1 files changed, 93 insertions, 0 deletions
diff --git a/src/SMAPI/Framework/StateTracking/FieldWatchers/NetCollectionWatcher.cs b/src/SMAPI/Framework/StateTracking/FieldWatchers/NetCollectionWatcher.cs
new file mode 100644
index 00000000..f92edb90
--- /dev/null
+++ b/src/SMAPI/Framework/StateTracking/FieldWatchers/NetCollectionWatcher.cs
@@ -0,0 +1,93 @@
+using System.Collections.Generic;
+using Netcode;
+
+namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
+{
+    /// <summary>A watcher which detects changes to a Netcode collection.</summary>
+    internal class NetCollectionWatcher<TValue> : BaseDisposableWatcher, ICollectionWatcher<TValue>
+        where TValue : INetObject<INetSerializable>
+    {
+        /*********
+        ** Properties
+        *********/
+        /// <summary>The field being watched.</summary>
+        private readonly NetCollection<TValue> Field;
+
+        /// <summary>The pairs added since the last reset.</summary>
+        private readonly List<TValue> AddedImpl = new List<TValue>();
+
+        /// <summary>The pairs demoved since the last reset.</summary>
+        private readonly List<TValue> RemovedImpl = new List<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 NetCollectionWatcher(NetCollection<TValue> field)
+        {
+            this.Field = field;
+            field.OnValueAdded += this.OnValueAdded;
+            field.OnValueRemoved += this.OnValueRemoved;
+        }
+
+        /// <summary>Update the current value if needed.</summary>
+        public void Update()
+        {
+            this.AssertNotDisposed();
+        }
+
+        /// <summary>Set the current value as the baseline.</summary>
+        public void Reset()
+        {
+            this.AssertNotDisposed();
+
+            this.AddedImpl.Clear();
+            this.RemovedImpl.Clear();
+        }
+
+        /// <summary>Stop watching the field and release all references.</summary>
+        public override void Dispose()
+        {
+            if (!this.IsDisposed)
+            {
+                this.Field.OnValueAdded -= this.OnValueAdded;
+                this.Field.OnValueRemoved -= this.OnValueRemoved;
+            }
+
+            base.Dispose();
+        }
+
+
+        /*********
+        ** Private methods
+        *********/
+        /// <summary>A callback invoked when an entry is added to the collection.</summary>
+        /// <param name="value">The added value.</param>
+        private void OnValueAdded(TValue value)
+        {
+            this.AddedImpl.Add(value);
+        }
+
+        /// <summary>A callback invoked when an entry is removed from the collection.</summary>
+        /// <param name="value">The added value.</param>
+        private void OnValueRemoved(TValue value)
+        {
+            this.RemovedImpl.Add(value);
+        }
+    }
+}