1 files changed, 83 insertions, 0 deletions
diff --git a/src/SMAPI/Framework/StateTracking/FieldWatchers/ComparableListWatcher.cs b/src/SMAPI/Framework/StateTracking/FieldWatchers/ComparableListWatcher.cs
new file mode 100644
index 00000000..2ea6609a
--- /dev/null
+++ b/src/SMAPI/Framework/StateTracking/FieldWatchers/ComparableListWatcher.cs
@@ -0,0 +1,83 @@
+using System.Collections.Generic;
+using System.Linq;
+
+namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers
+{
+    /// <summary>A watcher which detects changes to a collection of values using a specified <see cref="IEqualityComparer{T}"/> instance.</summary>
+    /// <typeparam name="TValue">The value type within the collection.</typeparam>
+    internal class ComparableListWatcher<TValue> : BaseDisposableWatcher, ICollectionWatcher<TValue>
+    {
+        /*********
+        ** Properties
+        *********/
+        /// <summary>The collection to watch.</summary>
+        private readonly ICollection<TValue> CurrentValues;
+
+        /// <summary>The values during the previous update.</summary>
+        private HashSet<TValue> LastValues;
+
+        /// <summary>The pairs added since the last reset.</summary>
+        private readonly List<TValue> AddedImpl = new List<TValue>();
+
+        /// <summary>The pairs removed since the last reset.</summary>
+        private readonly List<TValue> RemovedImpl = new List<TValue>();
+
+
+        /*********
+        ** Accessors
+        *********/
+        /// <summary>Whether the value 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="values">The collection to watch.</param>
+        /// <param name="comparer">The equality comparer which indicates whether two values are the same.</param>
+        public ComparableListWatcher(ICollection<TValue> values, IEqualityComparer<TValue> comparer)
+        {
+            this.CurrentValues = values;
+            this.LastValues = new HashSet<TValue>(comparer);
+        }
+
+        /// <summary>Update the current value if needed.</summary>
+        public void Update()
+        {
+            this.AssertNotDisposed();
+
+            // optimise for zero items
+            if (this.CurrentValues.Count == 0)
+            {
+                if (this.LastValues.Count > 0)
+                {
+                    this.AddedImpl.AddRange(this.LastValues);
+                    this.LastValues.Clear();
+                }
+                return;
+            }
+
+            // detect changes
+            HashSet<TValue> curValues = new HashSet<TValue>(this.CurrentValues, this.LastValues.Comparer);
+            this.RemovedImpl.AddRange(from value in this.LastValues where !curValues.Contains(value) select value);
+            this.AddedImpl.AddRange(from value in curValues where !this.LastValues.Contains(value) select value);
+            this.LastValues = curValues;
+        }
+
+        /// <summary>Set the current value as the baseline.</summary>
+        public void Reset()
+        {
+            this.AssertNotDisposed();
+
+            this.AddedImpl.Clear();
+            this.RemovedImpl.Clear();
+        }
+    }
+}