summary>The maximum number of consecutive attempts SMAPI should make to recover from an update error.</summary>
         private readonly Countdown UpdateCrashTimer = new Countdown(60); // 60 ticks = roughly one second
 
-        /// <summary>The number of ticks until SMAPI should notify mods that the game has loaded.</summary>
-        /// <remarks>Skipping a few frames ensures the game finishes initializing the world before mods try to change it.</remarks>
-        private readonly Countdown AfterLoadTimer = new Countdown(5);
-
         /// <summary>Whether custom content was removed from the save data to avoid a crash.</summary>
         private bool IsSaveContentRemoved;
 
-        /// <summary>Whether the game is saving and SMAPI has already raised <see cref="IGameLoopEvents.Saving"/>.</summary>
-        private bool IsBetweenSaveEvents;
-
-        /// <summary>Whether the game is creating the save file and SMAPI has already raised <see cref="IGameLoopEvents.SaveCreating"/>.</summary>
-        private bool IsBetweenCreateEvents;
-
-        /// <summary>Whether the player just returned to the title screen.</summary>
-        private bool JustReturnedToTitle;
-
         /// <summary>Asset interceptors added or removed since the last tick.</summary>
         private readonly List<AssetInterceptorChange> ReloadAssetInterceptorsQueue = new List<AssetInterceptorChange>();
 
@@ -191,7 +173,7 @@ namespace StardewModdingAPI.Framework
             if (File.Exists(Constants.ApiUserConfigPath))
                 JsonConvert.PopulateObject(File.ReadAllText(Constants.ApiUserConfigPath), this.Settings);
 
-            this.LogManager = new LogManager(logPath: logPath, colorConfig: this.Settings.ConsoleColors, writeToConsole: writeToConsole, isVerbose: this.Settings.VerboseLogging, isDeveloperMode: this.Settings.DeveloperMode);
+            this.LogManager = new LogManager(logPath: logPath, colorConfig: this.Settings.ConsoleColors, writeToConsole: writeToConsole, isVerbose: this.Settings.VerboseLogging, isDeveloperMode: this.Settings.DeveloperMode, getScreenIdForLog: this.GetScreenIdForLog);
 
             SCore.PerformanceMonitor = new PerformanceMonitor(this.Monitor);
             this.EventManager = new EventManager(this.ModRegistry, SCore.PerformanceMonitor);
@@ -250,22 +232,22 @@ namespace StardewModdingAPI.Framework
                 LocalizedContentManager.OnLanguageChange += locale => this.OnLocaleChanged();
 
                 // override game
-                var multiplayer = new SMultiplayer(this.Monitor, this.EventManager, this.Toolkit.JsonHelper, this.ModRegistry, this.Reflection, this.OnModMessageReceived, this.Settings.LogNetworkTraffic);
-                var modHooks = new SModHooks(this.OnNewDayAfterFade);
+                this.Multiplayer = new SMultiplayer(this.Monitor, this.EventManager, this.Toolkit.JsonHelper, this.ModRegistry, this.Reflection, this.OnModMessageReceived, this.Settings.LogNetworkTraffic);
                 SGame.CreateContentManagerImpl = this.CreateContentManager; // must be static since the game accesses it before the SGame constructor is called
-                this.Game = new SGame(
+                this.Game = new SGameRunner(
                     monitor: this.Monitor,
                     reflection: this.Reflection,
                     eventManager: this.EventManager,
-                    modHooks: modHooks,
-                    multiplayer: multiplayer,
+                    modHooks: new SModHooks(this.OnNewDayAfterFade),
+                    multiplayer: this.Multiplayer,
                     exitGameImmediately: this.ExitGameImmediately,
 
                     onGameContentLoaded: this.OnGameContentLoaded,
                     onGameUpdating: this.OnGameUpdating,
+                    onPlayerInstanceUpdating: this.OnPlayerInstanceUpdating,
                     onGameExiting: this.OnGameExiting
                 );
-                StardewValley.Program.gamePtr = this.Game;
+                StardewValley.GameRunner.instance = this.Game;
 
                 // apply game patches
                 new GamePatcher(this.Monitor).Apply(
@@ -422,12 +404,6 @@ namespace StardewModdingAPI.Framework
         /// <summary>Raised after the game finishes initializing.</summary>
         private void OnGameInitialized()
         {
-            // set initial state
-            this.Input.TrueUpdate();
-
-            // init watchers
-            this.Watchers = new WatcherCore(this.Input, this.Game.GetObservableLocations());
-
             // validate XNB integrity
             if (!this.ValidateContentIntegrity())
                 this.Monitor.Log("SMAPI found problems in your game's content files which are likely to cause errors or crashes. Consider uninstalling XNB mods or reinstalling the game.", LogLevel.Error);
@@ -460,8 +436,6 @@ namespace StardewModdingAPI.Framework
         /// <param name="runGameUpdate">Invoke the game's update logic.</param>
         private void OnGameUpdating(GameTime gameTime, Action runGameUpdate)
         {
-            var events = this.EventManager;
-
             try
             {
                 /*********
@@ -471,15 +445,6 @@ namespace StardewModdingAPI.Framework
                 SCore.DeprecationManager.PrintQueued();
                 SCore.PerformanceMonitor.PrintQueuedAlerts();
 
-                // reapply overrides
-                if (this.JustReturnedToTitle)
-                {
-                    if (!(Game1.mapDisplayDevice is SDisplayDevice))
-                        Game1.mapDisplayDevice = this.GetMapDisplayDevice();
-
-                    this.JustReturnedToTitle = false;
-                }
-
                 /*********
                 ** First-tick initialization
                 *********/
@@ -490,25 +455,151 @@ namespace StardewModdingAPI.Framework
                 }
 
                 /*********
+                ** Special cases
+                *********/
+                // Abort if SMAPI is exiting.
+                if (this.CancellationToken.IsCancellationRequested)
+                {
+                    this.Monitor.Log("SMAPI shutting down: aborting update.");
+                    return;
+                }
+
+                /*********
+                ** Reload assets when interceptors are added/removed
+                *********/
+                if (this.ReloadAssetInterceptorsQueue.Any())
+                {
+                    // get unique interceptors
+                    AssetInterceptorChange[] interceptors = this.ReloadAssetInterceptorsQueue
+                        .GroupBy(p => p.Instance, new ObjectReferenceComparer<object>())
+                        .Select(p => p.First())
+                        .ToArray();
+                    this.ReloadAssetInterceptorsQueue.Clear();
+
+                    // log summary
+                    this.Monitor.Log("Invalidating cached assets for new editors & loaders...");
+                    this.Monitor.Log(
+                        "   changed: "
+                        + string.Join(", ",
+                            interceptors
+                                .GroupBy(p => p.Mod)
+                                .OrderBy(p => p.Key.DisplayName)
+                                .Select(modGroup =>
+                                    $"{modGroup.Key.DisplayName} ("
+                                    + string.Join(", ", modGroup.GroupBy(p => p.WasAdded).ToDictionary(p => p.Key, p => p.Count()).Select(p => $"{(p.Key ? "added" : "removed")} {p.Value}"))
+                                    + ")"
+                                )
+                        )
+                    );
+
+                    // reload affected assets
+                    this.ContentCore.InvalidateCache(asset => interceptors.Any(p => p.CanIntercept(asset)));
+                }
+
+                /*********
+                ** Execute commands
+                *********/
+                while (this.CommandQueue.TryDequeue(out string rawInput))
+                {
+                    // parse command
+                    string name;
+                    string[] args;
+                    Command command;
+                    try
+                    {
+                        if (!this.CommandManager.TryParse(rawInput, out name, out args, out command))
+                        {
+                            this.Monitor.Log("Unknown command; type 'help' for a list of available commands.", LogLevel.Error);
+                            continue;
+                        }
+                    }
+                    catch (Exception ex)
+                    {
+                        this.Monitor.Log($"Failed parsing that command:\n{ex.GetLogSummary()}", LogLevel.Error);
+                        continue;
+                    }
+
+                    // execute command
+                    try
+                    {
+                        command.Callback.Invoke(name, args);
+                    }
+                    catch (Exception ex)
+                    {
+                        if (command.Mod != null)
+                            command.Mod.LogAsMod($"Mod failed handling that command:\n{ex.GetLogSummary()}", LogLevel.Error);
+                        else
+                            this.Monitor.Log($"Failed handling that command:\n{ex.GetLogSummary()}", LogLevel.Error);
+                    }
+                }
+
+                /*********
+                ** Show in-game warnings (for main player only)
+                *********/
+                // save content removed
+                if (this.IsSaveContentRemoved && Context.IsWorldReady)
+                {
+                    this.IsSaveContentRemoved = false;
+                    Game1.addHUDMessage(new HUDMessage(this.Translator.Get("warn.invalid-content-removed"), HUDMessage.error_type));
+                }
+
+                /*********
+                ** Run game update
+                *********/
+                runGameUpdate();
+
+                /*********
+                ** Reset crash timer
+                *********/
+                this.UpdateCrashTimer.Reset();
+            }
+            catch (Exception ex)
+            {
+                // log error
+                this.Monitor.Log($"An error occured in the overridden update loop: {ex.GetLogSummary()}", LogLevel.Error);
+
+                // exit if irrecoverable
+                if (!this.UpdateCrashTimer.Decrement())
+                    this.ExitGameImmediately("The game crashed when updating, and SMAPI was unable to recover the game.");
+            }
+            finally
+            {
+                SCore.TicksElapsed++;
+            }
+        }
+
+        /// <summary>Raised when the game instance for a local player is updating (once per <see cref="OnGameUpdating"/> per player).</summary>
+        /// <param name="instance">The game instance being updated.</param>
+        /// <param name="gameTime">A snapshot of the game timing state.</param>
+        /// <param name="runUpdate">Invoke the game's update logic.</param>
+        private void OnPlayerInstanceUpdating(SGame instance, GameTime gameTime, Action runUpdate)
+        {
+            var events = this.EventManager;
+
+            try
+            {
+                // reapply overrides
+                if (this.JustReturnedToTitle)
+                {
+                    if (!(Game1.mapDisplayDevice is SDisplayDevice))
+                        Game1.mapDisplayDevice = this.GetMapDisplayDevice();
+
+                    this.JustReturnedToTitle = false;
+                }
+
+                /*********
                 ** Update input
                 *********/
                 // This should *always* run, even when suppressing mod events, since the game uses
                 // this too. For example, doing this after mod event suppression would prevent the
                 // user from doing anything on the overnight shipping screen.
-                SInputState inputState = this.Input;
+                SInputState inputState = instance.Input;
                 if (this.Game.IsActive)
                     inputState.TrueUpdate();
 
                 /*********
                 ** Special cases
                 *********/
-                // Abort if SMAPI is exiting.
-                if (this.CancellationToken.IsCancellationRequested)
-                {
-                    this.Monitor.Log("SMAPI shutting down: aborting update.");
-                    return;
-                }
-
                 // Run async tasks synchronously to avoid issues due to mod events triggering
                 // concurrently with game code.
                 bool saveParsed = false;
@@ -544,10 +635,10 @@ namespace StardewModdingAPI.Framework
                     this.Monitor.Log("Game loader done.");
                 }
 
-                if (SGame.NewDayTask?.Status == TaskStatus.Created)
+                if (instance.NewDayTask?.Status == TaskStatus.Created)
                 {
                     this.Monitor.Log("New day task synchronizing...");
-                    SGame.NewDayTask.RunSynchronously();
+                    instance.NewDayTask.RunSynchronously();
                     this.Monitor.Log("New day task done.");
                 }
 
@@ -560,11 +651,10 @@ namespace StardewModdingAPI.Framework
                 // a small chance that the task will finish after we defer but before the game checks,
                 // which means technically events should be raised, but the effects of missing one
                 // update tick are negligible and not worth the complications of bypassing Game1.Update.
-                if (SGame.NewDayTask != null || Game1.gameMode == Game1.loadingMode)
+                if (instance.NewDayTask != null || Game1.gameMode == Game1.loadingMode)
                 {
                     events.UnvalidatedUpdateTicking.RaiseEmpty();
-                    SCore.TicksElapsed++;
-                    runGameUpdate();
+                    runUpdate();
                     events.UnvalidatedUpdateTicked.RaiseEmpty();
                     return;
                 }
@@ -578,132 +668,52 @@ namespace StardewModdingAPI.Framework
                 if (Context.IsSaving)
                 {
                     // raise before-create
-                    if (!Context.IsWorldReady && !this.IsBetweenCreateEvents)
+                    if (!Context.IsWorldReady && !instance.IsBetweenCreateEvents)
                     {
-                        this.IsBetweenCreateEvents = true;
+                        instance.IsBetweenCreateEvents = true;
                         this.Monitor.Log("Context: before save creation.");
                         events.SaveCreating.RaiseEmpty();
                     }
 
                     // raise before-save
-                    if (Context.IsWorldReady && !this.IsBetweenSaveEvents)
+                    if (Context.IsWorldReady && !instance.IsBetweenSaveEvents)
                     {
-                        this.IsBetweenSaveEvents = true;
+                        instance.IsBetweenSaveEvents = true;
                         this.Monitor.Log("Context: before save.");
                         events.Saving.RaiseEmpty();
                     }
 
                     // suppress non-save events
                     events.UnvalidatedUpdateTicking.RaiseEmpty();
-                    SCore.TicksElapsed++;
-                    runGameUpdate();
+                    runUpdate();
                     events.UnvalidatedUpdateTicked.RaiseEmpty();
                     return;
                 }
 
                 /*********
-                ** Reload assets when interceptors are added/removed
-                *********/
-                if (this.ReloadAssetInterceptorsQueue.Any())
-                {
-                    // get unique interceptors
-                    AssetInterceptorChange[] interceptors = this.ReloadAssetInterceptorsQueue
-                        .GroupBy(p => p.Instance, new ObjectReferenceComparer<object>())
-                        .Select(p => p.First())
-                        .ToArray();
-                    this.ReloadAssetInterceptorsQueue.Clear();
-
-                    // log summary
-                    this.Monitor.Log("Invalidating cached assets for new editors & loaders...");
-                    this.Monitor.Log(
-                        "   changed: "
-                        + string.Join(", ",
-                            interceptors
-                                .GroupBy(p => p.Mod)
-                                .OrderBy(p => p.Key.DisplayName)
-                                .Select(modGroup =>
-                                    $"{modGroup.Key.DisplayName} ("
-                                    + string.Join(", ", modGroup.GroupBy(p => p.WasAdded).ToDictionary(p => p.Key, p => p.Count()).Select(p => $"{(p.Key ? "added" : "removed")} {p.Value}"))
-                                    + ")"
-                                )
-                        )
-                    );
-
-                    // reload affected assets
-                    this.ContentCore.InvalidateCache(asset => interceptors.Any(p => p.CanIntercept(asset)));
-                }
-
-                /*********
-                ** Execute commands
-                *********/
-                while (this.CommandQueue.TryDequeue(out string rawInput))
-                {
-                    // parse command
-                    string name;
-                    string[] args;
-                    Command command;
-                    try
-                    {
-                        if (!this.CommandManager.TryParse(rawInput, out name, out args, out command))
-                        {
-                            this.Monitor.Log("Unknown command; type 'help' for a list of available commands.", LogLevel.Error);
-                            continue;
-                        }
-                    }
-                    catch (Exception ex)
-                    {
-                        this.Monitor.Log($"Failed parsing that command:\n{ex.GetLogSummary()}", LogLevel.Error);
-                        continue;
-                    }
-
-                    // execute command
-                    try
-                    {
-                        command.Callback.Invoke(name, args);
-                    }
-                    catch (Exception ex)
-                    {
-                        if (command.Mod != null)
-                            command.Mod.LogAsMod($"Mod failed handling that command:\n{ex.GetLogSummary()}", LogLevel.Error);
-                        else
-                            this.Monitor.Log($"Failed handling that command:\n{ex.GetLogSummary()}", LogLevel.Error);
-                    }
-                }
-
-                /*********
                 ** Update context
                 *********/
                 bool wasWorldReady = Context.IsWorldReady;
                 if ((Context.IsWorldReady && !Context.IsSaveLoaded) || Game1.exitToTitle)
                 {
                     Context.IsWorldReady = false;
-                    this.AfterLoadTimer.Reset();
+                    instance.AfterLoadTimer.Reset();
                 }
-                else if (Context.IsSaveLoaded && this.AfterLoadTimer.Current > 0 && Game1.currentLocation != null)
+                else if (Context.IsSaveLoaded && instance.AfterLoadTimer.Current > 0 && Game1.currentLocation != null)
                 {
                     if (Game1.dayOfMonth != 0) // wait until new-game intro finishes (world not fully initialized yet)
-                        this.AfterLoadTimer.Decrement();
-                    Context.IsWorldReady = this.AfterLoadTimer.Current == 0;
+                        instance.AfterLoadTimer.Decrement();
+                    Context.IsWorldReady = instance.AfterLoadTimer.Current == 0;
                 }
 
                 /*********
                 ** Update watchers
                 **   (Watchers need to be updated, checked, and reset in one go so we can detect any changes mods make in event handlers.)
                 *********/
-                this.Watchers.Update();
-                this.WatcherSnapshot.Update(this.Watchers);
-                this.Watchers.Reset();
-                WatcherSnapshot state = this.WatcherSnapshot;
-
-                /*********
-                ** Display in-game warnings
-                *********/
-                // save content removed
-                if (this.IsSaveContentRemoved && Context.IsWorldReady)
-                {
-                    this.IsSaveContentRemoved = false;
-                    Game1.addHUDMessage(new HUDMessage(this.Translator.Get("warn.invalid-content-removed"), HUDMessage.error_type));
-                }
+                instance.Watchers.Update();
+                instance.WatcherSnapshot.Update(instance.Watchers);
+                instance.Watchers.Reset();
+                WatcherSnapshot state = instance.WatcherSnapshot;
 
                 /*********
                 ** Pre-update events
@@ -712,19 +722,19 @@ namespace StardewModdingAPI.Framework
                     /*********
                     ** Save created/loaded events
                     *********/
-                    if (this.IsBetweenCreateEvents)
+                    if (instance.IsBetweenCreateEvents)
                     {
                         // raise after-create
-                        this.IsBetweenCreateEvents = false;
+                        instance.IsBetweenCreateEvents = false;
                         this.Monitor.Log($"Context: after save creation, starting {Game1.currentSeason} {Game1.dayOfMonth} Y{Game1.year}.");
                         this.OnLoadStageChanged(LoadStage.CreatedSaveFile);
                         events.SaveCreated.RaiseEmpty();
                     }
 
-                    if (this.IsBetweenSaveEvents)
+                    if (instance.IsBetweenSaveEvents)
                     {
                         // raise after-save
-                        this.IsBetweenSaveEvents = false;
+                        instance.IsBetweenSaveEvents = false;
                         this.Monitor.Log($"Context: after save, starting {Game1.currentSeason} {Game1.dayOfMonth} Y{Game1.year}.");
                         events.Saved.RaiseEmpty();
                         events.DayStarted.RaiseEmpty();
@@ -785,7 +795,7 @@ namespace StardewModdingAPI.Framework
                         bool isChatInput = Game1.IsChatting || (Context.IsMultiplayer && Context.IsWorldReady && Game1.activeClickableMenu == null && Game1.currentMinigame == null && inputState.IsAnyDown(Game1.options.chatButton));
                         if (!isChatInput)
                         {
-                            ICursorPosition cursor = this.Input.CursorPosition;
+                            ICursorPosition cursor = instance.Input.CursorPosition;
 
                             // raise cursor moved event
                             if (state.Cursor.IsChanged)
@@ -950,9 +960,8 @@ namespace StardewModdingAPI.Framework
                     /*********
                     ** Game update
                     *********/
-                    // game launched
-                    bool isFirstTick = SCore.TicksElapsed == 0;
-                    if (isFirstTick)
+                    // game launched (not raised for secondary players in split-screen mode)
+                    if (instance.IsFirstTick && !Context.IsGameLaunched)
                     {
                         Context.IsGameLaunched = true;
                         events.GameLaunched.Raise(new GameLaunchedEventArgs());
@@ -974,9 +983,8 @@ namespace StardewModdingAPI.Framework
                         events.OneSecondUpdateTicking.RaiseEmpty();
                     try
                     {
-                        this.Input.ApplyOverrides(); // if mods added any new overrides since the update, process them now
-                        SCore.TicksElapsed++;
-                        runGameUpdate();
+                        instance.Input.ApplyOverrides(); // if mods added any new overrides since the update, process them now
+                        runUpdate();
                     }
                     catch (Exception ex)
                     {
@@ -1113,6 +1121,13 @@ namespace StardewModdingAPI.Framework
             return this.ContentCore.CreateGameContentManager("(generated)");
         }
 
+        /// <summary>Get the current game instance. This may not be the main player if playing in split-screen.</summary>
+        private SGame GetCurrentGameInstance()
+        {
+            return Game1.game1 as SGame
+                ?? throw new InvalidOperationException("The current game instance wasn't created by SMAPI.");
+        }
+
         /// <summary>Look for common issues with the game's XNB content, and log warnings if anything looks broken or outdated.</summary>
         /// <returns>Returns whether all integrity checks passed.</returns>
         private bool ValidateContentIntegrity()
@@ -1601,7 +1616,7 @@ namespace StardewModdingAPI.Framework
                         IModRegistry modRegistryHelper = new ModRegistryHelper(manifest.UniqueID, this.ModRegistry, proxyFactory, monitor);
                         IMultiplayerHelper multiplayerHelper = new MultiplayerHelper(manifest.UniqueID, this.Multiplayer);
 
-                        modHelper = new ModHelper(manifest.UniqueID, mod.DirectoryPath, this.Input, events, contentHelper, contentPackHelper, commandHelper, dataHelper, modRegistryHelper, reflectionHelper, multiplayerHelper, translationHelper);
+                        modHelper = new ModHelper(manifest.UniqueID, mod.DirectoryPath, () => this.GetCurrentGameInstance().Input, events, contentHelper, contentPackHelper, commandHelper, dataHelper, modRegistryHelper, reflectionHelper, multiplayerHelper, translationHelper);
                     }
 
                     // init mod
@@ -1813,5 +1828,14 @@ namespace StardewModdingAPI.Framework
             this.Monitor.LogFatal(message);
             this.CancellationToken.Cancel();
         }
+
+        /// <summary>Get the screen ID that should be logged to distinguish between players in split-screen mode, if any.</summary>
+        private int? GetScreenIdForLog()
+        {
+            if (Context.ScreenId != 0 || (Context.IsWorldReady && Context.IsSplitScreen))
+                return Context.ScreenId;
+
+            return null;
+        }
     }
 }
diff --git a/src/SMAPI/Framework/SGame.cs b/src/SMAPI/Framework/SGame.cs
index 1c769c3f..aeb48f5c 100644
--- a/src/SMAPI/Framework/SGame.cs
+++ b/src/SMAPI/Framework/SGame.cs
@@ -5,9 +5,11 @@ using System.Text;
 using System.Threading.Tasks;
 using Microsoft.Xna.Framework;
 using Microsoft.Xna.Framework.Graphics;
+using StardewModdingAPI.Events;
 using StardewModdingAPI.Framework.Events;
 using StardewModdingAPI.Framework.Input;
 using StardewModdingAPI.Framework.Reflection;
+using StardewModdingAPI.Framework.StateTracking.Snapshots;
 using StardewModdingAPI.Framework.Utilities;
 using StardewValley;
 using StardewValley.BellsAndWhistles;
@@ -41,30 +43,47 @@ namespace StardewModdingAPI.Framework
         /// <summary>Immediately exit the game without saving. This should only be invoked when an irrecoverable fatal error happens that risks save corruption or game-breaking bugs.</summary>
         private readonly Action<string> ExitGameImmediately;
 
-        /// <summary>Raised after the game finishes loading its initial content.</summary>
-        private readonly Action OnGameContentLoaded;
+        /// <summary>The initial override for <see cref="Input"/>. This value is null after initialization.</summary>
+        private SInputState InitialInput;
 
-        /// <summary>Raised when the game is updating its state (roughly 60 times per second).</summary>
-        private readonly Action<GameTime, Action> OnGameUpdating;
+        /// <summary>The initial override for <see cref="Multiplayer"/>. This value is null after initialization.</summary>
+        private SMultiplayer InitialMultiplayer;
 
-        /// <summary>Raised before the game exits.</summary>
-        private readonly Action OnGameExiting;
+        /// <summary>Raised when the instance is updating its state (roughly 60 times per second).</summary>
+        private readonly Action<SGame, GameTime, Action> OnUpdating;
 
 
         /*********
         ** Accessors
         *********/
         /// <summary>Manages input visible to the game.</summary>
-        public static SInputState Input => (SInputState)Game1.input;
-
-        /// <summary>The game's core multiplayer utility.</summary>
-        public static SMultiplayer Multiplayer => (SMultiplayer)Game1.multiplayer;
+        public SInputState Input => (SInputState)Game1.input;
 
         /// <summary>The game background task which initializes a new day.</summary>
-        public static Task NewDayTask => Game1._newDayTask;
+        public Task NewDayTask => Game1._newDayTask;
+
+        /// <summary>Monitors the entire game state for changes.</summary>
+        public WatcherCore Watchers { get; private set; }
+
+        /// <summary>A snapshot of the current <see cref="Watchers"/> state.</summary>
+        public WatcherSnapshot WatcherSnapshot { get; } = new WatcherSnapshot();
+
+        /// <summary>Whether the current update tick is the first one for this instance.</summary>
+        public bool IsFirstTick = true;
+
+        /// <summary>The number of ticks until SMAPI should notify mods that the game has loaded.</summary>
+        /// <remarks>Skipping a few frames ensures the game finishes initializing the world before mods try to change it.</remarks>
+        public Countdown AfterLoadTimer { get; } = new Countdown(5);
+
+        /// <summary>Whether the game is saving and SMAPI has already raised <see cref="IGameLoopEvents.Saving"/>.</summary>
+        public bool IsBetweenSaveEvents { get; set; }
+
+        /// <summary>Whether the game is creating the save file and SMAPI has already raised <see cref="IGameLoopEvents.SaveCreating"/>.</summary>
+        public bool IsBetweenCreateEvents { get; set; }
 
         /// <summary>Construct a content manager to read game content files.</summary>
         /// <remarks>This must be static because the game accesses it before the <see cref="SGame"/> constructor is called.</remarks>
+        [NonInstancedStatic]
         public static Func<IServiceProvider, string, LocalizedContentManager> CreateContentManagerImpl;
 
 
@@ -72,63 +91,40 @@ namespace StardewModdingAPI.Framework
         ** Public methods
         *********/
         /// <summary>Construct an instance.</summary>
+        /// <param name="playerIndex">The player index.</param>
+        /// <param name="instanceIndex">The instance index.</param>
         /// <param name="monitor">Encapsulates monitoring and logging for SMAPI.</param>
         /// <param name="reflection">Simplifies access to private game code.</param>
         /// <param name="eventManager">Manages SMAPI events for mods.</param>
+        /// <param name="input">Manages the game's input state.</param>
         /// <param name="modHooks">Handles mod hooks provided by the game.</param>
         /// <param name="multiplayer">The core multiplayer logic.</param>
         /// <param name="exitGameImmediately">Immediately exit the game without saving. This should only be invoked when an irrecoverable fatal error happens that risks save corruption or game-breaking bugs.</param>
-        /// <param name="onGameContentLoaded">Raised after the game finishes loading its initial content.</param>
-        /// <param name="onGameUpdating">Raised when the game is updating its state (roughly 60 times per second).</param>
-        /// <param name="onGameExiting">Raised before the game exits.</param>
-        public SGame(Monitor monitor, Reflector reflection, EventManager eventManager, SModHooks modHooks, SMultiplayer multiplayer, Action<string> exitGameImmediately, Action onGameContentLoaded, Action<GameTime, Action> onGameUpdating, Action onGameExiting)
+        /// <param name="onUpdating">Raised when the instance is updating its state (roughly 60 times per second).</param>
+        public SGame(PlayerIndex playerIndex, int instanceIndex, Monitor monitor, Reflector reflection, EventManager eventManager, SInputState input, SModHooks modHooks, SMultiplayer multiplayer, Action<string> exitGameImmediately, Action<SGame, GameTime, Action> onUpdating)
+            : base(playerIndex, instanceIndex)
         {
             // init XNA
             Game1.graphics.GraphicsProfile = GraphicsProfile.HiDef;
 
             // hook into game
-            Game1.input = new SInputState();
-            Game1.multiplayer = multiplayer;
+            Game1.input = this.InitialInput = input;
+            Game1.multiplayer = this.InitialMultiplayer = multiplayer;
             Game1.hooks = modHooks;
-            Game1.locations = new ObservableCollection<GameLocation>();
+            this._locations = new ObservableCollection<GameLocation>();
 
             // init SMAPI
             this.Monitor = monitor;
             this.Events = eventManager;
             this.Reflection = reflection;
             this.ExitGameImmediately = exitGameImmediately;
-            this.OnGameContentLoaded = onGameContentLoaded;
-            this.OnGameUpdating = onGameUpdating;
-            this.OnGameExiting = onGameExiting;
-        }
-
-        /// <summary>Get the observable location list.</summary>
-        public ObservableCollection<GameLocation> GetObservableLocations()
-        {
-            return (ObservableCollection<GameLocation>)Game1.locations;
+            this.OnUpdating = onUpdating;
         }
 
 
         /*********
         ** Protected methods
         *********/
-        /// <summary>Load content when the game is launched.</summary>
-        protected override void LoadContent()
-        {
-            base.LoadContent();
-
-            this.OnGameContentLoaded();
-        }
-
-        /// <summary>Perform cleanup logic when the game exits.</summary>
-        /// <param name="sender">The event sender.</param>
-        /// <param name="args">The event args.</param>
-        /// <remarks>This overrides the logic in <see cref="Game1.exitEvent"/> to let SMAPI clean up before exit.</remarks>
-        protected override void OnExiting(object sender, EventArgs args)
-        {
-            this.OnGameExiting();
-        }
-
         /// <summary>Construct a content manager to read game content files.</summary>
         /// <param name="serviceProvider">The service provider to use to locate services.</param>
         /// <param name="rootDirectory">The root directory to search for content.</param>
@@ -140,11 +136,42 @@ namespace StardewModdingAPI.Framework
             return SGame.CreateContentManagerImpl(serviceProvider, rootDirectory);
         }
 
-        /// <summary>The method called when the game is updating its state (roughly 60 times per second).</summary>
+        /// <summary>Initialize the instance when the game starts.</summary>
+        protected override void Initialize()
+        {
+            base.Initialize();
+
+            // The game resets public static fields after the class is constructed (see
+            // GameRunner.SetInstanceDefaults), so SMAPI needs to re-override them here.
+            Game1.input = this.InitialInput;
+            Game1.multiplayer = this.InitialMultiplayer;
+
+            // The Initial* fields should no longer be used after this point, since mods may
+            // further override them after initialization.
+            this.InitialInput = null;
+            this.InitialMultiplayer = null;
+        }
+
+        /// <summary>The method called when the instance is updating its state (roughly 60 times per second).</summary>
         /// <param name="gameTime">A snapshot of the game timing state.</param>
         protected override void Update(GameTime gameTime)
         {
-            this.OnGameUpdating(gameTime, () => base.Update(gameTime));
+            // set initial state
+            if (this.IsFirstTick)
+            {
+                this.Input.TrueUpdate();
+                this.Watchers = new WatcherCore(this.Input, (ObservableCollection<GameLocation>)this._locations);
+            }
+
+            // update
+            try
+            {
+                this.OnUpdating(this, gameTime, () => base.Update(gameTime));
+            }
+            finally
+            {
+                this.IsFirstTick = false;
+            }
         }
 
         /// <summary>The method called to draw everything to the screen.</summary>
@@ -171,7 +198,7 @@ namespace StardewModdingAPI.Framework
                     return;
                 }
 
-                // recover sprite batch
+                // recover draw state
                 try
                 {
                     if (Game1.spriteBatch.IsOpen(this.Reflection))
diff --git a/src/SMAPI/Framework/SGameRunner.cs b/src/SMAPI/Framework/SGameRunner.cs
new file mode 100644
index 00000000..ae06f513
--- /dev/null
+++ b/src/SMAPI/Framework/SGameRunner.cs
@@ -0,0 +1,156 @@
+using System;
+using System.Collections.Generic;
+using Microsoft.Xna.Framework;
+using Microsoft.Xna.Framework.Graphics;
+using StardewModdingAPI.Framework.Events;
+using StardewModdingAPI.Framework.Input;
+using StardewModdingAPI.Framework.Reflection;
+using StardewValley;
+
+namespace StardewModdingAPI.Framework
+{
+    /// <summary>SMAPI's extension of the game's core <see cref="GameRunner"/>, used to inject SMAPI components.</summary>
+    internal class SGameRunner : GameRunner
+    {
+        /*********
+        ** Fields
+        *********/
+        /// <summary>Encapsulates monitoring and logging for SMAPI.</summary>
+        private readonly Monitor Monitor;
+
+        /// <summary>Manages SMAPI events for mods.</summary>
+        private readonly EventManager Events;
+
+        /// <summary>Simplifies access to private game code.</summary>
+        private readonly Reflector Reflection;
+
+        /// <summary>Immediately exit the game without saving. This should only be invoked when an irrecoverable fatal error happens that risks save corruption or game-breaking bugs.</summary>
+        private readonly Action<string> ExitGameImmediately;
+
+        /// <summary>The core SMAPI mod hooks.</summary>
+        private readonly SModHooks ModHooks;
+
+        /// <summary>The core multiplayer logic.</summary>
+        private readonly SMultiplayer Multiplayer;
+
+        /// <summary>Raised after the game finishes loading its initial content.</summary>
+        private readonly Action OnGameContentLoaded;
+
+        /// <summary>Raised when XNA is updating (roughly 60 times per second).</summary>
+        private readonly Action<GameTime, Action> OnGameUpdating;
+
+        /// <summary>Raised when the game instance for a local split-screen player is updating (once per <see cref="OnGameUpdating"/> per player).</summary>
+        private readonly Action<SGame, GameTime, Action> OnPlayerInstanceUpdating;
+
+        /// <summary>Raised before the game exits.</summary>
+        private readonly Action OnGameExiting;
+
+
+        /*********
+        ** Public methods
+        *********/
+        /// <summary>Construct an instance.</summary>
+        /// <param name="monitor">Encapsulates monitoring and logging for SMAPI.</param>
+        /// <param name="reflection">Simplifies access to private game code.</param>
+        /// <param name="eventManager">Manages SMAPI events for mods.</param>
+        /// <param name="modHooks">Handles mod hooks provided by the game.</param>
+        /// <param name="multiplayer">The core multiplayer logic.</param>
+        /// <param name="exitGameImmediately">Immediately exit the game without saving. This should only be invoked when an irrecoverable fatal error happens that risks save corruption or game-breaking bugs.</param>
+        /// <param name="onGameContentLoaded">Raised after the game finishes loading its initial content.</param>
+        /// <param name="onGameUpdating">Raised when XNA is updating its state (roughly 60 times per second).</param>
+        /// <param name="onPlayerInstanceUpdating">Raised when the game instance for a local split-screen player is updating (once per <see cref="OnGameUpdating"/> per player).</param>
+        /// <param name="onGameExiting">Raised before the game exits.</param>
+        public SGameRunner(Monitor monitor, Reflector reflection, EventManager eventManager, SModHooks modHooks, SMultiplayer multiplayer, Action<string> exitGameImmediately, Action onGameContentLoaded, Action<GameTime, Action> onGameUpdating, Action<SGame, GameTime, Action> onPlayerInstanceUpdating, Action onGameExiting)
+        {
+            // init XNA
+            Game1.graphics.GraphicsProfile = GraphicsProfile.HiDef;
+
+            // hook into game
+            this.ModHooks = modHooks;
+
+            // init SMAPI
+            this.Monitor = monitor;
+            this.Events = eventManager;
+            this.Reflection = reflection;
+            this.Multiplayer = multiplayer;
+            this.ExitGameImmediately = exitGameImmediately;
+            this.OnGameContentLoaded = onGameContentLoaded;
+            this.OnGameUpdating = onGameUpdating;
+            this.OnPlayerInstanceUpdating = onPlayerInstanceUpdating;
+            this.OnGameExiting = onGameExiting;
+        }
+
+        /// <summary>Create a game instance for a local player.</summary>
+        /// <param name="playerIndex">The player index.</param>
+        /// <param name="instanceIndex">The instance index.</param>
+        public override Game1 CreateGameInstance(PlayerIndex playerIndex = PlayerIndex.One, int instanceIndex = 0)
+        {
+            SInputState inputState = new SInputState();
+            return new SGame(playerIndex, instanceIndex, this.Monitor, this.Reflection, this.Events, inputState, this.ModHooks, this.Multiplayer, this.ExitGameImmediately, this.OnPlayerInstanceUpdating);
+        }
+
+        /// <inheritdoc />
+        public override void AddGameInstance(PlayerIndex playerIndex)
+        {
+            base.AddGameInstance(playerIndex);
+
+            EarlyConstants.LogScreenId = Context.ScreenId;
+            this.UpdateForSplitScreenChanges();
+        }
+
+        /// <inheritdoc />
+        public override void RemoveGameInstance(Game1 instance)
+        {
+            base.RemoveGameInstance(instance);
+
+            if (this.gameInstances.Count <= 1)
+                EarlyConstants.LogScreenId = null;
+            this.UpdateForSplitScreenChanges();
+        }
+
+
+        /*********
+        ** Protected methods
+        *********/
+        /// <summary>Load content when the game is launched.</summary>
+        protected override void LoadContent()
+        {
+            base.LoadContent();
+
+            this.OnGameContentLoaded();
+        }
+
+        /// <summary>Perform cleanup logic when the game exits.</summary>
+        /// <param name="sender">The event sender.</param>
+        /// <param name="args">The event args.</param>
+        /// <remarks>This overrides the logic in <see cref="Game1.exitEvent"/> to let SMAPI clean up before exit.</remarks>
+        protected override void OnExiting(object sender, EventArgs args)
+        {
+            this.OnGameExiting();
+        }
+
+        /// <summary>The method called when the game is updating its state (roughly 60 times per second).</summary>
+        /// <param name="gameTime">A snapshot of the game timing state.</param>
+        protected override void Update(GameTime gameTime)
+        {
+            this.OnGameUpdating(gameTime, () => base.Update(gameTime));
+        }
+
+        private void UpdateForSplitScreenChanges()
+        {
+            HashSet<int> oldScreenIds = new HashSet<int>(Context.ActiveScreenIds);
+
+            // track active screens
+            Context.ActiveScreenIds.Clear();
+            foreach (var screen in this.gameInstances)
+                Context.ActiveScreenIds.Add(screen.instanceId);
+
+            // remember last removed screen
+            foreach (int id in oldScreenIds)
+            {
+                if (!Context.ActiveScreenIds.Contains(id))
+                    Context.LastRemovedScreenId = id;
+            }
+        }
+    }
+}
diff --git a/src/SMAPI/Utilities/PerScreen.cs b/src/SMAPI/Utilities/PerScreen.cs
new file mode 100644
index 00000000..55dae0d8
--- /dev/null
+++ b/src/SMAPI/Utilities/PerScreen.cs
@@ -0,0 +1,79 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace StardewModdingAPI.Utilities
+{
+    /// <summary>Manages a separate value for each player in split-screen mode. This can safely be used in non-split-screen mode too, it'll just have a single state in that case.</summary>
+    /// <typeparam name="T">The state class.</typeparam>
+    public class PerScreen<T>
+    {
+        /*********
+        ** Fields
+        *********/
+        /// <summary>Create the initial value for a player.</summary>
+        private readonly Func<T> CreateNewState;
+
+        /// <summary>The tracked values for each player.</summary>
+        private readonly IDictionary<int, T> States = new Dictionary<int, T>();
+
+        /// <summary>The last <see cref="Context.LastRemovedScreenId"/> value for which this instance was updated.</summary>
+        private int LastRemovedScreenId;
+
+
+        /*********
+        ** Accessors
+        *********/
+        /// <summary>The value for the current player.</summary>
+        /// <remarks>The value is initialized the first time it's requested for that player, unless it's set manually first.</remarks>
+        public T Value
+        {
+            get
+            {
+                this.RemoveDeadPlayers();
+                return this.States.TryGetValue(Context.ScreenId, out T state)
+                    ? state
+                    : this.States[Context.ScreenId] = this.CreateNewState();
+            }
+            set
+            {
+                this.RemoveDeadPlayers();
+                this.States[Context.ScreenId] = value;
+            }
+        }
+
+
+        /*********
+        ** Public methods
+        *********/
+        /// <summary>Construct an instance.</summary>
+        public PerScreen()
+            : this(null) { }
+
+        /// <summary>Construct an instance.</summary>
+        /// <param name="createNewState">Create the initial state for a player screen.</param>
+        public PerScreen(Func<T> createNewState)
+        {
+            this.CreateNewState = createNewState ?? (() => default);
+        }
+
+
+        /*********
+        ** Private methods
+        *********/
+        /// <summary>Remove players who are no longer have a split-screen index.</summary>
+        /// <returns>Returns whether any players were removed.</returns>
+        private void RemoveDeadPlayers()
+        {
+            if (this.LastRemovedScreenId == Context.LastRemovedScreenId)
+                return;
+
+            this.LastRemovedScreenId = Context.LastRemovedScreenId;
+            foreach (int id in this.States.Keys.ToArray())
+            {
+                if (!Context.HasScreenId(id))
+                    this.States.Remove(id);
+            }
+        }
+    }
+}