Stage 1

MinecraftClient/MainThreadExecutionScope.cs

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+using System;
+using System.Threading;
+
+namespace MinecraftClient
+{
+    internal static class MainThreadExecutionScope
+    {
+        private sealed class ScopeNode(object owner, ScopeNode? parent) : IDisposable
+        {
+            public object Owner { get; } = owner;
+            public ScopeNode? Parent { get; } = parent;
+
+            public void Dispose()
+            {
+                if (!ReferenceEquals(s_currentScope.Value, this))
+                    throw new InvalidOperationException("Main-thread execution scope disposed out of order.");
+
+                s_currentScope.Value = Parent;
+            }
+        }
+
+        private static readonly AsyncLocal<ScopeNode?> s_currentScope = new();
+
+        public static IDisposable Enter(object owner)
+        {
+            ScopeNode scopeNode = new(owner, s_currentScope.Value);
+            s_currentScope.Value = scopeNode;
+            return scopeNode;
+        }
+
+        public static bool IsActive(object owner)
+        {
+            ScopeNode? scopeNode = s_currentScope.Value;
+            while (scopeNode is not null)
+            {
+                if (ReferenceEquals(scopeNode.Owner, owner))
+                    return true;
+
+                scopeNode = scopeNode.Parent;
+            }
+
+            return false;
+        }
+    }
+}