1 files changed, 79 insertions, 0 deletions
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);
+            }
+        }
+    }
+}