Merge pull request #1172 from kekekeks/dispatcher-priority

Dispatcher priority and support for DeferredRenderer in GTK3 backend

samples/ControlCatalog/MainWindow.xaml.cs

@@ -11,6 +11,7 @@ namespace ControlCatalog
         {
             this.InitializeComponent();
             this.AttachDevTools();
+            //Renderer.DrawFps = true;
             //Renderer.DrawDirtyRects = Renderer.DrawFps = true;
         }
 

src/Android/Avalonia.Android/AndroidThreadingInterface.cs

@@ -12,6 +12,7 @@ using Android.Runtime;
 using Android.Views;
 using Android.Widget;
 using Avalonia.Platform;
+using Avalonia.Threading;
 
 namespace Avalonia.Android
 {
@@ -78,13 +79,13 @@ namespace Avalonia.Android
 
         private void EnsureInvokeOnMainThread(Action action) => _handler.Post(action);
 
-        public void Signal()
+        public void Signal(DispatcherPriority prio)
         {
-            EnsureInvokeOnMainThread(() => Signaled?.Invoke());
+            EnsureInvokeOnMainThread(() => Signaled?.Invoke(null));
         }
 
         public bool CurrentThreadIsLoopThread => Looper.MainLooper.Thread.Equals(Java.Lang.Thread.CurrentThread());
-        public event Action Signaled;
+        public event Action<DispatcherPriority?> Signaled;
     }
 }
  

src/Avalonia.Base/Platform/IPlatformThreadingInterface.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Threading;
+using Avalonia.Threading;
 
 namespace Avalonia.Platform
 {
@@ -21,10 +22,10 @@ namespace Avalonia.Platform
         /// <returns>An <see cref="IDisposable"/> used to stop the timer.</returns>
         IDisposable StartTimer(TimeSpan interval, Action tick);
 
-        void Signal();
+        void Signal(DispatcherPriority priority);
 
         bool CurrentThreadIsLoopThread { get; }
 
-        event Action Signaled;
+        event Action<DispatcherPriority?> Signaled;
     }
 }

src/Avalonia.Base/Threading/AvaloniaSynchronizationContext.cs

@@ -36,7 +36,7 @@ namespace Avalonia.Threading
         /// <inheritdoc/>
         public override void Post(SendOrPostCallback d, object state)
         {
-           Dispatcher.UIThread.InvokeAsync(() => d(state));
+           Dispatcher.UIThread.InvokeAsync(() => d(state), DispatcherPriority.Send);
         }
 
         /// <inheritdoc/>
@@ -45,7 +45,7 @@ namespace Avalonia.Threading
             if (Dispatcher.UIThread.CheckAccess())
                 d(state);
             else
-                Dispatcher.UIThread.InvokeTaskAsync(() => d(state)).Wait();
+                Dispatcher.UIThread.InvokeTaskAsync(() => d(state), DispatcherPriority.Send).Wait();
         }
     }
 }

src/Avalonia.Base/Threading/Dispatcher.cs

@@ -60,7 +60,7 @@ namespace Avalonia.Threading
         public void MainLoop(CancellationToken cancellationToken)
         {
             var platform = AvaloniaLocator.Current.GetService<IPlatformThreadingInterface>();
-            cancellationToken.Register(platform.Signal);
+            cancellationToken.Register(() => platform.Signal(DispatcherPriority.Send));
             platform.RunLoop(cancellationToken);
         }
 
@@ -69,7 +69,7 @@ namespace Avalonia.Threading
         /// </summary>
         public void RunJobs()
         {
-            _jobRunner?.RunJobs();
+            _jobRunner?.RunJobs(null);
         }
 
         /// <inheritdoc/>

src/Avalonia.Base/Threading/DispatcherPriority.cs

@@ -10,10 +10,10 @@ namespace Avalonia.Threading
     public enum DispatcherPriority
     {
         /// <summary>
-        /// The job will not be processed.
+        /// Minimum possible priority
         /// </summary>
-        Inactive = 0,
-
+        MinValue = 1,
+        
         /// <summary>
         /// The job will be processed when the system is idle.
         /// </summary>
@@ -48,20 +48,30 @@ namespace Avalonia.Threading
         /// The job will be processed with the same priority as render.
         /// </summary>
         Render = 7,
-
+        
+        /// <summary>
+        /// The job will be processed with the same priority as render.
+        /// </summary>
+        Layout = 8,
+        
         /// <summary>
         /// The job will be processed with the same priority as data binding.
         /// </summary>
-        DataBind = 8,
+        DataBind = 9,
 
         /// <summary>
         /// The job will be processed with normal priority.
         /// </summary>
-        Normal = 9,
+        Normal = 10,
 
         /// <summary>
         /// The job will be processed before other asynchronous operations.
         /// </summary>
-        Send = 10,
+        Send = 11,
+        
+        /// <summary>
+        /// Maximum possible priority
+        /// </summary>
+        MaxValue = 11
     }
 }

src/Avalonia.Base/Threading/JobRunner.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Threading.Tasks;
 using Avalonia.Platform;
 
@@ -13,29 +14,45 @@ namespace Avalonia.Threading
     /// </summary>
     internal class JobRunner
     {
-        private readonly Queue<Job> _queue = new Queue<Job>();
+
+
         private IPlatformThreadingInterface _platform;
 
+        private Queue<Job>[] _queues = Enumerable.Range(0, (int) DispatcherPriority.MaxValue + 1)
+            .Select(_ => new Queue<Job>()).ToArray();
+
         public JobRunner(IPlatformThreadingInterface platform)
         {
             _platform = platform;
         }
 
+        Job GetNextJob(DispatcherPriority minimumPriority)
+        {
+            for (int c = (int) DispatcherPriority.MaxValue; c >= (int) minimumPriority; c--)
+            {
+                var q = _queues[c];
+                lock (q)
+                {
+                    if (q.Count > 0)
+                        return q.Dequeue();
+                }
+            }
+            return null;
+        }
+
         /// <summary>
         /// Runs continuations pushed on the loop.
         /// </summary>
-        public void RunJobs()
+        /// <param name="priority">Priority to execute jobs for. Pass null if platform doesn't have internal priority system</param>
+        public void RunJobs(DispatcherPriority? priority)
         {
+            var minimumPriority = priority ?? DispatcherPriority.MinValue;
             while (true)
             {
-                Job job;
-
-                lock (_queue)
-                {
-                    if (_queue.Count == 0)
-                        return;
-                    job = _queue.Dequeue();
-                }
+                var job = GetNextJob(minimumPriority);
+                if (job == null)
+                    return;
+                
 
                 if (job.TaskCompletionSource == null)
                 {
@@ -77,7 +94,6 @@ namespace Avalonia.Threading
         /// <param name="priority">The priority with which to invoke the method.</param>
         internal void Post(Action action, DispatcherPriority priority)
         {
-            // TODO: Respect priority.
             AddJob(new Job(action, priority, true));
         }
 
@@ -92,13 +108,14 @@ namespace Avalonia.Threading
         private void AddJob(Job job)
         {
             var needWake = false;
-            lock (_queue)
+            var queue = _queues[(int) job.Priority];
+            lock (queue)
             {
-                needWake = _queue.Count == 0;
-                _queue.Enqueue(job);
+                needWake = queue.Count == 0;
+                queue.Enqueue(job);
             }
             if (needWake)
-                _platform?.Signal();
+                _platform?.Signal(job.Priority);
         }
 
         /// <summary>

src/Avalonia.Base/Threading/SingleThreadDispatcher.cs

@@ -1,60 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading;
-using System.Threading.Tasks;
-using Avalonia.Platform;
-
-namespace Avalonia.Threading
-{
-    public class SingleThreadDispatcher : Dispatcher
-    {
-        class ThreadingInterface : IPlatformThreadingInterface
-        {
-            private readonly AutoResetEvent _evnt = new AutoResetEvent(false);
-            private readonly JobRunner _timerJobRunner;
-
-            public ThreadingInterface()
-            {
-                _timerJobRunner = new JobRunner(this);
-            }
-
-            public void RunLoop(CancellationToken cancellationToken)
-            {
-                while (!cancellationToken.IsCancellationRequested)
-                {
-                    _evnt.WaitOne();
-                    if (cancellationToken.IsCancellationRequested)
-                        return;
-                    Signaled?.Invoke();
-                    _timerJobRunner.RunJobs();
-                }
-            }
-
-            public IDisposable StartTimer(TimeSpan interval, Action tick)
-                => AvaloniaLocator.Current.GetService<IRuntimePlatform>().StartSystemTimer(interval,
-                    () => _timerJobRunner.Post(tick, DispatcherPriority.Normal));
-
-            public void Signal() => _evnt.Set();
-            //TODO: Actually perform a check
-            public bool CurrentThreadIsLoopThread => true;
-
-            public event Action Signaled;
-        }
-
-        public SingleThreadDispatcher() : base(new ThreadingInterface())
-        {
-        }
-
-        public static Dispatcher StartNew(CancellationToken token)
-        {
-            var dispatcher = new SingleThreadDispatcher();
-            AvaloniaLocator.Current.GetService<IRuntimePlatform>().PostThreadPoolItem(() =>
-            {
-                dispatcher.MainLoop(token);
-            });
-            return dispatcher;
-        }
-    }
-}

src/Avalonia.Base/Utilities/WeakTimer.cs

@@ -23,7 +23,6 @@ namespace Avalonia.Utilities
             _timer = new DispatcherTimer();
             
             _timer.Tick += delegate { OnTick(); };
-            _timer.Start();
         }
 
         private void OnTick()

src/Avalonia.Controls/Primitives/Track.cs

@@ -165,7 +165,7 @@ namespace Avalonia.Controls.Primitives
 
                 if (increaseButton != null)
                 {
-                    increaseButton.Arrange(new Rect(0, firstHeight + thumbHeight, finalSize.Width, remaining - firstHeight));
+                    increaseButton.Arrange(new Rect(0, firstHeight + thumbHeight, finalSize.Width, Math.Max(remaining - firstHeight, 0)));
                 }
             }
 

src/Avalonia.Layout/LayoutManager.cs

@@ -203,7 +203,7 @@ namespace Avalonia.Layout
         {
             if (!_queued && !_running)
             {
-                Dispatcher.UIThread.InvokeAsync(ExecuteLayoutPass, DispatcherPriority.Render);
+                Dispatcher.UIThread.InvokeAsync(ExecuteLayoutPass, DispatcherPriority.Layout);
                 _queued = true;
             }
         }

src/Avalonia.Visuals/Rendering/DefaultRenderLoop.cs

@@ -15,7 +15,7 @@ namespace Avalonia.Rendering
     /// </remarks>
     public class DefaultRenderLoop : IRenderLoop
     {
-        private IPlatformThreadingInterface _threading;
+        private IRuntimePlatform _runtime;
         private int _subscriberCount;
         private EventHandler<EventArgs> _tick;
         private IDisposable _subscription;
@@ -78,12 +78,12 @@ namespace Avalonia.Rendering
         /// </remarks>
         protected virtual IDisposable StartCore(Action tick)
         {
-            if (_threading == null)
+            if (_runtime == null)
             {
-                _threading = AvaloniaLocator.Current.GetService<IPlatformThreadingInterface>();
+                _runtime = AvaloniaLocator.Current.GetService<IRuntimePlatform>();
             }
 
-            return _threading.StartTimer(TimeSpan.FromSeconds(1.0 / FramesPerSecond), tick);
+            return _runtime.StartSystemTimer(TimeSpan.FromSeconds(1.0 / FramesPerSecond), tick);
         }
 
         /// <summary>

src/Gtk/Avalonia.Gtk3/Avalonia.Gtk3.csproj

@@ -1,7 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
-    <EnableDefaultCompileItems>False</EnableDefaultCompileItems>
     <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)' == 'Debug' ">
@@ -25,34 +24,6 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="..\..\Shared\SharedAssemblyInfo.cs" Link="Properties\SharedAssemblyInfo.cs" />
-    <Compile Include="KeyTransform.cs" />
-    <Compile Include="Interop\CairoSurface.cs" />
-    <Compile Include="ClipboardImpl.cs" />
-    <Compile Include="CursorFactory.cs" />
-    <Compile Include="FramebufferManager.cs" />
-    <Compile Include="GdkCursor.cs" />
-    <Compile Include="GdkKey.cs" />
-    <Compile Include="Gtk3Platform.cs" />
-    <Compile Include="Interop\GException.cs" />
-    <Compile Include="Interop\GObject.cs" />
-    <Compile Include="Interop\ICustomGtk3NativeLibraryResolver.cs" />
-    <Compile Include="Interop\DynLoader.cs" />
-    <Compile Include="Interop\GlibTimeout.cs" />
-    <Compile Include="Interop\Native.cs" />
-    <Compile Include="Interop\NativeException.cs" />
-    <Compile Include="Interop\Pixbuf.cs" />
-    <Compile Include="Interop\Resolver.cs" />
-    <Compile Include="Interop\Signal.cs" />
-    <Compile Include="ImageSurfaceFramebuffer.cs" />
-    <Compile Include="PlatformIconLoader.cs" />
-    <Compile Include="PopupImpl.cs" />
-    <Compile Include="Properties\AssemblyInfo.cs" />
-    <Compile Include="SystemDialogs.cs" />
-    <Compile Include="WindowBaseImpl.cs" />
-    <Compile Include="Interop\Utf8Buffer.cs" />
-    <Compile Include="WindowImpl.cs" />
-    <Compile Include="ScreenImpl.cs" />
-    <Compile Include="GtkScreen.cs" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\Avalonia.Base\Avalonia.Base.csproj" />

src/Gtk/Avalonia.Gtk3/FramebufferManager.cs

@@ -23,11 +23,11 @@ namespace Avalonia.Gtk3
 
         public ILockedFramebuffer Lock()
         {
-            if(_window.CurrentCairoContext == IntPtr.Zero)
-                throw new InvalidOperationException("Window is not in drawing state");
-            var width = (int) _window.ClientSize.Width;
-            var height = (int) _window.ClientSize.Height;
-            return new ImageSurfaceFramebuffer(_window.CurrentCairoContext, _window.GtkWidget, width, height);
+            // This method may be called from non-UI thread, don't touch anything that calls back to GTK/GDK
+            var s = _window.ClientSize;
+            var width = (int) s.Width;
+            var height = (int) s.Height;
+            return new ImageSurfaceFramebuffer(_window, width, height);
         }
     }
 }

src/Gtk/Avalonia.Gtk3/Gtk3Platform.cs

@@ -12,6 +12,7 @@ using Avalonia.Input.Platform;
 using Avalonia.Platform;
 using Avalonia.Rendering;
 using Avalonia.Gtk3;
+using Avalonia.Threading;
 
 namespace Avalonia.Gtk3
 {
@@ -21,6 +22,7 @@ namespace Avalonia.Gtk3
         internal static readonly MouseDevice Mouse = new MouseDevice();
         internal static readonly KeyboardDevice Keyboard = new KeyboardDevice();
         internal static IntPtr App { get; set; }
+        public static bool UseDeferredRendering = true;
         public static void Initialize()
         {
             Resolver.Resolve();
@@ -65,37 +67,42 @@ namespace Avalonia.Gtk3
 
         public IDisposable StartTimer(TimeSpan interval, Action tick)
         {
-            return GlibTimeout.StarTimer((uint) interval.TotalMilliseconds, tick);
+            var msec = interval.TotalMilliseconds;
+            if (msec <= 0)
+                throw new ArgumentException("Don't know how to create a timer with zero or negative interval");
+            var imsec = (uint) msec;
+            if (imsec == 0)
+                imsec = 1;
+            return GlibTimeout.StarTimer(imsec, tick);
         }
 
-        private bool _signaled = false;
+        private bool[] _signaled = new bool[(int) DispatcherPriority.MaxValue + 1];
         object _lock = new object();
-
-        public void Signal()
+        public void Signal(DispatcherPriority prio)
         {
+            var idx = (int) prio;
             lock(_lock)
-                if (!_signaled)
+                if (!_signaled[idx])
                 {
-                    _signaled = true;
-                    GlibTimeout.Add(0, () =>
+                    _signaled[idx] = true;
+                    GlibTimeout.Add(GlibPriority.FromDispatcherPriority(prio), 0, () =>
                     {
                         lock (_lock)
                         {
-                            _signaled = false;
+                            _signaled[idx] = false;
                         }
-                        Signaled?.Invoke();
+                        Signaled?.Invoke(prio);
                         return false;
                     });
                 }
         }
-        public event Action Signaled;
+        public event Action<DispatcherPriority?> Signaled;
 
 
         [ThreadStatic]
         private static bool s_tlsMarker;
 
         public bool CurrentThreadIsLoopThread => s_tlsMarker;
-
     }
 }
 
@@ -103,10 +110,11 @@ namespace Avalonia
 {
     public static class Gtk3AppBuilderExtensions
     {
-        public static T UseGtk3<T>(this AppBuilderBase<T> builder, ICustomGtk3NativeLibraryResolver resolver = null) 
+        public static T UseGtk3<T>(this AppBuilderBase<T> builder, bool deferredRendering = true, ICustomGtk3NativeLibraryResolver resolver = null) 
             where T : AppBuilderBase<T>, new()
         {
             Resolver.Custom = resolver;
+            Gtk3Platform.UseDeferredRendering = deferredRendering;
             return builder.UseWindowingSubsystem(Gtk3Platform.Initialize, "GTK3");
         }
     }

src/Gtk/Avalonia.Gtk3/IDeferredRenderOperation.cs

@@ -0,0 +1,9 @@
+using System;
+
+namespace Avalonia.Gtk3
+{
+    public interface IDeferredRenderOperation : IDisposable
+    {
+        void RenderNow();
+    }
+}

src/Gtk/Avalonia.Gtk3/ImageSurfaceFramebuffer.cs

@@ -1,26 +1,28 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
 using Avalonia.Controls.Platform.Surfaces;
 using Avalonia.Gtk3.Interop;
 using Avalonia.Platform;
+using Avalonia.Threading;
 
 
 namespace Avalonia.Gtk3
 {
     class ImageSurfaceFramebuffer : ILockedFramebuffer
     {
-        private IntPtr _context;
+        private readonly WindowBaseImpl _impl;
         private readonly GtkWidget _widget;
         private CairoSurface _surface;
         private int _factor;
-
-        public ImageSurfaceFramebuffer(IntPtr context, GtkWidget widget, int width, int height)
+        private object _lock = new object();
+        public ImageSurfaceFramebuffer(WindowBaseImpl impl, int width, int height)
         {
-            _context = context;
-            _widget = widget;
+            _impl = impl;
+            _widget = impl.GtkWidget;
             _factor = (int)(Native.GtkWidgetGetScaleFactor?.Invoke(_widget) ?? 1u);
             width *= _factor;
             height *= _factor;
@@ -32,18 +34,97 @@ namespace Avalonia.Gtk3
             RowBytes = Native.CairoImageSurfaceGetStride(_surface);
             Native.CairoSurfaceFlush(_surface);
         }
+
+        static void Draw(IntPtr context, CairoSurface surface, double factor)
+        {
+            
+            Native.CairoSurfaceMarkDirty(surface);
+            Native.CairoScale(context, 1d / factor, 1d / factor);
+            Native.CairoSetSourceSurface(context, surface, 0, 0);
+            Native.CairoPaint(context);
+
+        }
+        /*
+        static Stopwatch St =Stopwatch.StartNew();
+        private static int _frames;
+        private static int _fps;*/
+        static void DrawToWidget(GtkWidget widget, CairoSurface surface, int width, int height, double factor)
+        {
+            if(surface == null || widget.IsClosed)
+                return;
+            var window = Native.GtkWidgetGetWindow(widget);
+            if(window == IntPtr.Zero)
+                return;
+            var rc = new GdkRectangle {Width = width, Height = height};
+            Native.GdkWindowBeginPaintRect(window, ref rc);
+            var context = Native.GdkCairoCreate(window);
+            Draw(context, surface, factor);
+            /*
+            _frames++;
+            var el = St.Elapsed;
+            if (el.TotalSeconds > 1)
+            {
+                _fps = (int) (_frames / el.TotalSeconds);
+                _frames = 0;
+                St = Stopwatch.StartNew();
+            }
+            
+            Native.CairoSetSourceRgba(context, 1, 0, 0, 1);
+            Native.CairoMoveTo(context, 20, 20);
+            Native.CairoSetFontSize(context, 30);
+            using (var txt = new Utf8Buffer("FPS: " + _fps))
+                Native.CairoShowText(context, txt);
+            */
+            
+            Native.CairoDestroy(context);
+            Native.GdkWindowEndPaint(window);
+        }
+        
+        class RenderOp : IDeferredRenderOperation
+        {
+            private readonly GtkWidget _widget;
+            private CairoSurface _surface;
+            private readonly double _factor;
+            private readonly int _width;
+            private readonly int _height;
+
+            public RenderOp(GtkWidget widget, CairoSurface _surface, double factor, int width, int height)
+            {
+                _widget = widget;
+                this._surface = _surface;
+                _factor = factor;
+                _width = width;
+                _height = height;
+            }
+
+            public void Dispose()
+            {
+                _surface?.Dispose();
+                _surface = null;
+            }
+
+            public void RenderNow()
+            {
+                DrawToWidget(_widget, _surface, _width, _height, _factor);
+            }
+        }
         
         public void Dispose()
         {
-            if(_context == IntPtr.Zero || _surface == null)
-                return;
-            Native.CairoSurfaceMarkDirty(_surface);
-            Native.CairoScale(_context, 1d / _factor, 1d / _factor);
-            Native.CairoSetSourceSurface(_context, _surface, 0, 0);
-            Native.CairoPaint(_context);
-            _context = IntPtr.Zero;
-            _surface.Dispose();
-            _surface = null;
+            lock (_lock)
+            {
+                if (Dispatcher.UIThread.CheckAccess())
+                {
+                    if (_impl.CurrentCairoContext != IntPtr.Zero)
+                        Draw(_impl.CurrentCairoContext, _surface, _factor);
+                    else
+                        DrawToWidget(_widget, _surface, Width, Height, _factor);
+                    _surface.Dispose();
+                }
+                else
+                    _impl.SetNextRenderOperation(new RenderOp(_widget, _surface, _factor, Width, Height));
+                _surface = null;
+            }
         }
 
         public IntPtr Address { get; }

src/Gtk/Avalonia.Gtk3/Interop/CairoSurface.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Runtime.InteropServices;
 
 namespace Avalonia.Gtk3.Interop

src/Gtk/Avalonia.Gtk3/Interop/GlibPriority.cs

@@ -0,0 +1,48 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Avalonia.Threading;
+
+namespace Avalonia.Gtk3.Interop
+{
+    static class GlibPriority
+    {
+        public static int High = -100;
+        public static int Default = 0;
+        public static int HighIdle = 100;
+        public static int GtkResize = HighIdle + 10;
+        public static int GtkPaint = HighIdle + 20;
+        public static int DefaultIdle = 200;
+        public static int Low = 300;
+        public static int GdkEvents = Default;
+        public static int GdkRedraw = HighIdle + 20;
+
+        public static int FromDispatcherPriority(DispatcherPriority prio)
+        {
+            if (prio == DispatcherPriority.Send)
+                return High;
+            if (prio == DispatcherPriority.Normal)
+                return Default;
+            if (prio == DispatcherPriority.DataBind)
+                return Default + 1;
+            if (prio == DispatcherPriority.Layout)
+                return Default + 2;
+            if (prio == DispatcherPriority.Render)
+                return Default + 3;
+            if (prio == DispatcherPriority.Loaded)
+                return GtkPaint + 20;
+            if (prio == DispatcherPriority.Input)
+                return GtkPaint + 21;
+            if (prio == DispatcherPriority.Background)
+                return DefaultIdle + 1;
+            if (prio == DispatcherPriority.ContextIdle)
+                return DefaultIdle + 2;
+            if (prio == DispatcherPriority.ApplicationIdle)
+                return DefaultIdle + 3;
+            if (prio == DispatcherPriority.SystemIdle)
+                return DefaultIdle + 4;
+            throw new ArgumentException("Unknown priority");
+
+        }
+    }
+}

src/Gtk/Avalonia.Gtk3/Interop/GlibTimeout.cs

@@ -4,6 +4,7 @@ using System.Linq;
 using System.Runtime.InteropServices;
 using System.Text;
 using System.Threading.Tasks;
+using Avalonia.Threading;
 
 namespace Avalonia.Gtk3.Interop
 {
@@ -30,10 +31,11 @@ namespace Avalonia.Gtk3.Interop
         }
 
 
-        public static void Add(uint interval, Func<bool> callback)
+        public static void Add(int priority, uint interval, Func<bool> callback)
         {
             var handle = GCHandle.Alloc(callback);
-            Native.GTimeoutAdd(interval, PinnedHandler, GCHandle.ToIntPtr(handle));
+            //Native.GTimeoutAdd(interval, PinnedHandler, GCHandle.ToIntPtr(handle));
+            Native.GTimeoutAddFull(priority, interval, PinnedHandler, GCHandle.ToIntPtr(handle), IntPtr.Zero);
         }
 
         class Timer : IDisposable
@@ -48,8 +50,10 @@ namespace Avalonia.Gtk3.Interop
 
         public static IDisposable StarTimer(uint interval, Action tick)
         {
+            if (interval == 0)
+                throw new ArgumentException("Don't know how to create a timer with zero or negative interval");
             var timer = new Timer ();
-            GlibTimeout.Add(interval,
+            GlibTimeout.Add(GlibPriority.FromDispatcherPriority(DispatcherPriority.Background), interval,
                 () =>
                 {
                     if (timer.Stopped)

src/Gtk/Avalonia.Gtk3/Interop/Native.cs

@@ -169,6 +169,9 @@ namespace Avalonia.Gtk3.Interop
 
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Cairo)]
             public delegate void cairo_surface_mark_dirty(CairoSurface surface);
+            
+            [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Cairo)]
+            public delegate void cairo_surface_write_to_png(CairoSurface surface, Utf8Buffer path);
 
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Cairo)]
             public delegate void cairo_surface_flush(CairoSurface surface);
@@ -178,15 +181,36 @@ namespace Avalonia.Gtk3.Interop
 
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Cairo)]
             public delegate void cairo_set_source_surface(IntPtr cr, CairoSurface surface, double x, double y);
+            
+            [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Cairo)]
+            public delegate void cairo_set_source_rgba(IntPtr cr, double r, double g, double b, double a);
 
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Cairo)]
             public delegate void cairo_scale(IntPtr context, double sx, double sy);
 
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Cairo)]
             public delegate void cairo_paint(IntPtr context);
+            
+            [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Cairo)]
+            public delegate void cairo_show_text(IntPtr context, Utf8Buffer text);   
+            
+            [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Cairo)]
+            public delegate void cairo_set_font_size(IntPtr context, double size);  
+            
+            [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Cairo)]
+            public delegate void cairo_select_font_face(IntPtr context, Utf8Buffer face, int slant, int weight);
+            
+            [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Cairo)]
+            public delegate void cairo_move_to(IntPtr context, double x, double y);   
+            
+            [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Cairo)]
+            public delegate void cairo_destroy(IntPtr context);
 
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gtk)]
             public delegate void gtk_widget_queue_draw_area(GtkWidget widget, int x, int y, int width, int height);
+            
+            [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gtk)]
+            public delegate void gtk_widget_add_tick_callback(GtkWidget widget, TickCallback callback, IntPtr userData, IntPtr destroy);
 
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gtk)]
             public delegate GtkImContext gtk_im_multicontext_new();
@@ -236,6 +260,15 @@ namespace Avalonia.Gtk3.Interop
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gdk)]
             public delegate void gdk_window_begin_resize_drag(IntPtr window, WindowEdge edge, gint button, gint root_x, gint root_y, guint32 timestamp);
 
+            [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gdk)]
+            public delegate void gdk_window_process_updates(IntPtr window, bool updateChildren);
+            
+            [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gdk)]
+            public delegate void gdk_window_begin_paint_rect(IntPtr window, ref GdkRectangle rect);
+            
+            [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gdk)]
+            public delegate void gdk_window_end_paint(IntPtr window);
+
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gdk)]
             public delegate void gdk_event_request_motions(IntPtr ev);
 
@@ -273,6 +306,9 @@ namespace Avalonia.Gtk3.Interop
             public delegate bool gdk_pixbuf_save_to_bufferv(Pixbuf pixbuf, out IntPtr buffer, out IntPtr buffer_size,
                             Utf8Buffer type, IntPtr option_keys, IntPtr option_values, out IntPtr error);
 
+            [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gdk)]
+            public delegate IntPtr gdk_cairo_create(IntPtr window);
+
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gobject)]
             public delegate void g_object_unref(IntPtr instance);
             
@@ -287,7 +323,10 @@ namespace Avalonia.Gtk3.Interop
             
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Glib)]
             public delegate ulong g_timeout_add(uint interval, timeout_callback callback, IntPtr data);
-            
+
+            [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Glib)]
+            public delegate ulong g_timeout_add_full(int prio, uint interval, timeout_callback callback, IntPtr data, IntPtr destroy);
+
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Glib)]
             public delegate ulong g_free(IntPtr data);
             
@@ -320,6 +359,11 @@ namespace Avalonia.Gtk3.Interop
 
             [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
             public delegate void GtkClipboardTextReceivedFunc(IntPtr clipboard, IntPtr utf8string, IntPtr userdata);
+
+            [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
+            public delegate bool TickCallback(IntPtr widget, IntPtr clock, IntPtr userdata);
+
+
         }
 
         public static D.gdk_display_get_n_screens GdkDisplayGetNScreens;
@@ -366,6 +410,7 @@ namespace Avalonia.Gtk3.Interop
         public static D.g_signal_connect_object GSignalConnectObject;
         public static D.g_signal_handler_disconnect GSignalHandlerDisconnect;
         public static D.g_timeout_add GTimeoutAdd;
+        public static D.g_timeout_add_full GTimeoutAddFull;
         public static D.g_free GFree;
         public static D.g_slist_free GSlistFree;
         public static D.g_memory_input_stream_new_from_data GMemoryInputStreamNewFromData;
@@ -373,6 +418,7 @@ namespace Avalonia.Gtk3.Interop
         public static D.gtk_widget_set_events GtkWidgetSetEvents;
         public static D.gdk_window_invalidate_rect GdkWindowInvalidateRect;
         public static D.gtk_widget_queue_draw_area GtkWidgetQueueDrawArea;
+        public static D.gtk_widget_add_tick_callback GtkWidgetAddTickCallback;
         public static D.gtk_widget_activate GtkWidgetActivate;
         public static D.gtk_clipboard_get_for_display GtkClipboardGetForDisplay;
         public static D.gtk_clipboard_request_text GtkClipboardRequestText;
@@ -398,6 +444,10 @@ namespace Avalonia.Gtk3.Interop
         public static D.gdk_window_begin_move_drag GdkWindowBeginMoveDrag;
         public static D.gdk_window_begin_resize_drag GdkWindowBeginResizeDrag;
         public static D.gdk_event_request_motions GdkEventRequestMotions;
+        public static D.gdk_window_process_updates GdkWindowProcessUpdates;
+        public static D.gdk_window_begin_paint_rect GdkWindowBeginPaintRect;
+        public static D.gdk_window_end_paint GdkWindowEndPaint;
+        
 
         public static D.gdk_pixbuf_new_from_file GdkPixbufNewFromFile;
         public static D.gtk_icon_theme_get_default GtkIconThemeGetDefault;
@@ -406,16 +456,24 @@ namespace Avalonia.Gtk3.Interop
         public static D.gdk_window_set_cursor GdkWindowSetCursor;
         public static D.gdk_pixbuf_new_from_stream GdkPixbufNewFromStream;
         public static D.gdk_pixbuf_save_to_bufferv GdkPixbufSaveToBufferv;
-
+        public static D.gdk_cairo_create GdkCairoCreate;
+        
         public static D.cairo_image_surface_create CairoImageSurfaceCreate;
         public static D.cairo_image_surface_get_data CairoImageSurfaceGetData;
         public static D.cairo_image_surface_get_stride CairoImageSurfaceGetStride;
         public static D.cairo_surface_mark_dirty CairoSurfaceMarkDirty;
+        public static D.cairo_surface_write_to_png CairoSurfaceWriteToPng;
         public static D.cairo_surface_flush CairoSurfaceFlush;
         public static D.cairo_surface_destroy CairoSurfaceDestroy;
         public static D.cairo_set_source_surface CairoSetSourceSurface;
+        public static D.cairo_set_source_rgba CairoSetSourceRgba;
         public static D.cairo_scale CairoScale;
         public static D.cairo_paint CairoPaint;
+        public static D.cairo_show_text CairoShowText;
+        public static D.cairo_select_font_face CairoSelectFontFace;
+        public static D.cairo_set_font_size CairoSetFontSize;
+        public static D.cairo_move_to CairoMoveTo;
+        public static D.cairo_destroy CairoDestroy;
     }
 
     public enum GtkWindowType

src/Gtk/Avalonia.Gtk3/Interop/Resolver.cs

@@ -140,13 +140,11 @@ namespace Avalonia.Gtk3.Interop
             var nativeHandleNames = new[] { "gdk_win32_window_get_handle", "gdk_x11_window_get_xid", "gdk_quartz_window_get_nswindow" };
             foreach (var name in nativeHandleNames)
             {
-                try
-                {
-                    Native.GetNativeGdkWindowHandle = (Native.D.gdk_get_native_handle)Marshal
-                        .GetDelegateForFunctionPointer(loader.GetProcAddress(dlls[GtkDll.Gdk], name, false), typeof(Native.D.gdk_get_native_handle));
-                    break;
-                }
-                catch { }
+                var ptr = loader.GetProcAddress(dlls[GtkDll.Gdk], name, true);
+                if (ptr == IntPtr.Zero)
+                    continue;
+                Native.GetNativeGdkWindowHandle = (Native.D.gdk_get_native_handle) Marshal
+                    .GetDelegateForFunctionPointer(ptr, typeof(Native.D.gdk_get_native_handle));
             }
             if (Native.GetNativeGdkWindowHandle == null)
                 throw new Exception($"Unable to locate any of [{string.Join(", ", nativeHandleNames)}] in libgdk");

src/Gtk/Avalonia.Gtk3/ScreenImpl.cs

@@ -10,7 +10,7 @@ namespace Avalonia.Gtk3
     {
         public int ScreenCount
         {
-            get => _allScreens.Length;
+            get => AllScreens.Length;
         }
         
         private Screen[] _allScreens;
@@ -23,7 +23,7 @@ namespace Avalonia.Gtk3
                     IntPtr display = Native.GdkGetDefaultDisplay();
                     GdkScreen screen = Native.GdkDisplayGetDefaultScreen(display);
                     short primary = Native.GdkScreenGetPrimaryMonitor(screen);
-                    Screen[] screens = new Screen[ScreenCount];
+                    Screen[] screens = new Screen[Native.GdkScreenGetNMonitors(screen)];
                     for (short i = 0; i < screens.Length; i++)
                     {
                         GdkRectangle workArea = new GdkRectangle(), geometry = new GdkRectangle();

src/Gtk/Avalonia.Gtk3/WindowBaseImpl.cs

@@ -10,6 +10,7 @@ using Avalonia.Input;
 using Avalonia.Input.Raw;
 using Avalonia.Platform;
 using Avalonia.Rendering;
+using Avalonia.Threading;
 
 namespace Avalonia.Gtk3
 {
@@ -25,6 +26,9 @@ namespace Avalonia.Gtk3
         private double _lastScaling;
         private uint _lastKbdEvent;
         private uint _lastSmoothScrollEvent;
+        private GCHandle _gcHandle;
+        private object _lock = new object();
+        private IDeferredRenderOperation _nextRenderOperation;
 
         public WindowBaseImpl(GtkWindow gtkWidget)
         {
@@ -50,11 +54,25 @@ namespace Avalonia.Gtk3
             Connect<Native.D.signal_generic>("destroy", OnDestroy);
             Native.GtkWidgetRealize(gtkWidget);
             _lastSize = ClientSize;
+            GlibTimeout.Add(0, 16, () =>
+            {
+                Invalidate(default(Rect));
+                return true;
+            });
+            if (Gtk3Platform.UseDeferredRendering)
+            {
+                Native.GtkWidgetSetDoubleBuffered(gtkWidget, false);
+                _gcHandle = GCHandle.Alloc(this);
+                Native.GtkWidgetAddTickCallback(GtkWidget, PinnedStaticCallback, GCHandle.ToIntPtr(_gcHandle), IntPtr.Zero);
+                
+            }
         }
 
         private bool OnConfigured(IntPtr gtkwidget, IntPtr ev, IntPtr userdata)
         {
-            var size = ClientSize;
+            int w, h;
+            Native.GtkWindowGetSize(GtkWidget, out w, out h);
+            var size = ClientSize = new Size(w, h);
             if (_lastSize != size)
             {
                 Resized?.Invoke(size);
@@ -222,12 +240,52 @@ namespace Avalonia.Gtk3
 
         private bool OnDraw(IntPtr gtkwidget, IntPtr cairocontext, IntPtr userdata)
         {
-            CurrentCairoContext = cairocontext;
-            Paint?.Invoke(new Rect(ClientSize));
-            CurrentCairoContext = IntPtr.Zero;
+            if (!Gtk3Platform.UseDeferredRendering)
+            {
+                CurrentCairoContext = cairocontext;
+                Paint?.Invoke(new Rect(ClientSize));
+                CurrentCairoContext = IntPtr.Zero;
+            }
+            return true;
+        }
+
+        private static Native.D.TickCallback PinnedStaticCallback = StaticTickCallback;
+
+        static bool StaticTickCallback(IntPtr widget, IntPtr clock, IntPtr userData)
+        {
+            var impl = (WindowBaseImpl) GCHandle.FromIntPtr(userData).Target;
+            impl.OnRenderTick();
             return true;
         }
 
+        public void SetNextRenderOperation(IDeferredRenderOperation op)
+        {
+            lock (_lock)
+            {
+                _nextRenderOperation?.Dispose();
+                _nextRenderOperation = op;
+            }
+        }
+
+        private void OnRenderTick()
+        {
+            IDeferredRenderOperation op = null;
+            lock (_lock)
+            {
+                if (_nextRenderOperation != null)
+                {
+                    op = _nextRenderOperation;
+                    _nextRenderOperation = null;
+                }
+            }
+            if (op != null)
+            {
+                op?.RenderNow();
+                op?.Dispose();
+            }
+        }
+
+
         public void Dispose()
         {
             //We are calling it here, since signal handler will be detached
@@ -236,6 +294,10 @@ namespace Avalonia.Gtk3
             foreach(var d in Disposables.AsEnumerable().Reverse())
                 d.Dispose();
             Disposables.Clear();
+            if (_gcHandle.IsAllocated)
+            {
+                _gcHandle.Free();
+            }
         }
 
         public Size MaxClientSize
@@ -270,7 +332,8 @@ namespace Avalonia.Gtk3
         {
             if(GtkWidget.IsClosed)
                 return;
-            Native.GtkWidgetQueueDrawArea(GtkWidget, (int)rect.X, (int)rect.Y, (int)rect.Width, (int)rect.Height);
+            var s = ClientSize;
+            Native.GtkWidgetQueueDrawArea(GtkWidget, 0, 0, (int) s.Width, (int) s.Height);
         }
 
         public void SetInputRoot(IInputRoot inputRoot) => _inputRoot = inputRoot;
@@ -323,17 +386,7 @@ namespace Avalonia.Gtk3
         }
 
 
-        public Size ClientSize
-        {
-            get
-            {
-                if (GtkWidget.IsClosed)
-                    return new Size();
-                int w, h;
-                Native.GtkWindowGetSize(GtkWidget, out w, out h);
-                return new Size(w, h);
-            }
-        }
+        public Size ClientSize { get; private set; }
 
         public void Resize(Size value)
         {
@@ -363,7 +416,10 @@ namespace Avalonia.Gtk3
 
         public IRenderer CreateRenderer(IRenderRoot root)
         {
-            return new ImmediateRenderer(root);
+            var loop = AvaloniaLocator.Current.GetService<IRenderLoop>();
+            return Gtk3Platform.UseDeferredRendering
+                ? (IRenderer) new DeferredRenderer(root, loop)
+                : new ImmediateRenderer(root);
         }
     }
 }

src/Linux/Avalonia.LinuxFramebuffer/PlatformThreadingInterface.cs

@@ -6,6 +6,7 @@ using System.Threading;
 using System.Threading.Tasks;
 using Avalonia.Platform;
 using Avalonia.Rendering;
+using Avalonia.Threading;
 
 namespace Avalonia.LinuxFramebuffer
 {
@@ -30,7 +31,7 @@ namespace Avalonia.LinuxFramebuffer
             while (true)
             {
                 if (0 == WaitHandle.WaitAny(handles))
-                    Signaled?.Invoke();
+                    Signaled?.Invoke(null);
                 else
                 {
                     while (true)
@@ -97,7 +98,7 @@ namespace Avalonia.LinuxFramebuffer
 
         }
 
-        public void Signal()
+        public void Signal(DispatcherPriority prio)
         {
             _signaled.Set();
         }
@@ -105,7 +106,7 @@ namespace Avalonia.LinuxFramebuffer
         [ThreadStatic] private static bool TlsCurrentThreadIsLoopThread;
 
         public bool CurrentThreadIsLoopThread => TlsCurrentThreadIsLoopThread;
-        public event Action Signaled;
+        public event Action<DispatcherPriority?> Signaled;
         public event EventHandler<EventArgs> Tick;
 
     }

src/OSX/Avalonia.MonoMac/PlatformThreadingInterface.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Threading;
 using Avalonia.Platform;
+using Avalonia.Threading;
 using MonoMac.AppKit;
 using MonoMac.CoreGraphics;
 using MonoMac.Foundation;
@@ -13,12 +14,12 @@ namespace Avalonia.MonoMac
         public static PlatformThreadingInterface Instance { get; } = new PlatformThreadingInterface();
         public bool CurrentThreadIsLoopThread => NSThread.Current.IsMainThread;
 
-        public event Action Signaled;
+        public event Action<DispatcherPriority?> Signaled;
 
         public IDisposable StartTimer(TimeSpan interval, Action tick)
             => NSTimer.CreateRepeatingScheduledTimer(interval, () => tick());
 
-        public void Signal()
+        public void Signal(DispatcherPriority prio)
         {
             lock (this)
             {
@@ -34,7 +35,7 @@ namespace Avalonia.MonoMac
                         return;
                     _signaled = false;
                 }
-                Signaled?.Invoke();
+                Signaled?.Invoke(null);
             });
         }
 

src/Windows/Avalonia.Win32/Win32Platform.cs

@@ -15,6 +15,7 @@ using Avalonia.Win32.Input;
 using Avalonia.Win32.Interop;
 using Avalonia.Controls;
 using Avalonia.Rendering;
+using Avalonia.Threading;
 #if NETSTANDARD
 using Win32Exception = Avalonia.Win32.NetStandard.AvaloniaWin32Exception;
 #else
@@ -137,7 +138,7 @@ namespace Avalonia.Win32
         private static readonly int SignalW = unchecked((int) 0xdeadbeaf);
         private static readonly int SignalL = unchecked((int)0x12345678);
 
-        public void Signal()
+        public void Signal(DispatcherPriority prio)
         {
             UnmanagedMethods.PostMessage(
                 _hwnd,
@@ -148,14 +149,14 @@ namespace Avalonia.Win32
 
         public bool CurrentThreadIsLoopThread => _uiThread == UnmanagedMethods.GetCurrentThreadId();
 
-        public event Action Signaled;
+        public event Action<DispatcherPriority?> Signaled;
 
         [SuppressMessage("Microsoft.StyleCop.CSharp.NamingRules", "SA1305:FieldNamesMustNotUseHungarianNotation", Justification = "Using Win32 naming for consistency.")]
         private IntPtr WndProc(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam)
         {
             if (msg == (int) UnmanagedMethods.WindowsMessage.WM_DISPATCH_WORK_ITEM && wParam.ToInt64() == SignalW && lParam.ToInt64() == SignalL)
             {
-                Signaled?.Invoke();
+                Signaled?.Invoke(null);
             }
             return UnmanagedMethods.DefWindowProc(hWnd, msg, wParam, lParam);
         }

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -16,6 +16,7 @@ using Avalonia.Win32.Input;
 using Avalonia.Win32.Interop;
 using static Avalonia.Win32.Interop.UnmanagedMethods;
 using Avalonia.Rendering;
+using Avalonia.Threading;
 #if NETSTANDARD
 using Win32Exception = Avalonia.Win32.NetStandard.AvaloniaWin32Exception;
 #endif

src/iOS/Avalonia.iOS/PlatformThreadingInterface.cs

@@ -9,6 +9,7 @@ using CoreAnimation;
 using Foundation;
 using Avalonia.Platform;
 using Avalonia.Shared.PlatformSupport;
+using Avalonia.Threading;
 
 namespace Avalonia.iOS
 {
@@ -18,7 +19,7 @@ namespace Avalonia.iOS
         public static PlatformThreadingInterface Instance { get; } = new PlatformThreadingInterface();
         public bool CurrentThreadIsLoopThread => NSThread.Current.IsMainThread;
         
-        public event Action Signaled;
+        public event Action<DispatcherPriority?> Signaled;
         public void RunLoop(CancellationToken cancellationToken)
         {
             //Mobile platforms are using external main loop
@@ -53,7 +54,7 @@ namespace Avalonia.iOS
         public IDisposable StartTimer(TimeSpan interval, Action tick)
             => NSTimer.CreateRepeatingScheduledTimer(interval, _ => tick());
 
-        public void Signal()
+        public void Signal(DispatcherPriority prio)
         {
             lock (this)
             {
@@ -65,7 +66,7 @@ namespace Avalonia.iOS
             {
                 lock (this)
                     _signaled = false;
-                Signaled?.Invoke();
+                Signaled?.Invoke(null);
             });
         }
     }

tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_Threading.cs

@@ -6,6 +6,7 @@ using System.Reactive.Subjects;
 using System.Threading;
 using System.Threading.Tasks;
 using Avalonia.Platform;
+using Avalonia.Threading;
 using Avalonia.UnitTests;
 using Xunit;
 
@@ -147,14 +148,14 @@ namespace Avalonia.Base.UnitTests
 
             public bool CurrentThreadIsLoopThread { get; set; }
 
-            public event Action Signaled;
+            public event Action<DispatcherPriority?> Signaled;
 
             public void RunLoop(CancellationToken cancellationToken)
             {
                 throw new NotImplementedException();
             }
 
-            public void Signal()
+            public void Signal(DispatcherPriority prio)
             {
                 throw new NotImplementedException();
             }

tests/Avalonia.RenderTests/TestBase.cs

@@ -13,7 +13,7 @@ using Avalonia.Platform;
 using System.Threading.Tasks;
 using System;
 using System.Threading;
-
+using Avalonia.Threading;
 #if AVALONIA_SKIA
 using Avalonia.Skia;
 #else
@@ -149,14 +149,14 @@ namespace Avalonia.Direct2D1.RenderTests
 
             public Thread MainThread { get; set; }
 
-            public event Action Signaled;
+            public event Action<DispatcherPriority?> Signaled;
 
             public void RunLoop(CancellationToken cancellationToken)
             {
                 throw new NotImplementedException();
             }
 
-            public void Signal()
+            public void Signal(DispatcherPriority prio)
             {
                 throw new NotImplementedException();
             }