From bb3b7a9f2907ad404c38f8824bce874e3c54e45a Mon Sep 17 00:00:00 2001
From: Eric Fulton <emfult@gmail.com>
Date: Wed, 1 Jul 2026 17:59:54 -0700
Subject: [PATCH] Fix collection-modified crash in ManualMotionDispatcher when
 scheduling during Update

ManualMotionDispatcher.Update and Reset enumerated the runners dictionary
directly. A motion callback invoked during iteration (Bind/OnComplete/
OnCancel/loop callbacks) can schedule the first motion of a new type, which
lazily adds a runner via GetOrCreateRunner and mutates the dictionary
mid-enumeration, throwing "InvalidOperationException: Collection was modified;
enumeration operation may not execute."

Iterate a reused buffer snapshot of runners.Values instead of the live
dictionary. The buffer is reused across calls and filled via AddRange's
ICollection.CopyTo fast path, so this remains allocation-free after warmup.
---
 .../Runtime/ManualMotionDispatcher.cs         | 22 +++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/src/LitMotion/Assets/LitMotion/Runtime/ManualMotionDispatcher.cs b/src/LitMotion/Assets/LitMotion/Runtime/ManualMotionDispatcher.cs
index e7708667..036134f0 100644
--- a/src/LitMotion/Assets/LitMotion/Runtime/ManualMotionDispatcher.cs
+++ b/src/LitMotion/Assets/LitMotion/Runtime/ManualMotionDispatcher.cs
@@ -39,6 +39,14 @@ public sealed class ManualMotionDispatcher
         readonly ManualMotionDispatcherScheduler scheduler;
         readonly Dictionary<Type, IUpdateRunner> runners = new();
 
+        // Reused buffer so Update/Reset iterate a snapshot of the runners instead of the live
+        // dictionary. A motion callback fired during iteration can schedule the first motion of a
+        // new type, which lazily adds a runner (see GetOrCreateRunner) and would otherwise throw
+        // "InvalidOperationException: Collection was modified" mid-enumeration. Reused rather than
+        // allocated per call; AddRange(runners.Values) copies via ICollection.CopyTo and the
+        // Dictionary caches its ValueCollection, so this stays allocation-free after warmup.
+        readonly List<IUpdateRunner> runnerBuffer = new();
+
         public ManualMotionDispatcher()
         {
             scheduler = new(this);
@@ -79,10 +87,13 @@ public void Update(double deltaTime)
         {
             time += deltaTime;
 
-            foreach (var kv in runners)
+            runnerBuffer.Clear();
+            runnerBuffer.AddRange(runners.Values);
+            for (int i = 0; i < runnerBuffer.Count; i++)
             {
-                kv.Value.Update(time, time, time);
+                runnerBuffer[i].Update(time, time, time);
             }
+            runnerBuffer.Clear();
         }
 
         /// <summary>
@@ -90,10 +101,13 @@ public void Update(double deltaTime)
         /// </summary>
         public void Reset()
         {
-            foreach (var kv in runners)
+            runnerBuffer.Clear();
+            runnerBuffer.AddRange(runners.Values);
+            for (int i = 0; i < runnerBuffer.Count; i++)
             {
-                kv.Value.Reset();
+                runnerBuffer[i].Reset();
             }
+            runnerBuffer.Clear();
 
             time = 0;
         }