Stop calling getNextDrawable on buffer swap to avoid CPU stalls

src/Veldrid/MTL/MTLCommandList.cs

@@ -11,7 +11,7 @@ internal unsafe class MTLCommandList : CommandList
     {
         private readonly MTLGraphicsDevice _gd;
         private MTLCommandBuffer _cb;
-        private MTLFramebufferBase _mtlFramebuffer;
+        private MTLFramebuffer _mtlFramebuffer;
         private uint _viewportCount;
         private bool _currentFramebufferEverActive;
         private MTLRenderCommandEncoder _rce;
@@ -695,7 +695,7 @@ protected override void SetFramebufferCore(Framebuffer fb)
             }
 
             EnsureNoRenderPass();
-            _mtlFramebuffer = Util.AssertSubtype<Framebuffer, MTLFramebufferBase>(fb);
+            _mtlFramebuffer = Util.AssertSubtype<Framebuffer, MTLFramebuffer>(fb);
             _viewportCount = Math.Max(1u, (uint)fb.ColorTargets.Count);
             Util.EnsureArrayMinimumSize(ref _viewports, _viewportCount);
             Util.ClearArray(_viewports);
@@ -942,10 +942,8 @@ private bool EnsureRenderPass()
 
         private bool BeginCurrentRenderPass()
         {
-            if (!_mtlFramebuffer.IsRenderable)
-            {
+            if (_mtlFramebuffer is MTLSwapchainFramebuffer swapchainFramebuffer && !swapchainFramebuffer.EnsureDrawableAvailable())
                 return false;
-            }
 
             MTLRenderPassDescriptor rpDesc = _mtlFramebuffer.CreateRenderPassDescriptor();
             for (uint i = 0; i < _clearColors.Length; i++)

src/Veldrid/MTL/MTLFramebuffer.cs

@@ -3,19 +3,23 @@
 
 namespace Veldrid.MTL
 {
-    internal class MTLFramebuffer : MTLFramebufferBase
+    internal class MTLFramebuffer : Framebuffer
     {
-        public override bool IsRenderable => true;
         private bool _disposed;
 
         public MTLFramebuffer(MTLGraphicsDevice gd, ref FramebufferDescription description)
-            : base(gd, ref description)
+            : base(description.DepthTarget, description.ColorTargets)
         {
         }
 
-        public override MTLRenderPassDescriptor CreateRenderPassDescriptor()
+        public MTLFramebuffer()
+        {
+        }
+
+        public MTLRenderPassDescriptor CreateRenderPassDescriptor()
         {
             MTLRenderPassDescriptor ret = MTLRenderPassDescriptor.New();
+
             for (int i = 0; i < ColorTargets.Count; i++)
             {
                 FramebufferAttachment colorTarget = ColorTargets[i];
@@ -50,6 +54,7 @@ public override MTLRenderPassDescriptor CreateRenderPassDescriptor()
             return ret;
         }
 
+        public override string Name { get; set; }
         public override bool IsDisposed => _disposed;
 
         public override void Dispose()

src/Veldrid/MTL/MTLGraphicsDevice.cs

@@ -304,9 +304,9 @@ private protected override void SwapBuffersCore(Swapchain swapchain)
                     submitCB.presentDrawable(currentDrawablePtr);
                     submitCB.commit();
                 }
-            }
 
-            mtlSC.GetNextDrawable();
+                mtlSC.InvalidateDrawable();
+            }
         }
 
         private protected override void UpdateBufferCore(DeviceBuffer buffer, uint bufferOffsetInBytes, IntPtr source, uint sizeInBytes)

src/Veldrid/MTL/MTLSwapchain.cs

@@ -106,18 +106,16 @@ public MTLSwapchain(MTLGraphicsDevice gd, ref SwapchainDescription description)
 
             SetSyncToVerticalBlank(_syncToVerticalBlank);
 
-            GetNextDrawable();
-
             _framebuffer = new MTLSwapchainFramebuffer(
                 gd,
                 this,
-                width,
-                height,
                 description.DepthFormat,
                 format);
+
+            getNextDrawable();
         }
 
-        public void GetNextDrawable()
+        private bool getNextDrawable()
         {
             if (!_drawable.IsNull)
             {
@@ -127,7 +125,15 @@ public void GetNextDrawable()
             using (NSAutoreleasePool.Begin())
             {
                 _drawable = _metalLayer.nextDrawable();
-                ObjectiveCRuntime.retain(_drawable.NativePtr);
+
+                if (!_drawable.IsNull)
+                {
+                    ObjectiveCRuntime.retain(_drawable.NativePtr);
+                    _framebuffer.UpdateTextures(_drawable, _metalLayer.drawableSize);
+                    return true;
+                }
+
+                return false;
             }
         }
 
@@ -143,13 +149,21 @@ public override void Resize(uint width, uint height)
                 _metalLayer.frame = _uiView.frame;
             }
 
-            _framebuffer.Resize(width, height);
             _metalLayer.drawableSize = new CGSize(width, height);
             if (_uiView.NativePtr != IntPtr.Zero)
             {
                 _metalLayer.frame = _uiView.frame;
             }
-            GetNextDrawable();
+
+            getNextDrawable();
+        }
+
+        public bool EnsureDrawableAvailable() => !_drawable.IsNull || getNextDrawable();
+
+        public void InvalidateDrawable()
+        {
+            ObjectiveCRuntime.release(_drawable.NativePtr);
+            _drawable = default;
         }
 
         private void SetSyncToVerticalBlank(bool value)

src/Veldrid/MTL/MTLSwapchainFramebuffer.cs

@@ -5,54 +5,46 @@
 
 namespace Veldrid.MTL
 {
-    internal class MTLSwapchainFramebuffer : MTLFramebufferBase
+    internal class MTLSwapchainFramebuffer : MTLFramebuffer
     {
         private readonly MTLGraphicsDevice _gd;
-        private readonly MTLPlaceholderTexture _placeholderTexture;
+        private MTLTexture _colorTexture;
         private MTLTexture _depthTexture;
         private readonly MTLSwapchain _parentSwapchain;
-        private bool _disposed;
+        private readonly PixelFormat _colorFormat;
 
-        public override uint Width => _placeholderTexture.Width;
-        public override uint Height => _placeholderTexture.Height;
+        public override uint Width => _colorTexture.Width;
+        public override uint Height => _colorTexture.Height;
 
         public override OutputDescription OutputDescription { get; }
 
-        private readonly FramebufferAttachment[] _colorTargets;
-        private readonly FramebufferAttachment? _depthTarget;
+        private FramebufferAttachment[] _colorTargets;
+        private FramebufferAttachment? _depthTarget;
+
         private readonly PixelFormat? _depthFormat;
 
         public override IReadOnlyList<FramebufferAttachment> ColorTargets => _colorTargets;
         public override FramebufferAttachment? DepthTarget => _depthTarget;
 
-        public override bool IsDisposed => _disposed;
-
         public MTLSwapchainFramebuffer(
             MTLGraphicsDevice gd,
             MTLSwapchain parent,
-            uint width,
-            uint height,
             PixelFormat? depthFormat,
             PixelFormat colorFormat)
-            : base()
         {
             _gd = gd;
             _parentSwapchain = parent;
+            _colorFormat = colorFormat;
 
             OutputAttachmentDescription? depthAttachment = null;
             if (depthFormat != null)
             {
                 _depthFormat = depthFormat;
                 depthAttachment = new OutputAttachmentDescription(depthFormat.Value);
-                RecreateDepthTexture(width, height);
-                _depthTarget = new FramebufferAttachment(_depthTexture, 0);
             }
             OutputAttachmentDescription colorAttachment = new OutputAttachmentDescription(colorFormat);
 
             OutputDescription = new OutputDescription(depthAttachment, colorAttachment);
-            _placeholderTexture = new MTLPlaceholderTexture(colorFormat);
-            _placeholderTexture.Resize(width, height);
-            _colorTargets = new[] { new FramebufferAttachment(_placeholderTexture, 0) };
         }
 
         private void RecreateDepthTexture(uint width, uint height)
@@ -66,41 +58,24 @@ private void RecreateDepthTexture(uint width, uint height)
             _depthTexture = Util.AssertSubtype<Texture, MTLTexture>(
                 _gd.ResourceFactory.CreateTexture(TextureDescription.Texture2D(
                     width, height, 1, 1, _depthFormat.Value, TextureUsage.DepthStencil)));
+            _depthTarget = new FramebufferAttachment(_depthTexture, 0);
         }
 
-        public void Resize(uint width, uint height)
+        public void UpdateTextures(CAMetalDrawable drawable, CGSize size)
         {
-            _placeholderTexture.Resize(width, height);
+            _colorTexture = new MTLTexture(drawable, size, _colorFormat);
+            _colorTargets = new[] { new FramebufferAttachment(_colorTexture, 0) };
 
             if (_depthFormat.HasValue)
-            {
-                RecreateDepthTexture(width, height);
-            }
+                RecreateDepthTexture((uint)size.width, (uint)size.height);
         }
 
-        public override bool IsRenderable => !_parentSwapchain.CurrentDrawable.IsNull;
-
-        public override MTLRenderPassDescriptor CreateRenderPassDescriptor()
-        {
-            MTLRenderPassDescriptor ret = MTLRenderPassDescriptor.New();
-            var color0 = ret.colorAttachments[0];
-            color0.texture = _parentSwapchain.CurrentDrawable.texture;
-            color0.loadAction = MTLLoadAction.Load;
-
-            if (_depthTarget != null)
-            {
-                var depthAttachment = ret.depthAttachment;
-                depthAttachment.texture = _depthTexture.DeviceTexture;
-                depthAttachment.loadAction = MTLLoadAction.Load;
-            }
-
-            return ret;
-        }
+        public bool EnsureDrawableAvailable() => _parentSwapchain.EnsureDrawableAvailable();
 
         public override void Dispose()
         {
             _depthTexture?.Dispose();
-            _disposed = true;
+            base.Dispose();
         }
     }
 }

src/Veldrid/MTL/MTLTexture.cs

@@ -118,6 +118,23 @@ public MTLTexture(ulong nativeTexture, ref TextureDescription description)
                     (Usage & TextureUsage.Cubemap) != 0);
         }
 
+        public MTLTexture(CAMetalDrawable drawable, CGSize size, PixelFormat format)
+        {
+            DeviceTexture = drawable.texture;
+            Width = (uint)size.width;
+            Height = (uint)size.height;
+            Depth = 1;
+            ArrayLayers = 1;
+            MipLevels = 1;
+            Format = format;
+            Usage = TextureUsage.RenderTarget;
+            Type = TextureType.Texture2D;
+            SampleCount = TextureSampleCount.Count1;
+
+            MTLPixelFormat = MTLFormats.VdToMTLPixelFormat(Format, false);
+            MTLTextureType = MTLTextureType.Type2D;
+        }
+
         internal uint GetSubresourceSize(uint mipLevel, uint arrayLayer)
         {
             uint blockSize = FormatHelpers.IsCompressedFormat(Format) ? 4u : 1u;