reduce allocations in collection watchers

src/SMAPI/Framework/StateTracking/FieldWatchers/ComparableListWatcher.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using StardewValley.Extensions;
 
 namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers;
 
@@ -15,6 +16,9 @@ internal class ComparableListWatcher<TValue> : BaseDisposableWatcher, ICollectio
     /// <summary>The values during the previous update.</summary>
     private HashSet<TValue> LastValues;
 
+    /// <summary>A pooled set used to update <see cref="LastValues"/> each update.</summary>
+    private HashSet<TValue> NewValues;
+
     /// <summary>The pairs added since the last reset.</summary>
     private readonly List<TValue> AddedImpl = [];
 
@@ -50,6 +54,7 @@ internal class ComparableListWatcher<TValue> : BaseDisposableWatcher, ICollectio
         this.Name = name;
         this.CurrentValues = values;
         this.LastValues = new HashSet<TValue>(comparer);
+        this.NewValues = new HashSet<TValue>(comparer);
     }
 
     /// <inheritdoc />
@@ -68,19 +73,24 @@ internal class ComparableListWatcher<TValue> : BaseDisposableWatcher, ICollectio
             return;
         }
 
+        // get new values
+        this.NewValues.Clear();
+        this.NewValues.AddRange(this.CurrentValues);
+
         // detect changes
-        HashSet<TValue> curValues = new HashSet<TValue>(this.CurrentValues, this.LastValues.Comparer);
         foreach (TValue value in this.LastValues)
         {
-            if (!curValues.Contains(value))
+            if (!this.NewValues.Contains(value))
                 this.RemovedImpl.Add(value);
         }
-        foreach (TValue value in curValues)
+        foreach (TValue value in this.NewValues)
         {
             if (!this.LastValues.Contains(value))
                 this.AddedImpl.Add(value);
         }
-        this.LastValues = curValues;
+
+        // save result
+        (this.LastValues, this.NewValues) = (this.NewValues, this.LastValues); // reuse the now-unused previous 'LastValues' set on the next update
     }
 
     /// <inheritdoc />

src/SMAPI/Framework/StateTracking/FieldWatchers/InventoryWatcher.cs

@@ -1,6 +1,7 @@
 using System.Collections.Generic;
 using StardewModdingAPI.Framework.StateTracking.Comparers;
 using StardewValley;
+using StardewValley.Extensions;
 using StardewValley.Inventories;
 
 namespace StardewModdingAPI.Framework.StateTracking.FieldWatchers;
@@ -15,10 +16,16 @@ internal class InventoryWatcher : BaseDisposableWatcher, ICollectionWatcher<Item
     private readonly Inventory Inventory;
 
     /// <summary>The pairs added since the last reset.</summary>
-    private readonly ISet<Item> AddedImpl = new HashSet<Item>(new ObjectReferenceComparer<Item>());
+    private readonly HashSet<Item> AddedImpl = new(new ObjectReferenceComparer<Item>());
 
     /// <summary>The pairs removed since the last reset.</summary>
-    private readonly ISet<Item> RemovedImpl = new HashSet<Item>(new ObjectReferenceComparer<Item>());
+    private readonly HashSet<Item> RemovedImpl = new(new ObjectReferenceComparer<Item>());
+
+    /// <summary>A pooled set used to track the previous inventory when detecting changes.</summary>
+    private readonly HashSet<Item> PooledOldSet = new(new ObjectReferenceComparer<Item>());
+
+    /// <summary>A pooled set used to track the new inventory when detecting changes.</summary>
+    private readonly HashSet<Item> PooledNewSet = new(new ObjectReferenceComparer<Item>());
 
 
     /*********
@@ -87,17 +94,21 @@ internal class InventoryWatcher : BaseDisposableWatcher, ICollectionWatcher<Item
     /// <param name="newValues">The new list of values.</param>
     private void OnInventoryReplaced(Inventory inventory, IList<Item> oldValues, IList<Item> newValues)
     {
-        ISet<Item> oldSet = new HashSet<Item>(oldValues, new ObjectReferenceComparer<Item>());
-        ISet<Item> changed = new HashSet<Item>(newValues, new ObjectReferenceComparer<Item>());
+        this.PooledOldSet.Clear();
+        this.PooledOldSet.AddRange(oldValues);
 
-        foreach (Item value in oldSet)
+        this.PooledNewSet.Clear();
+        this.PooledNewSet.AddRange(newValues);
+
+        foreach (Item value in this.PooledOldSet)
         {
-            if (!changed.Contains(value))
+            if (!this.PooledNewSet.Contains(value))
                 this.Remove(value);
         }
-        foreach (Item value in changed)
+
+        foreach (Item value in this.PooledNewSet)
         {
-            if (!oldSet.Contains(value))
+            if (!this.PooledOldSet.Contains(value))
                 this.Add(value);
         }
     }

src/SMAPI/Framework/StateTracking/PlayerTracker.cs

@@ -20,6 +20,12 @@ internal class PlayerTracker : IDisposable
     /// <summary>The player's inventory change as of the last update.</summary>
     private readonly Dictionary<Item, int> CurrentInventory = [];
 
+    /// <summary>A pooled set used to track the added items when detecting changes.</summary>
+    private readonly HashSet<Item> PooledAdded;
+
+    /// <summary>A pooled set used to track the removed items when detecting changes.</summary>
+    private readonly HashSet<Item> PooledRemoved;
+
     /// <summary>The player's last valid location.</summary>
     private GameLocation? LastValidLocation;
 
@@ -67,6 +73,11 @@ internal class PlayerTracker : IDisposable
         // track watchers for convenience
         this.Watchers.Add(this.LocationWatcher);
         this.Watchers.AddRange(this.SkillWatchers.Values);
+
+        // init pooled sets
+        var comparer = new ObjectReferenceComparer<Item>();
+        this.PooledAdded = new HashSet<Item>(comparer);
+        this.PooledRemoved = new HashSet<Item>(comparer);
     }
 
     /// <summary>Update the current values if needed.</summary>
@@ -109,22 +120,22 @@ internal class PlayerTracker : IDisposable
     /// <returns>Returns whether anything changed.</returns>
     public bool TryGetInventoryChanges([NotNullWhen(true)] out SnapshotItemListDiff? changes)
     {
-        HashSet<Item> added = new(new ObjectReferenceComparer<Item>());
-        HashSet<Item> removed = new(new ObjectReferenceComparer<Item>());
+        this.PooledAdded.Clear();
+        this.PooledRemoved.Clear();
 
         foreach (Item item in this.PreviousInventory.Keys)
         {
             if (!this.CurrentInventory.ContainsKey(item))
-                removed.Add(item);
+                this.PooledRemoved.Add(item);
         }
 
         foreach (Item item in this.CurrentInventory.Keys)
         {
             if (!this.PreviousInventory.ContainsKey(item))
-                added.Add(item);
+                this.PooledAdded.Add(item);
         }
 
-        return SnapshotItemListDiff.TryGetChanges(added: added, removed: removed, stackSizes: this.PreviousInventory, out changes);
+        return SnapshotItemListDiff.TryGetChanges(added: this.PooledAdded, removed: this.PooledRemoved, stackSizes: this.PreviousInventory, out changes);
     }
 
     /// <summary>Release watchers and resources.</summary>