[whatwg] Canvas.getContext error handling

Kenneth Russell kbr at google.com
Mon Apr 11 17:55:51 PDT 2011

On Sat, Apr 9, 2011 at 7:55 PM, Glenn Maynard <glenn at zewt.org> wrote:
> getContext doesn't specify error handling.  WebGL solves this oddly: if an
> error occurs, it dispatches an event with error details at the canvas.  It's
> odd for a synchronous API to report error information with an event; it
> would make a lot more sense to raise an exception.  However, getContext
> doesn't specify error handling for the "Return a new object for contextId"
> algorithms.
> The "primary context" should only be set by getContext after "Return a new
> object for contextId" completes successfully; it shouldn't be set on error.
> The cached return value used in step 5 should also only happen after
> success; don't cache a null response.  This way, you can retry getContext on
> failure, and getContext is a straightforward no-op after an error.
> (I don't know if the WebGL folks could be convinced to change to throwing an
> exception or if they want to continue returning null and firing an event.)
> Related thread:
> https://www.khronos.org/webgl/public-mailing-list/archives/1104/msg00027.html

Just for the record, I'm sure the WebGL working group would be
amenable to making changes in this area. However, there is a general
problem: if getContext() throws an exception, how does the caller know
whether a later call to getContext() might succeed, or will always

I don't remember all of the discussions in the WebGL working group
which led to the currently defined behavior, but I think that the fact
that 3D graphics contexts can be spontaneously lost, and recovered,
factored into the current design.


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