Determine if OffscreenCanvas is paintable in different rendering contexts

third_party/WebKit/Source/modules/offscreencanvas/OffscreenCanvasModules.cpp

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "modules/offscreencanvas/OffscreenCanvasModules.h"
 
 #include "core/html/canvas/CanvasContextCreationAttributes.h"
 #include "core/offscreencanvas/OffscreenCanvas.h"
+#include "modules/imagebitmap/ImageBitmapRenderingContext.h"
 #include "modules/offscreencanvas2d/OffscreenCanvasRenderingContext2D.h"
+#include "modules/webgl/WebGLRenderingContextBase.h"
+#include "platform/graphics/ImageBuffer.h"
 
 namespace blink {
 
 void OffscreenCanvasModules::getContext(ScriptState* scriptState, OffscreenCanvas& offscreenCanvas, const String& id, const CanvasContextCreationAttributes& attributes, ExceptionState& exceptionState, OffscreenRenderingContext& result)
 {
     if (offscreenCanvas.isNeutered()) {
         exceptionState.throwDOMException(InvalidStateError, "OffscreenCanvas object is detached");
         return;
     }
 
     CanvasRenderingContext* context = offscreenCanvas.getCanvasRenderingContext(scriptState, id, attributes);
     if (context)
         context->setOffscreenCanvasGetContextResult(result);
 }
 
+bool OffscreenCanvasModules::isPaintable(OffscreenCanvas& offscreenCanvas)

[Justin Novosad, 2016/08/19 14:55:47]

There is a cleaner way to implement this: put a pure virtual method
isPaintable() on CanvasRenderingContext.  It will allow you to keep
drawingBuffer() encapsulated in WebGLRenderingContextBase, and it will allow you
to implement isPaintable in core (in OffscreenCanvas proper), and it avoid
sticking explicit knowledge of all the context types in OffscreenCAnvas, which
is better for maintainability.

+{
+    CanvasRenderingContext* context = offscreenCanvas.renderingContext();
+    if (!context)
+        return ImageBuffer::canCreateImageBuffer(offscreenCanvas.size());
+
+    if (context->is2d())
+        return toOffscreenCanvasRenderingContext2D(context)->imageBuffer();
+    if (context->is3d())
+        return toWebGLRenderingContextBase(context)->drawingBuffer();
+    if (context->getContextType() == CanvasRenderingContext::ContextImageBitmap)
+        return toImageBitmapRenderingContext(context)->image();
+
+    NOTREACHED();
+    return false;
+}
+
 } // namespace blink