Prevent FIELD redraw flicker

engine/Age.Engine/Hosting/ScriptPresentationBarrier.cs

@@ -0,0 +1,47 @@
+using System;
+using System.Threading;
+
+namespace Age.Engine.Hosting;
+
+/// <summary>
+/// Keeps one VM opcode burst atomic with respect to the interactive compositor. The VM owns the
+/// write side while script code is running; the render thread samples through the read side only
+/// after the VM reaches a presentation, sleep, or input service boundary.
+/// </summary>
+public sealed class ScriptPresentationBarrier
+{
+    private readonly ReaderWriterLockSlim _lock = new(LockRecursionPolicy.NoRecursion);
+    private int _scriptDepth;
+
+    public void EnterScript()
+    {
+        if (_scriptDepth++ == 0)
+            _lock.EnterWriteLock();
+    }
+
+    public void ExitScript()
+    {
+        if (_scriptDepth <= 0)
+            throw new InvalidOperationException("No script burst is active.");
+        if (--_scriptDepth == 0)
+            _lock.ExitWriteLock();
+    }
+
+    /// <summary>Temporarily expose the completed burst state while a host service is parked.</summary>
+    public bool SuspendScript()
+    {
+        if (_scriptDepth <= 0 || !_lock.IsWriteLockHeld) return false;
+        _lock.ExitWriteLock();
+        return true;
+    }
+
+    public void ResumeScript(bool suspended)
+    {
+        if (suspended) _lock.EnterWriteLock();
+    }
+
+    public bool TryEnterPresentation()
+        => !_lock.IsWriteLockHeld && _lock.TryEnterReadLock(0);
+
+    public void ExitPresentation() => _lock.ExitReadLock();
+}