允许复用Context

在内存比较小的旧电脑上，在调用
var bitmap = new SKBitmap(BITMAP.bmWidth, BITMAP.bmHeight);时
会因为SKBitmap.TryAllocPixels 随机出现System.Exception: Unable to allocate pixels for the bitmap.   复用Context，减小SKBitmap.TryAllocPixels 调用

CPF.Skia/SkiaDrawingContext.cs

@@ -128,6 +128,39 @@ namespace CPF.Skia
             canvas = new SKCanvas(this.bitmap);
         }
 
+        public override bool ResetHdcCanvas(IRenderTarget renderTarget)
+        {
+            IntPtr hdc = IntPtr.Zero;
+            if (renderTarget is HDCRenderTarget hDC)
+            {
+                hdc = hDC.Hdc;
+            }
+            else if (renderTarget is OpenGlRenderTarget<IntPtr> open)
+            {
+                hdc = open.FailBackTarget;
+            }
+            if (hdc == IntPtr.Zero)
+            {
+                return false;
+            }
+
+            //获取dc里的位图信息，Windows平台
+            var hbitmap = UnmanagedMethods.GetCurrentObject(hdc, ObjectType.OBJ_BITMAP);
+            BITMAP BITMAP = new BITMAP();
+            UnmanagedMethods.GetObject(hbitmap, Marshal.SizeOf(typeof(BITMAP)), BITMAP);
+            if (bitmap == null || bitmap.Width != BITMAP.bmWidth || bitmap.Height != BITMAP.bmHeight)
+            {
+                return false;
+            }
+
+            canvas.Flush();
+            canvas.Dispose();
+            canvas = null;
+            bitmap.SetPixels(BITMAP.bmBits);
+            canvas = new SKCanvas(this.bitmap);
+            return true;
+        }
+
         //public SkiaDrawingContext(SKBitmap bitmap)
         //{
         //    this.bitmap = bitmap;

CPF.Windows/WindowImpl.cs

@@ -939,6 +939,10 @@ namespace CPF.Windows
         //    gl.PopMatrix();
         //    gl.Flush();
         //}
+
+        IRenderTarget lastRenderTarget;
+        DrawingContext cacheDC;
+
         void OnPaint(IntPtr hdc, Rect rect)
         {
             if (handle == IntPtr.Zero)
@@ -1013,10 +1017,26 @@ namespace CPF.Windows
                         wglContext.GetFramebufferInfo(out var fb, out var sam, out var sten);
                         rt = new OpenGlRenderTarget<IntPtr>(hdc, wglContext, (int)Bounds.Width, (int)Bounds.Height, fb, sam, sten);
                     }
-                    //Console.WriteLine(rt + " " + isLayered + "  " + useGPU + "  " + memDc);
-                    using (DrawingContext dc = DrawingContext.FromRenderTarget(rt))
+
+                    if (lastRenderTarget == null || lastRenderTarget.Width != rt.Width || lastRenderTarget.Height != rt.Height)
                     {
-                        root.RenderView(dc, rect);
+                        lastRenderTarget = rt;
+                        if (cacheDC != null)
+                        {
+                            cacheDC.Dispose();
+                            cacheDC = null;
+                        }
+                    }
+
+                    if (cacheDC == null || !cacheDC.ResetHdcCanvas(rt))
+                    {
+                        cacheDC = DrawingContext.FromRenderTarget(rt);
+                    }
+
+                    //Console.WriteLine(rt + " " + isLayered + "  " + useGPU + "  " + memDc);
+                    //using (DrawingContext dc = DrawingContext.FromRenderTarget(rt))
+                    {
+                        root.RenderView(cacheDC, rect);
                         //DrawRenderRectangle(dc);
                     }
                     //testPaint();

CPF/Drawing/DrawingContext.cs

@@ -243,6 +243,15 @@ namespace CPF.Drawing
             }
         }
 
+        /// <summary>
+        /// 重置画布
+        /// </summary>
+        /// <param name="renderTarget"></param>
+        public virtual bool ResetHdcCanvas(IRenderTarget renderTarget)
+        {
+            return false;
+        }
+
         public abstract DrawingFactory DrawingFactory { get; }
 
         public static DrawingContext FromBitmap(Bitmap bmp)