Simplified render loop, added guard drawing context wrapper

src/Gtk/Perspex.Cairo/Media/TileBrushes.cs

@@ -101,7 +101,7 @@ namespace Perspex.Cairo.Media
             var intermediateSize = CalculateIntermediateSize(tileMode, targetSize, destinationRect.Size);
             
 			using (var intermediate = new ImageSurface(Format.ARGB32, (int)intermediateSize.Width, (int)intermediateSize.Height))
-            using (var context = new Context(intermediate))
+            using (var ctx = new RenderTarget(intermediate).CreateDrawingContext())
             {
                 Rect drawRect;
                 var transform = CalculateIntermediateTransform(
@@ -111,12 +111,12 @@ namespace Perspex.Cairo.Media
                     scale,
                     translate,
                     out drawRect);
-                var renderer = new RenderTarget(intermediate);
 
-                context.Rectangle(drawRect.ToCairo());
-                context.Clip();
-                context.Transform(transform.ToCairo());
-                renderer.Render(visual, transform, drawRect);
+                using (ctx.PushClip(drawRect))
+                using (ctx.PushTransform(transform))
+                {
+                    ctx.Render(visual);
+                }
 
                 var result = new SurfacePattern(intermediate);
 

src/Perspex.SceneGraph/Media/ValidatingDrawingContext.cs

@@ -0,0 +1,76 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+using System.Reactive.Disposables;
+using Perspex.Media.Imaging;
+
+namespace Perspex.Media
+{
+    public class ValidatingDrawingContext : IDrawingContext
+    {
+        private readonly IDrawingContext _base;
+
+        public ValidatingDrawingContext(IDrawingContext @base)
+        {
+            _base = @base;
+        }
+
+        public void Dispose()
+        {
+            _base.Dispose();
+        }
+
+        public Matrix CurrentTransform => _base.CurrentTransform;
+        public void DrawImage(IBitmap source, double opacity, Rect sourceRect, Rect destRect)
+        {
+            _base.DrawImage(source, opacity, sourceRect, destRect);
+        }
+
+        public void DrawLine(Pen pen, Point p1, Point p2)
+        {
+            _base.DrawLine(pen, p1, p2);
+        }
+
+        public void DrawGeometry(Brush brush, Pen pen, Geometry geometry)
+        {
+            _base.DrawGeometry(brush, pen, geometry);
+        }
+
+        public void DrawRectangle(Pen pen, Rect rect, float cornerRadius = 0)
+        {
+            _base.DrawRectangle(pen, rect, cornerRadius);
+        }
+
+        public void DrawText(Brush foreground, Point origin, FormattedText text)
+        {
+            _base.DrawText(foreground, origin, text);
+        }
+
+        public void FillRectangle(Brush brush, Rect rect, float cornerRadius = 0)
+        {
+            _base.FillRectangle(brush, rect, cornerRadius);
+        }
+
+
+        Stack<IDisposable> _stateStack = new Stack<IDisposable>();
+
+        IDisposable Transform(IDisposable disposable)
+        {
+            _stateStack.Push(disposable);
+            return Disposable.Create(() =>
+            {
+                var current = _stateStack.Peek();
+                if (current != disposable)
+                    throw new InvalidOperationException("Invalid push/pop order");
+                current.Dispose();
+                _stateStack.Pop();
+            });
+        }
+
+        public IDisposable PushClip(Rect clip) => Transform(_base.PushClip(clip));
+
+        public IDisposable PushOpacity(double opacity) => Transform(_base.PushOpacity(opacity));
+
+        public IDisposable PushTransform(Matrix matrix) => Transform(_base.PushTransform(matrix));
+    }
+}

src/Perspex.SceneGraph/Perspex.SceneGraph.csproj

@@ -99,6 +99,7 @@
     <Compile Include="Media\Transform.cs" />
     <Compile Include="Media\TileBrush.cs" />
     <Compile Include="Media\ImageBush.cs" />
+    <Compile Include="Media\ValidatingDrawingContext.cs" />
     <Compile Include="Media\VisualBrush.cs" />
     <Compile Include="RelativePoint.cs" />
     <Compile Include="Platform\IFormattedTextImpl.cs" />

src/Perspex.SceneGraph/Rendering/RendererBase.cs

@@ -27,31 +27,7 @@ namespace Perspex.Rendering
                 ctx.Render(visual);
         }
 
-        /// <summary>
-        /// Renders the specified visual.
-        /// </summary>
-        /// <param name="renderTarget">IRenderer instance</param>
-        /// <param name="visual">The visual to render.</param>
-        /// <param name="translation">The current translation.</param>
-        /// <param name="transform">The current transform.</param>
-        public static void Render(this IRenderTarget renderTarget, IVisual visual, Matrix translation, Matrix transform)
-        {
-            using (var ctx = renderTarget.CreateDrawingContext())
-                ctx.Render(visual, translation, transform);
-        }
 
-        /// <summary>
-        /// Renders the specified visual with the specified transform and clip.
-        /// </summary>
-        /// <param name="renderTarget">IRenderer instance</param>
-        /// <param name="visual">The visual to render.</param>
-        /// <param name="transform">The transform.</param>
-        /// <param name="clip">An optional clip rectangle.</param>
-        public static void Render(this IRenderTarget renderTarget, IVisual visual, Matrix transform, Rect? clip = null)
-        {
-            using (var context = renderTarget.CreateDrawingContext())
-                context.Render(visual, transform, clip);
-        }
 
         /// <summary>
         /// Renders the specified visual.
@@ -60,65 +36,29 @@ namespace Perspex.Rendering
         /// 
         /// <param name="context">The drawing context.</param>
         public static void Render(this IDrawingContext context, IVisual visual)
-        {
-            context.Render(visual, Matrix.Identity);
-        }
-
-        /// <summary>
-        /// Renders the specified visual with the specified transform and clip.
-        /// </summary>
-        /// <param name="visual">The visual to render.</param>
-        /// <param name="context">The drawing context.</param>
-        /// <param name="transform">The transform.</param>
-        /// <param name="clip">An optional clip rectangle.</param>
-        public static void Render(this IDrawingContext context, IVisual visual, Matrix transform, Rect? clip = null)
-        {
-            using (clip.HasValue ? context.PushClip(clip.Value) : null)
-            {
-                context.Render(visual, Matrix.Identity, transform);
-            }
-        }
-        
-        /// <summary>
-        /// Renders the specified visual.
-        /// </summary>
-        /// <param name="visual">The visual to render.</param>
-        /// <param name="context">The drawing context.</param>
-        /// <param name="translation">The current translation.</param>
-        /// <param name="transform">The current transform.</param>
-        public static void Render(this IDrawingContext context, IVisual visual, Matrix translation, Matrix transform)
         {
             var opacity = visual.Opacity;
-
             if (visual.IsVisible && opacity > 0)
             {
-                // Translate any existing transform into this controls coordinate system.
-                Matrix offset = Matrix.CreateTranslation(visual.Bounds.Position);
-                transform = offset * transform * -offset;
+                var m = Matrix.CreateTranslation(visual.Bounds.Position);
 
-                // Update the current offset.
-                translation *= Matrix.CreateTranslation(visual.Bounds.Position);
+                var renderTransform = Matrix.Identity;
 
-                // Apply the control's render transform, if any.
                 if (visual.RenderTransform != null)
                 {
-                    offset = Matrix.CreateTranslation(visual.TransformOrigin.ToPixels(visual.Bounds.Size));
-                    transform *= -offset * visual.RenderTransform.Value * offset;
+                    var offset = -Matrix.CreateTranslation(visual.TransformOrigin.ToPixels(visual.Bounds.Size));
+                    renderTransform = offset*visual.RenderTransform.Value;
                 }
+                m = context.CurrentTransform.Invert()*renderTransform*m*context.CurrentTransform;
 
-                // Draw the control and its children.
-                var m = transform * translation;
-                var d = context.PushTransform(m);
-
+                using (context.PushTransform(m))
                 using (context.PushOpacity(opacity))
                 using (visual.ClipToBounds ? context.PushClip(new Rect(visual.Bounds.Size)) : null)
                 {
                     visual.Render(context);
-                    d.Dispose();
-
                     foreach (var child in visual.VisualChildren.OrderBy(x => x.ZIndex))
                     {
-                        context.Render(child, translation, transform);
+                        context.Render(child);
                     }
                 }
             }

src/Windows/Perspex.Direct2D1/Media/VisualBrushImpl.cs

@@ -53,8 +53,12 @@ namespace Perspex.Direct2D1.Media
                     out drawRect);
                 var renderer = new RenderTarget(intermediate);
 
-                renderer.Render(visual, transform, drawRect);
-
+                using (var ctx = renderer.CreateDrawingContext())
+                using (ctx.PushClip(drawRect))
+                using (ctx.PushTransform(transform))
+                {
+                    ctx.Render(visual);
+                }
                 this.PlatformBrush = new BitmapBrush(
                     target,
                     intermediate.Bitmap,

src/Windows/Perspex.Direct2D1/RenderTarget.cs

@@ -91,13 +91,23 @@ namespace Perspex.Direct2D1
             window.Resize(new Size2(width, height));
         }
 
+        IDrawingContext Wrap(IDrawingContext ctx)
+        {
+#if DEBUG
+            return new ValidatingDrawingContext(ctx);
+#endif
+#pragma warning disable 162
+            return ctx;
+#pragma warning restore 162
+        }
+
         /// <summary>
         /// Creates a drawing context for a rendering session.
         /// </summary>
         /// <returns>An <see cref="IDrawingContext"/>.</returns>
         public IDrawingContext CreateDrawingContext()
         {
-            return new DrawingContext(_renderTarget, DirectWriteFactory);
+            return Wrap(new DrawingContext(_renderTarget, DirectWriteFactory));
         }
 
         public void Dispose()