Implement HTMLCanvasElement.probablySupportsContext

[mskrzypkows]

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #11403

• There are tests for these changes OR
• These changes do not require tests because _____

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This change is

[highfive]

Heads up! This PR modifies the following files:

• @KiChjang: components/script/dom/htmlcanvaselement.rs, components/script/dom/webidls/HTMLCanvasElement.webidl

[nox]

r? @emilio

I think this change is incorrect, because GetContext will cache the context, so if you try to check whether you can create a 2d context, and then try to get for real a canvas context, you won't get it.

[dzbarsky]

just return self.GetContext(cx, id, attributes).is_some() (and you don't actualy need the return keyword)

[emilio]

@nox is right here, we can't do this that way.

In fact, there's no real support for this method in any other browser (and it's arguable), see this for example.

In any case, if we plan to implement this, we should use an heuristic, and not calling GetContext() (since the whole point of this API is avoiding the GetContext() overhead).

In my opinion, we should return true if the id is one of those we support (i.e. webgl, experimental-webgl or 2d), and maybe we want to make it return false if getting the WebGLContextAttributes we see that antialias is true (it's something we don't support right now), but that's an extra thing to change when we support it, so... I'd just check the id.

That being said, I'm a bit skeptical about the utility of this method.

-S-awaiting-review +S-needs-code-changes

Previously, nox (Anthony Ramine) wrote…

r? @emilio

I think this change is incorrect, because GetContext will cache the context, so if you try to check whether you can create a 2d context, and then try to get for real a canvas context, you won't get it.

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Review status: 0 of 3 files reviewed at latest revision, 1 unresolved discussion.

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Comments from Reviewable

[Ms2ger]

We should probably figure out why no other browser implemented this; it's not particularly likely to be implementation difficulty.

[bors-servo]

☔ The latest upstream changes (presumably #11504) made this pull request unmergeable. Please resolve the merge conflicts.

[mskrzypkows]

Here is whatwg thread: https://lists.w3.org/Archives/Public/public-whatwg-archive/2013Jun/0098.html and last message: https://lists.w3.org/Archives/Public/public-whatwg-archive/2013Sep/0248.html
As I understand from the discussion , not all people agree that this API is really useful. Should I continue the task?

[mskrzypkows]

Is there any decision about this task?

[emilio]

I just filled whatwg/html#1459

[emilio]

The spec has been modified in order to no longer include this method.

Thanks for the contribution though @mskrzypkows, it's been crucial to raise the issue! :-)