Bug 1172796 - Part 3: Implement OffscreenCanvas constructor. r=roc r=smaug

dom/canvas/OffscreenCanvas.cpp

@@ -70,6 +70,18 @@ OffscreenCanvas::WrapObject(JSContext* aCx,
   return OffscreenCanvasBinding::Wrap(aCx, this, aGivenProto);
 }
 
+/* static */ already_AddRefed<OffscreenCanvas>
+OffscreenCanvas::Constructor(const GlobalObject& aGlobal,
+                             uint32_t aWidth,
+                             uint32_t aHeight,
+                             ErrorResult& aRv)
+{
+  RefPtr<OffscreenCanvas> offscreenCanvas =
+    new OffscreenCanvas(aWidth, aHeight,
+                        layers::LayersBackend::LAYERS_NONE, nullptr);
+  return offscreenCanvas.forget();
+}
+
 void
 OffscreenCanvas::ClearResources()
 {
@@ -167,6 +179,12 @@ OffscreenCanvas::CreateContext(CanvasContextType aContextType)
 void
 OffscreenCanvas::CommitFrameToCompositor()
 {
+  if (!mCanvasRenderer) {
+    // This offscreen canvas doesn't associate to any HTML canvas element.
+    // So, just bail out.
+    return;
+  }
+
   // The attributes has changed, we have to notify main
   // thread to change canvas size.
   if (mAttrDirty) {

dom/canvas/OffscreenCanvas.h

@@ -63,6 +63,12 @@ public:
   virtual JSObject* WrapObject(JSContext* aCx,
                                JS::Handle<JSObject*> aGivenProto) override;
 
+  static already_AddRefed<OffscreenCanvas>
+  Constructor(const GlobalObject& aGlobal,
+              uint32_t aWidth,
+              uint32_t aHeight,
+              ErrorResult& aRv);
+
   void ClearResources();
 
   uint32_t Width() const

dom/webidl/OffscreenCanvas.webidl

@@ -5,14 +5,10 @@
  *
  * For more information on this interface, please see
  * https://wiki.whatwg.org/wiki/OffscreenCanvas
- *
- * Current implementation focus on transfer canvas from main thread to worker.
- * So there are some spec doesn't implement, such as [Constructor], toBlob() and
- * transferToImageBitmap in OffscreenCanvas. Bug 1172796 will implement
- * remaining spec.
  */
 
-[Exposed=(Window,Worker),
+[Constructor(unsigned long width, unsigned long height),
+ Exposed=(Window,Worker),
  Func="mozilla::dom::OffscreenCanvas::PrefEnabled"]
 interface OffscreenCanvas : EventTarget {
   [Pure, SetterThrows]