Merge pull request #686 from wartech0/chest-tracking

Add chest items changed event

11 files changed, 440 insertions, 26 deletions

diff --git a/src/SMAPI/Framework/Events/EventManager.cs b/src/SMAPI/Framework/Events/EventManager.cs
index 18b00f69..892cbc7b 100644
--- a/src/SMAPI/Framework/Events/EventManager.cs
+++ b/src/SMAPI/Framework/Events/EventManager.cs
@@ -148,6 +148,9 @@ namespace StardewModdingAPI.Framework.Events
         /// <summary>Raised after objects are added or removed in a location.</summary>
         public readonly ManagedEvent<ObjectListChangedEventArgs> ObjectListChanged;
 
+        /// <summary>Raised after items are added or removed from a chest.</summary>
+        public readonly ManagedEvent<ChestInventoryChangedEventArgs> ChestInventoryChanged;
+
         /// <summary>Raised after terrain features (like floors and trees) are added or removed in a location.</summary>
         public readonly ManagedEvent<TerrainFeatureListChangedEventArgs> TerrainFeatureListChanged;
 
@@ -221,6 +224,7 @@ namespace StardewModdingAPI.Framework.Events
             this.LocationListChanged = ManageEventOf<LocationListChangedEventArgs>(nameof(IModEvents.World), nameof(IWorldEvents.BuildingListChanged));
             this.NpcListChanged = ManageEventOf<NpcListChangedEventArgs>(nameof(IModEvents.World), nameof(IWorldEvents.NpcListChanged));
             this.ObjectListChanged = ManageEventOf<ObjectListChangedEventArgs>(nameof(IModEvents.World), nameof(IWorldEvents.ObjectListChanged));
+            this.ChestInventoryChanged = ManageEventOf<ChestInventoryChangedEventArgs>(nameof(IModEvents.World), nameof(IWorldEvents.ChestInventoryChanged));
             this.TerrainFeatureListChanged = ManageEventOf<TerrainFeatureListChangedEventArgs>(nameof(IModEvents.World), nameof(IWorldEvents.TerrainFeatureListChanged));
 
             this.LoadStageChanged = ManageEventOf<LoadStageChangedEventArgs>(nameof(IModEvents.Specialized), nameof(ISpecializedEvents.LoadStageChanged));
diff --git a/src/SMAPI/Framework/Events/ModWorldEvents.cs b/src/SMAPI/Framework/Events/ModWorldEvents.cs
index b85002a3..2ae69669 100644
--- a/src/SMAPI/Framework/Events/ModWorldEvents.cs
+++ b/src/SMAPI/Framework/Events/ModWorldEvents.cs
@@ -51,6 +51,13 @@ namespace StardewModdingAPI.Framework.Events
             remove => this.EventManager.ObjectListChanged.Remove(value);
         }
 
+        /// <summary>Raised after items are added or removed from a chest.</summary>
+        public event EventHandler<ChestInventoryChangedEventArgs> ChestInventoryChanged
+        {
+            add => this.EventManager.ChestInventoryChanged.Add(value);
+            remove => this.EventManager.ChestInventoryChanged.Remove(value);
+        }
+
         /// <summary>Raised after terrain features (like floors and trees) are added or removed in a location.</summary>
         public event EventHandler<TerrainFeatureListChangedEventArgs> TerrainFeatureListChanged
         {
diff --git a/src/SMAPI/Framework/SGame.cs b/src/SMAPI/Framework/SGame.cs
index 4774233e..e2b22ba7 100644
--- a/src/SMAPI/Framework/SGame.cs
+++ b/src/SMAPI/Framework/SGame.cs
@@ -702,6 +702,16 @@ namespace StardewModdingAPI.Framework
                                 if (locState.Objects.IsChanged)
                                     events.ObjectListChanged.Raise(new ObjectListChangedEventArgs(location, locState.Objects.Added, locState.Objects.Removed));
 
+                                // chest items changed
+                                if (events.ChestInventoryChanged.HasListeners())
+                                {
+                                    foreach (var pair in locState.ChestItems)
+                                    {
+                                        SnapshotItemListDiff diff = pair.Value;
+                                        events.ChestInventoryChanged.Raise(new ChestInventoryChangedEventArgs(pair.Key, location, added: diff.Added, removed: diff.Removed, quantityChanged: diff.QuantityChanged));
+                                    }
+                                }
+
                                 // terrain features changed
                                 if (locState.TerrainFeatures.IsChanged)
                                     events.TerrainFeatureListChanged.Raise(new TerrainFeatureListChangedEventArgs(location, locState.TerrainFeatures.Added, locState.TerrainFeatures.Removed));
@@ -740,12 +750,13 @@ namespace StardewModdingAPI.Framework
                             }
 
                             // raise player inventory changed
-                            ItemStackChange[] changedItems = playerState.InventoryChanges.ToArray();
-                            if (changedItems.Any())
+                            if (playerState.Inventory.IsChanged)
                             {
+                                var inventory = playerState.Inventory;
+
                                 if (this.Monitor.IsVerbose)
                                     this.Monitor.Log("Events: player inventory changed.", LogLevel.Trace);
-                                events.InventoryChanged.Raise(new InventoryChangedEventArgs(player, changedItems));
+                                events.InventoryChanged.Raise(new InventoryChangedEventArgs(player, added: inventory.Added, removed: inventory.Removed, quantityChanged: inventory.QuantityChanged));
                             }
                         }
                     }
diff --git a/src/SMAPI/Framework/SnapshotItemListDiff.cs b/src/SMAPI/Framework/SnapshotItemListDiff.cs
new file mode 100644
index 00000000..e8ab1b1e
--- /dev/null
+++ b/src/SMAPI/Framework/SnapshotItemListDiff.cs
@@ -0,0 +1,66 @@
+using System.Collections.Generic;
+using System.Linq;
+using StardewModdingAPI.Events;
+using StardewValley;
+
+namespace StardewModdingAPI.Framework
+{
+    /// <summary>A snapshot of a tracked item list.</summary>
+    internal class SnapshotItemListDiff
+    {
+        /*********
+        ** Accessors
+        *********/
+        /// <summary>Whether the item list changed.</summary>
+        public bool IsChanged { get; }
+
+        /// <summary>The removed values.</summary>
+        public Item[] Removed { get; }
+
+        /// <summary>The added values.</summary>
+        public Item[] Added { get; }
+
+        /// <summary>The items whose stack sizes changed.</summary>
+        public ItemStackSizeChange[] QuantityChanged { get; }
+
+
+        /*********
+        ** Public methods
+        *********/
+        /// <summary>Update the snapshot.</summary>
+        /// <param name="added">The added values.</param>
+        /// <param name="removed">The removed values.</param>
+        /// <param name="sizesChanged">The items whose stack sizes changed.</param>
+        public SnapshotItemListDiff(Item[] added, Item[] removed, ItemStackSizeChange[] sizesChanged)
+        {
+            this.Removed = removed;
+            this.Added = added;
+            this.QuantityChanged = sizesChanged;
+
+            this.IsChanged = removed.Length > 0 || added.Length > 0 || sizesChanged.Length > 0;
+        }
+
+        /// <summary>Get a snapshot diff if anything changed in the given data.</summary>
+        /// <param name="added">The added item stacks.</param>
+        /// <param name="removed">The removed item stacks.</param>
+        /// <param name="stackSizes">The items with their previous stack sizes.</param>
+        /// <param name="changes">The inventory changes, or <c>null</c> if nothing changed.</param>
+        /// <returns>Returns whether anything changed.</returns>
+        public static bool TryGetChanges(ISet<Item> added, ISet<Item> removed, IDictionary<Item, int> stackSizes, out SnapshotItemListDiff changes)
+        {
+            KeyValuePair<Item, int>[] sizesChanged = stackSizes.Where(p => p.Key.Stack != p.Value).ToArray();
+            if (sizesChanged.Any() || added.Any() || removed.Any())
+            {
+                changes = new SnapshotItemListDiff(
+                    added: added.ToArray(),
+                    removed: removed.ToArray(),
+                    sizesChanged: sizesChanged.Select(p => new ItemStackSizeChange(p.Key, p.Value, p.Key.Stack)).ToArray()
+                );
+                return true;
+            }
+
+            changes = null;
+            return false;
+        }
+    }
+}
diff --git a/src/SMAPI/Framework/StateTracking/ChestTracker.cs b/src/SMAPI/Framework/StateTracking/ChestTracker.cs
new file mode 100644
index 00000000..65f58ee7
--- /dev/null
+++ b/src/SMAPI/Framework/StateTracking/ChestTracker.cs
@@ -0,0 +1,101 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using StardewModdingAPI.Framework.StateTracking.Comparers;
+using StardewModdingAPI.Framework.StateTracking.FieldWatchers;
+using StardewValley;
+using StardewValley.Objects;
+
+namespace StardewModdingAPI.Framework.StateTracking
+{
+    /// <summary>Tracks changes to a chest's items.</summary>
+    internal class ChestTracker : IDisposable
+    {
+        /*********
+        ** Fields
+        *********/
+        /// <summary>The item stack sizes as of the last update.</summary>
+        private readonly IDictionary<Item, int> StackSizes;
+
+        /// <summary>Items added since the last update.</summary>
+        private readonly HashSet<Item> Added = new HashSet<Item>(new ObjectReferenceComparer<Item>());
+
+        /// <summary>Items removed since the last update.</summary>
+        private readonly HashSet<Item> Removed = new HashSet<Item>(new ObjectReferenceComparer<Item>());
+
+        /// <summary>The underlying inventory watcher.</summary>
+        private readonly ICollectionWatcher<Item> InventoryWatcher;
+
+
+        /*********
+        ** Accessors
+        *********/
+        /// <summary>The chest being tracked.</summary>
+        public Chest Chest { get; }
+
+
+        /*********
+        ** Public methods
+        *********/
+        /// <summary>Construct an instance.</summary>
+        /// <param name="chest">The chest being tracked.</param>
+        public ChestTracker(Chest chest)
+        {
+            this.Chest = chest;
+            this.InventoryWatcher = WatcherFactory.ForNetList(chest.items);
+
+            this.StackSizes = this.Chest.items
+                .Where(n => n != null)
+                .Distinct()
+                .ToDictionary(n => n, n => n.Stack);
+        }
+
+        /// <summary>Update the current values if needed.</summary>
+        public void Update()
+        {
+            // update watcher
+            this.InventoryWatcher.Update();
+            foreach (Item item in this.InventoryWatcher.Added)
+                this.Added.Add(item);
+            foreach (Item item in this.InventoryWatcher.Removed)
+            {
+                if (!this.Added.Remove(item)) // item didn't change if it was both added and removed, so remove it from both lists
+                    this.Removed.Add(item);
+            }
+
+            // stop tracking removed stacks
+            foreach (Item item in this.Removed)
+                this.StackSizes.Remove(item);
+        }
+
+        /// <summary>Reset all trackers so their current values are the baseline.</summary>
+        public void Reset()
+        {
+            // update stack sizes
+            foreach (Item item in this.StackSizes.Keys.ToArray().Concat(this.Added))
+                this.StackSizes[item] = item.Stack;
+
+            // update watcher
+            this.InventoryWatcher.Reset();
+            this.Added.Clear();
+            this.Removed.Clear();
+        }
+
+        /// <summary>Get the inventory changes since the last update, if anything changed.</summary>
+        /// <param name="changes">The inventory changes, or <c>null</c> if nothing changed.</param>
+        /// <returns>Returns whether anything changed.</returns>
+        public bool TryGetInventoryChanges(out SnapshotItemListDiff changes)
+        {
+            return SnapshotItemListDiff.TryGetChanges(added: this.Added, removed: this.Removed, stackSizes: this.StackSizes, out changes);
+        }
+
+        /// <summary>Release watchers and resources.</summary>
+        public void Dispose()
+        {
+            this.StackSizes.Clear();
+            this.Added.Clear();
+            this.Removed.Clear();
+            this.InventoryWatcher.Dispose();
+        }
+    }
+}
diff --git a/src/SMAPI/Framework/StateTracking/FieldWatchers/NetListWatcher.cs b/src/SMAPI/Framework/StateTracking/FieldWatchers/NetListWatcher.cs
new file mode 100644
index 00000000..0b4d3030
--- /dev/null
+++ b/src/SMAPI/Framework/StateTracking/FieldWatchers/NetListWatcher.cs
@@ -0,0 +1,143 @@
+using System.Collections.Generic;
+using Netcode;
+using StardewModdingAPI.Framework.StateTracking.Comparers;
+
+namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
+{
+    /// <summary>A watcher which detects changes to a net list field.</summary>
+    /// <typeparam name="TValue">The list value type.</typeparam>
+    internal class NetListWatcher<TValue> : BaseDisposableWatcher, ICollectionWatcher<TValue>
+        where TValue : class, INetObject<INetSerializable>
+    {
+        /*********
+        ** Fields
+        *********/
+        /// <summary>The field being watched.</summary>
+        private readonly NetList<TValue, NetRef<TValue>> Field;
+
+        /// <summary>The pairs added since the last reset.</summary>
+        private readonly ISet<TValue> AddedImpl = new HashSet<TValue>(new ObjectReferenceComparer<TValue>());
+
+        /// <summary>The pairs removed since the last reset.</summary>
+        private readonly ISet<TValue> RemovedImpl = new HashSet<TValue>(new ObjectReferenceComparer<TValue>());
+
+
+        /*********
+        ** Accessors
+        *********/
+        /// <summary>Whether the collection changed since the last reset.</summary>
+        public bool IsChanged => this.AddedImpl.Count > 0 || this.RemovedImpl.Count > 0;
+
+        /// <summary>The values added since the last reset.</summary>
+        public IEnumerable<TValue> Added => this.AddedImpl;
+
+        /// <summary>The values removed since the last reset.</summary>
+        public IEnumerable<TValue> Removed => this.RemovedImpl;
+
+
+        /*********
+        ** Public methods
+        *********/
+        /// <summary>Construct an instance.</summary>
+        /// <param name="field">The field to watch.</param>
+        public NetListWatcher(NetList<TValue, NetRef<TValue>> field)
+        {
+            this.Field = field;
+            field.OnElementChanged += this.OnElementChanged;
+            field.OnArrayReplaced += this.OnArrayReplaced;
+        }
+
+        /// <summary>Set the current value as the baseline.</summary>
+        public void Reset()
+        {
+            this.AddedImpl.Clear();
+            this.RemovedImpl.Clear();
+        }
+
+        /// <summary>Update the current value if needed.</summary>
+        public void Update()
+        {
+            this.AssertNotDisposed();
+        }
+
+        /// <summary>Stop watching the field and release all references.</summary>
+        public override void Dispose()
+        {
+            if (!this.IsDisposed)
+            {
+                this.Field.OnElementChanged -= this.OnElementChanged;
+                this.Field.OnArrayReplaced -= this.OnArrayReplaced;
+            }
+
+            base.Dispose();
+        }
+
+
+        /*********
+        ** Private methods
+        *********/
+        /// <summary>A callback invoked when the value list is replaced.</summary>
+        /// <param name="list">The net field whose values changed.</param>
+        /// <param name="oldValues">The previous list of values.</param>
+        /// <param name="newValues">The new list of values.</param>
+        private void OnArrayReplaced(NetList<TValue, NetRef<TValue>> list, IList<TValue> oldValues, IList<TValue> newValues)
+        {
+            ISet<TValue> oldSet = new HashSet<TValue>(oldValues, new ObjectReferenceComparer<TValue>());
+            ISet<TValue> changed = new HashSet<TValue>(newValues, new ObjectReferenceComparer<TValue>());
+
+            foreach (TValue value in oldSet)
+            {
+                if (!changed.Contains(value))
+                    this.Remove(value);
+            }
+            foreach (TValue value in changed)
+            {
+                if (!oldSet.Contains(value))
+                    this.Add(value);
+            }
+        }
+
+        /// <summary>A callback invoked when an entry is replaced.</summary>
+        /// <param name="list">The net field whose values changed.</param>
+        /// <param name="index">The list index which changed.</param>
+        /// <param name="oldValue">The previous value.</param>
+        /// <param name="newValue">The new value.</param>
+        private void OnElementChanged(NetList<TValue, NetRef<TValue>> list, int index, TValue oldValue, TValue newValue)
+        {
+            this.Remove(oldValue);
+            this.Add(newValue);
+        }
+
+        /// <summary>Track an added item.</summary>
+        /// <param name="value">The value that was added.</param>
+        private void Add(TValue value)
+        {
+            if (value == null)
+                return;
+
+            if (this.RemovedImpl.Contains(value))
+            {
+                this.AddedImpl.Remove(value);
+                this.RemovedImpl.Remove(value);
+            }
+            else
+                this.AddedImpl.Add(value);
+        }
+
+        /// <summary>Track a removed item.</summary>
+        /// <param name="value">The value that was removed.</param>
+        private void Remove(TValue value)
+        {
+            if (value == null)
+                return;
+
+            if (this.AddedImpl.Contains(value))
+            {
+                this.AddedImpl.Remove(value);
+                this.RemovedImpl.Remove(value);
+            }
+            else
+                this.RemovedImpl.Add(value);
+        }
+    }
+}
diff --git a/src/SMAPI/Framework/StateTracking/FieldWatchers/WatcherFactory.cs b/src/SMAPI/Framework/StateTracking/FieldWatchers/WatcherFactory.cs
index 314ff7f5..bde43486 100644
--- a/src/SMAPI/Framework/StateTracking/FieldWatchers/WatcherFactory.cs
+++ b/src/SMAPI/Framework/StateTracking/FieldWatchers/WatcherFactory.cs
@@ -82,6 +82,14 @@ namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
             return new NetCollectionWatcher<T>(collection);
         }
 
+        /// <summary>Get a watcher for a net list.</summary>
+        /// <typeparam name="T">The value type.</typeparam>
+        /// <param name="collection">The net list.</param>
+        public static ICollectionWatcher<T> ForNetList<T>(NetList<T, NetRef<T>> collection) where T : class, INetObject<INetSerializable>
+        {
+            return new NetListWatcher<T>(collection);
+        }
+
         /// <summary>Get a watcher for a net dictionary.</summary>
         /// <typeparam name="TKey">The dictionary key type.</typeparam>
         /// <typeparam name="TValue">The dictionary value type.</typeparam>
diff --git a/src/SMAPI/Framework/StateTracking/LocationTracker.cs b/src/SMAPI/Framework/StateTracking/LocationTracker.cs
index 1f479e12..519fe8f4 100644
--- a/src/SMAPI/Framework/StateTracking/LocationTracker.cs
+++ b/src/SMAPI/Framework/StateTracking/LocationTracker.cs
@@ -5,8 +5,9 @@ using StardewModdingAPI.Framework.StateTracking.FieldWatchers;
 using StardewValley;
 using StardewValley.Buildings;
 using StardewValley.Locations;
+using StardewValley.Objects;
 using StardewValley.TerrainFeatures;
-using Object = StardewValley.Object;
+using SObject = StardewValley.Object;
 
 namespace StardewModdingAPI.Framework.StateTracking
 {
@@ -42,11 +43,14 @@ namespace StardewModdingAPI.Framework.StateTracking
         public ICollectionWatcher<NPC> NpcsWatcher { get; }
 
         /// <summary>Tracks added or removed objects.</summary>
-        public IDictionaryWatcher<Vector2, Object> ObjectsWatcher { get; }
+        public IDictionaryWatcher<Vector2, SObject> ObjectsWatcher { get; }
 
         /// <summary>Tracks added or removed terrain features.</summary>
         public IDictionaryWatcher<Vector2, TerrainFeature> TerrainFeaturesWatcher { get; }
 
+        /// <summary>Tracks items added or removed to chests.</summary>
+        public IDictionary<Vector2, ChestTracker> ChestWatchers { get; } = new Dictionary<Vector2, ChestTracker>();
+
 
         /*********
         ** Public methods
@@ -74,13 +78,8 @@ namespace StardewModdingAPI.Framework.StateTracking
                 this.ObjectsWatcher,
                 this.TerrainFeaturesWatcher
             });
-        }
 
-        /// <summary>Stop watching the player fields and release all references.</summary>
-        public void Dispose()
-        {
-            foreach (IWatcher watcher in this.Watchers)
-                watcher.Dispose();
+            this.UpdateChestWatcherList(added: location.Objects.Pairs, removed: new KeyValuePair<Vector2, SObject>[0]);
         }
 
         /// <summary>Update the current value if needed.</summary>
@@ -88,6 +87,11 @@ namespace StardewModdingAPI.Framework.StateTracking
         {
             foreach (IWatcher watcher in this.Watchers)
                 watcher.Update();
+
+            this.UpdateChestWatcherList(added: this.ObjectsWatcher.Added, removed: this.ObjectsWatcher.Removed);
+
+            foreach (var watcher in this.ChestWatchers)
+                watcher.Value.Update();
         }
 
         /// <summary>Set the current value as the baseline.</summary>
@@ -95,6 +99,46 @@ namespace StardewModdingAPI.Framework.StateTracking
         {
             foreach (IWatcher watcher in this.Watchers)
                 watcher.Reset();
+
+            foreach (var watcher in this.ChestWatchers)
+                watcher.Value.Reset();
+        }
+
+        /// <summary>Stop watching the player fields and release all references.</summary>
+        public void Dispose()
+        {
+            foreach (IWatcher watcher in this.Watchers)
+                watcher.Dispose();
+
+            foreach (var watcher in this.ChestWatchers.Values)
+                watcher.Dispose();
+        }
+
+
+        /*********
+        ** Private methods
+        *********/
+        /// <summary>Update the watcher list for added or removed chests.</summary>
+        /// <param name="added">The objects added to the location.</param>
+        /// <param name="removed">The objects removed from the location.</param>
+        private void UpdateChestWatcherList(IEnumerable<KeyValuePair<Vector2, SObject>> added, IEnumerable<KeyValuePair<Vector2, SObject>> removed)
+        {
+            // remove unused watchers
+            foreach (KeyValuePair<Vector2, SObject> pair in removed)
+            {
+                if (pair.Value is Chest && this.ChestWatchers.TryGetValue(pair.Key, out ChestTracker watcher))
+                {
+                    watcher.Dispose();
+                    this.ChestWatchers.Remove(pair.Key);
+                }
+            }
+
+            // add new watchers
+            foreach (KeyValuePair<Vector2, SObject> pair in added)
+            {
+                if (pair.Value is Chest chest && !this.ChestWatchers.ContainsKey(pair.Key))
+                    this.ChestWatchers.Add(pair.Key, new ChestTracker(chest));
+            }
         }
     }
 }
diff --git a/src/SMAPI/Framework/StateTracking/PlayerTracker.cs b/src/SMAPI/Framework/StateTracking/PlayerTracker.cs
index 6302a889..cf49a7c1 100644
--- a/src/SMAPI/Framework/StateTracking/PlayerTracker.cs
+++ b/src/SMAPI/Framework/StateTracking/PlayerTracker.cs
@@ -2,10 +2,9 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 using StardewModdingAPI.Enums;
-using StardewModdingAPI.Events;
+using StardewModdingAPI.Framework.StateTracking.Comparers;
 using StardewModdingAPI.Framework.StateTracking.FieldWatchers;
 using StardewValley;
-using ChangeType = StardewModdingAPI.Events.ChangeType;
 
 namespace StardewModdingAPI.Framework.StateTracking
 {
@@ -99,25 +98,32 @@ namespace StardewModdingAPI.Framework.StateTracking
             return this.Player.currentLocation ?? this.LastValidLocation;
         }
 
-        /// <summary>Get the player inventory changes between two states.</summary>
-        public IEnumerable<ItemStackChange> GetInventoryChanges()
+        /// <summary>Get the inventory changes since the last update, if anything changed.</summary>
+        /// <param name="changes">The inventory changes, or <c>null</c> if nothing changed.</param>
+        /// <returns>Returns whether anything changed.</returns>
+        public bool TryGetInventoryChanges(out SnapshotItemListDiff changes)
         {
-            IDictionary<Item, int> previous = this.PreviousInventory;
             IDictionary<Item, int> current = this.GetInventory();
-            foreach (Item item in previous.Keys.Union(current.Keys))
+
+            ISet<Item> added = new HashSet<Item>(new ObjectReferenceComparer<Item>());
+            ISet<Item> removed = new HashSet<Item>(new ObjectReferenceComparer<Item>());
+            foreach (Item item in this.PreviousInventory.Keys.Union(current.Keys))
             {
-                if (!previous.TryGetValue(item, out int prevStack))
-                    yield return new ItemStackChange { Item = item, StackChange = item.Stack, ChangeType = ChangeType.Added };
-                else if (!current.TryGetValue(item, out int newStack))
-                    yield return new ItemStackChange { Item = item, StackChange = -item.Stack, ChangeType = ChangeType.Removed };
-                else if (prevStack != newStack)
-                    yield return new ItemStackChange { Item = item, StackChange = newStack - prevStack, ChangeType = ChangeType.StackChange };
+                if (!this.PreviousInventory.ContainsKey(item))
+                    added.Add(item);
+                else if (!current.ContainsKey(item))
+                    removed.Add(item);
             }
+
+            return SnapshotItemListDiff.TryGetChanges(added: added, removed: removed, stackSizes: this.PreviousInventory, out changes);
         }
 
-        /// <summary>Stop watching the player fields and release all references.</summary>
+        /// <summary>Release watchers and resources.</summary>
         public void Dispose()
         {
+            this.PreviousInventory.Clear();
+            this.CurrentInventory?.Clear();
+
             foreach (IWatcher watcher in this.Watchers)
                 watcher.Dispose();
         }
diff --git a/src/SMAPI/Framework/StateTracking/Snapshots/LocationSnapshot.cs b/src/SMAPI/Framework/StateTracking/Snapshots/LocationSnapshot.cs
index d3029540..6ae52fd0 100644
--- a/src/SMAPI/Framework/StateTracking/Snapshots/LocationSnapshot.cs
+++ b/src/SMAPI/Framework/StateTracking/Snapshots/LocationSnapshot.cs
@@ -2,6 +2,7 @@ using System.Collections.Generic;
 using Microsoft.Xna.Framework;
 using StardewValley;
 using StardewValley.Buildings;
+using StardewValley.Objects;
 using StardewValley.TerrainFeatures;
 
 namespace StardewModdingAPI.Framework.StateTracking.Snapshots
@@ -33,6 +34,9 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
         /// <summary>Tracks added or removed terrain features.</summary>
         public SnapshotListDiff<KeyValuePair<Vector2, TerrainFeature>> TerrainFeatures { get; } = new SnapshotListDiff<KeyValuePair<Vector2, TerrainFeature>>();
 
+        /// <summary>Tracks changed chest inventories.</summary>
+        public IDictionary<Chest, SnapshotItemListDiff> ChestItems { get; } = new Dictionary<Chest, SnapshotItemListDiff>();
+
 
         /*********
         ** Public methods
@@ -48,12 +52,21 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
         /// <param name="watcher">The watcher to snapshot.</param>
         public void Update(LocationTracker watcher)
         {
+            // main lists
             this.Buildings.Update(watcher.BuildingsWatcher);
             this.Debris.Update(watcher.DebrisWatcher);
             this.LargeTerrainFeatures.Update(watcher.LargeTerrainFeaturesWatcher);
             this.Npcs.Update(watcher.NpcsWatcher);
             this.Objects.Update(watcher.ObjectsWatcher);
             this.TerrainFeatures.Update(watcher.TerrainFeaturesWatcher);
+
+            // chest inventories
+            this.ChestItems.Clear();
+            foreach (ChestTracker tracker in watcher.ChestWatchers.Values)
+            {
+                if (tracker.TryGetInventoryChanges(out SnapshotItemListDiff changes))
+                    this.ChestItems[tracker.Chest] = changes;
+            }
         }
     }
 }
diff --git a/src/SMAPI/Framework/StateTracking/Snapshots/PlayerSnapshot.cs b/src/SMAPI/Framework/StateTracking/Snapshots/PlayerSnapshot.cs
index 7bcd9f82..f0fb9485 100644
--- a/src/SMAPI/Framework/StateTracking/Snapshots/PlayerSnapshot.cs
+++ b/src/SMAPI/Framework/StateTracking/Snapshots/PlayerSnapshot.cs
@@ -11,6 +11,13 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
     internal class PlayerSnapshot
     {
         /*********
+        ** Fields
+        *********/
+        /// <summary>An empty item list diff.</summary>
+        private readonly SnapshotItemListDiff EmptyItemListDiff = new SnapshotItemListDiff(new Item[0], new Item[0], new ItemStackSizeChange[0]);
+
+
+        /*********
         ** Accessors
         *********/
         /// <summary>The player being tracked.</summary>
@@ -27,7 +34,7 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
                 .ToDictionary(skill => skill, skill => new SnapshotDiff<int>());
 
         /// <summary>Get a list of inventory changes.</summary>
-        public IEnumerable<ItemStackChange> InventoryChanges { get; private set; }
+        public SnapshotItemListDiff Inventory { get; private set; }
 
 
         /*********
@@ -47,7 +54,11 @@ namespace StardewModdingAPI.Framework.StateTracking.Snapshots
             this.Location.Update(watcher.LocationWatcher);
             foreach (var pair in this.Skills)
                 pair.Value.Update(watcher.SkillWatchers[pair.Key]);
-            this.InventoryChanges = watcher.GetInventoryChanges().ToArray();
+
+            this.Inventory = watcher.TryGetInventoryChanges(out SnapshotItemListDiff itemChanges)
+                ? itemChanges
+                : this.EmptyItemListDiff;
+
         }
     }
 }