1 files changed, 101 insertions, 0 deletions
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();
+        }
+    }
+}