1 files changed, 103 insertions, 0 deletions
diff --git a/src/SMAPI/Framework/StateTracking/FieldWatchers/NetDictionaryWatcher.cs b/src/SMAPI/Framework/StateTracking/FieldWatchers/NetDictionaryWatcher.cs
new file mode 100644
index 00000000..7a2bf84e
--- /dev/null
+++ b/src/SMAPI/Framework/StateTracking/FieldWatchers/NetDictionaryWatcher.cs
@@ -0,0 +1,103 @@
+using System.Collections.Generic;
+using Netcode;
+
+namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
+{
+    /// <summary>A watcher which detects changes to a net dictionary field.</summary>
+    /// <typeparam name="TKey">The dictionary key type.</typeparam>
+    /// <typeparam name="TValue">The dictionary value type.</typeparam>
+    /// <typeparam name="TField">The net type equivalent to <typeparamref name="TValue"/>.</typeparam>
+    /// <typeparam name="TSerialDict">The serializable dictionary type that can store the keys and values.</typeparam>
+    /// <typeparam name="TSelf">The net field instance type.</typeparam>
+    internal class NetDictionaryWatcher<TKey, TValue, TField, TSerialDict, TSelf> : BaseDisposableWatcher, IDictionaryWatcher<TKey, TValue>
+        where TField : class, INetObject<INetSerializable>, new()
+        where TSerialDict : IDictionary<TKey, TValue>, new()
+        where TSelf : NetDictionary<TKey, TValue, TField, TSerialDict, TSelf>
+    {
+        /*********
+        ** Properties
+        *********/
+        /// <summary>The pairs added since the last reset.</summary>
+        private readonly IDictionary<TKey, TValue> PairsAdded = new Dictionary<TKey, TValue>();
+
+        /// <summary>The pairs demoved since the last reset.</summary>
+        private readonly IDictionary<TKey, TValue> PairsRemoved = new Dictionary<TKey, TValue>();
+
+        /// <summary>The field being watched.</summary>
+        private readonly NetDictionary<TKey, TValue, TField, TSerialDict, TSelf> Field;
+
+
+        /*********
+        ** Accessors
+        *********/
+        /// <summary>Whether the collection changed since the last reset.</summary>
+        public bool IsChanged => this.PairsAdded.Count > 0 || this.PairsRemoved.Count > 0;
+
+        /// <summary>The values added since the last reset.</summary>
+        public IEnumerable<KeyValuePair<TKey, TValue>> Added => this.PairsAdded;
+
+        /// <summary>The values removed since the last reset.</summary>
+        public IEnumerable<KeyValuePair<TKey, TValue>> Removed => this.PairsRemoved;
+
+
+        /*********
+        ** Public methods
+        *********/
+        /// <summary>Construct an instance.</summary>
+        /// <param name="field">The field to watch.</param>
+        public NetDictionaryWatcher(NetDictionary<TKey, TValue, TField, TSerialDict, TSelf> 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.PairsAdded.Clear();
+            this.PairsRemoved.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 dictionary.</summary>
+        /// <param name="key">The entry key.</param>
+        /// <param name="value">The entry value.</param>
+        private void OnValueAdded(TKey key, TValue value)
+        {
+            this.PairsAdded[key] = value;
+        }
+
+        /// <summary>A callback invoked when an entry is removed from the dictionary.</summary>
+        /// <param name="key">The entry key.</param>
+        /// <param name="value">The entry value.</param>
+        private void OnValueRemoved(TKey key, TValue value)
+        {
+            if (!this.PairsRemoved.ContainsKey(key))
+                this.PairsRemoved[key] = value;
+        }
+    }
+}