[whatwg] [canvas] Proposal for supportsContext

Dean Jackson dino at apple.com
Mon Sep 10 12:32:26 PDT 2012

On Sep 10, 2012, at 12:28 PM, Tobie Langel <tobie.langel at gmail.com> wrote:

> On Sep 10, 2012, at 8:14 PM, Dean Jackson <dino at apple.com> wrote:
>> I propose adding a new method to HTMLCanvasElement:
>> interface HTMLCanvasElement : HTMLElement {
>> boolean supportsContext(DOMString contextId, any... arguments);
>> };
>> supportsContext takes the same parameters as getContext, and simply returns
>> true if the corresponding call to getContext would have returned a valid
>> context, false otherwise.
> What about enabling feature detection by providing a method per context?
> interface HTMLCanvasElement : HTMLElement {
>  object get2DContext();
>  object getWebGLContext(any... args);
> };
> That way, developers can use idiomatic JS for feature testing like
> pretty much everywhere else on the Web platform:
> if (canvas.get2DContext) {
>  // do stuff with 2D canvas
> }

This doesn't address the problem we are hitting, which is that
it is expensive to create a context. It's also a big change to
an existing operational API.

I'm looking for some way an implementation can suggest that
it supports a particular context without the user having to
create one. As James Robinson pointed out in the WebKit IRC
channel, even the implementation might not know for sure it can
create the context, but I think that's ok. The important
thing is that returning false from supportsContext means that
a corresponding call to getContext would fail.


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