Use dictionaries for context creation attributes. Eliminate custom bindings.

Source/core/html/HTMLCanvasElement.idl

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+typedef (CanvasRenderingContext2D or WebGLRenderingContext) RenderingContext;
+
 interface HTMLCanvasElement : HTMLElement {
 
     attribute long width;
     attribute long height;
 
     [Custom, RaisesException] DOMString toDataURL([TreatUndefinedAs=NullString, Default=Undefined] optional DOMString? type);
 
-    // The custom binding is needed to handle context creation attributes.
-    [Custom] any getContext([Default=Undefined] optional DOMString contextId);
+    // Note: this differs deliberately from the specified Web IDL for this function:
+    //     RenderingContext? getContext(DOMString contextId, any... arguments);
+    // in order to eliminate the custom binding. It is functionally equivalent.
+    //
+    // The PermissiveDictionaryConversion extended attribute is needed to allow the
+    // autogenerated code to match the behavior of the custom binding. Web IDL
+    // requires throwing TypeError if the incoming argument is not an object type
+    // (and is not undefined or null). The binding must ignore this.
+    //
+    // Also note: the only reason this must return type "any" is to allow the
+    // InspectorInstrumentation wrappers to be called for canvases. If that could be
+    // handled differently the return type could be changed to "RenderingContext?"
+    // and the [CallWith=ScriptState] extended attribute removed.
+    [CallWith=ScriptState] any getContext([Default=Undefined] optional DOMString contextId, [PermissiveDictionaryConversion] optional CanvasContextCreationAttributes attributes);

[bashi, 2014/12/12 08:31:33]

Let me ask a question: is it possible to use union types here instead of adding
PermissiveDictionaryConversion? (e.g. (CanvasContextCreationAttributes or any)).

I was wondering if we could use the standard features, but I prefer this
approach if using union types could make the CL more complicated.

[dshwang, 2014/12/12 14:46:37]

RenderingContext? getContext(...) doesn't require custom binding. The counter
example is Document.idl
RenderingContext getCSSCanvasContext(DOMString contextId, DOMString name, long
width, long height);

If you define "RenderingContext? getContext(...)", the binding code get the
CanvasRenderingContext2DOrWebGLRenderingContext& argument for return value like
Document::getCSSCanvasContext()
It has two benefits;
1. HTMLCanvasElement::getContext can be unified.
2. WrapCanvasContext is not needed.

[Ken Russell (switch to Gerrit), 2014/12/12 19:09:38]

The manually-written epilogue is required in order to call
InspectorInstrumentation's wrapCanvas2DRenderingContextForInstrumentation and
wrapWebGLRenderingContextForInstrumentation. Otherwise the inspector layout
tests break. It would be better if these hooks were implemented differently, but
this CL can't break that functionality.

[Ken Russell (switch to Gerrit), 2014/12/12 19:09:39]

Sorry, I just tested this and it's not possible. The code generator doesn't
allow "any" in a union type (not unexpected).

[dshwang, 2014/12/12 21:53:16]

Thank you for explaining. Can Document.idl be changed like this? If so, I guess
two HTMLCanvasElement::getContext can be unified. 

 };