Add support to Contextlost to Canvas 2d API #6797

yiyix · 2021-06-23T06:27:40Z

At least two implementers are interested (and none opposed):
- Chrome
- Firefox: API for dealing with 2D context loss #4809 (comment)
- WebKit: API for dealing with 2D context loss #4809 (comment)
Tests are written and can be reviewed and commented upon at:
We have added some internal tests, we are not sure how to trigger context lost cross platforms.
Implementation bugs are filed:
- Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=1164059
- Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1717768
- Safari: https://bugs.webkit.org/show_bug.cgi?id=227280

(See WHATWG Working Mode: Changes for more details.)

/canvas.html ( diff )
/indices.html ( diff )
/webappapis.html ( diff )

annevk · 2021-06-23T11:46:15Z

This does not (yet?) match the discussion in #4809, e.g., #4809 (comment). I think it should.

Why is isContextLost() a method and not a getter?

Why is the name for the second event contextrestored and not contextreset as discussed?

cc @whatwg/canvas

yiyix · 2021-07-06T06:51:58Z

This does not (yet?) match the discussion in #4809, e.g., #4809 (comment). I think it should.

Sorry, I missed that comment, I moved the event description to 'update the rendering' session.

Why is isContextLost() a method and not a getter?

I think this makes more sense as well! But an attribute cannot start with verb. Maybe i can add an attribute called ContextLost, ContextIsLost? What do you think?

Why is the name for the second event contextrestored and not contextreset as discussed?

In Ken's replay, he explained it is possible not to restore a lost event. So we decide to implement it as 2 events instead of 1.

cc @whatwg/canvas

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domenic · 2021-07-08T20:29:51Z

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+
+      <li><p>Reset <span data-x="concept-canvas-context-lost-flag">context lost flag</span>.</p></li>
+
+      <li><p>Fire an event <code data-x="event-contextrestored">contextrestored</code> at <var>canvas</var>.</p></li>


Interesting that this one doesn't queue a task...

I got the idea of how to write this session from webgl contextlost event spec: https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.15.2I. In context lost event, we need to queue a task to call context restore event after. In context restore event, it's done after it's restored, so we don't need to queue a task to call any other events.

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domenic · 2021-07-08T20:32:37Z

I forgot to say: an attribute called contextLost makes sense to me.

fserb · 2021-07-12T18:37:04Z

I forgot to say: an attribute called contextLost makes sense to me.

Originally that was our proposal (and implementation).

The problem is WebGL uses isContextLost() (on WebGLRenderingContext). So maybe keeping it the same on CanvasRenderingContext2D makes more sense? WDYT?

domenic · 2021-07-12T18:41:57Z

Oh, yeah, in that case staying consistent gets my vote.

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domenic · 2021-07-20T21:51:18Z

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+      <li><p>Let <var>canvas</var> be the value of <var>context</var>'s <var>canvas</var> or the <code>canvas</code> associated with <code>OffscreenCanvas</code> object.</p></li>
+
+      <li><p>If <var>context</var>'s <span data-x="concept-canvas-context-lost">context lost
+      flag</span> is not set, abort these steps.</p></li>


When would this condition be true? I.e. when would the user agent try to restore the backing storage for something without its context lost set to true?

BTW the phrasing should be "If context's context lost is false"

I got this line from webgl spec, https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.15.3. I don't know how to satisfy this line's condition. I thought about it when i put it down, I still keep it here to avoid canvas end up in some weird place. Do you think it's better to remove it?

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domenic · 2021-07-20T21:55:37Z

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  attribute <span>EventHandler</span> <span data-x="handler-oncontextmenu">oncontextmenu</span>;
+  attribute <span>EventHandler</span> <span data-x="handler-oncontextrestored">oncontextrestored</span>;


You should probably add oncontextlost and oncontextrestored to the OffscreenCanvas IDL too. Then you need a small table like the one at the bottom of https://html.spec.whatwg.org/#the-eventsource-interface inside the OffscreenCanvas section.

that's true, updated.

domenic

Overall looking good. I have some grammar nits but we can fix them later. The big question is around cancelability.

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annevk · 2021-08-04T07:14:02Z

I don't see point 2 of #4809 (comment) reflected in this PR. In particular I thought one goal was to create a shared abstraction with WebGL.

yiyix · 2021-08-09T20:08:21Z

I don't see point 2 of #4809 (comment) reflected in this PR. In particular I thought one goal was to create a shared abstraction with WebGL.

Could you clarify what you mean by creating a shared abstraction with WebGL? We are using the same interface and same event names as webgl, I can add some notes, if it's webgl context, then follow the webgl spec.

domenic

I pushed a commit with some minor editorial touchups; PTAL.

Then I have two more substantive questions which I left as inline comments.

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yiyix · 2021-08-11T23:41:29Z

Sorry, I am very new to github. When i try to download your previous commit, I somehow messed up my commit history and this includes every commit i did in the past. I reviewed the changed file and it seems to be up to date. If it is too confusing, I can start a new branch and do all commits there.

domenic

OK, this looks good to me! I filed web-platform-tests/wpt#30039 to see if eventually we can convert your tests into web platform tests, but that's not a blocker.

@annevk, do you want to take a final pass? Otherwise I'll merge in the next day or two.

IMO further unification with Web GL, if it's even desirable, should be done as a separate followup. In particular the semantics of the events are backward, so it's not clear how much they can really share.

yiyix changed the title ~~Contextlost~~ Add support to Contextlost to Canvas 2d API Jun 23, 2021

annevk added addition/proposal New features or enhancements topic: canvas labels Jun 23, 2021

domenic requested changes Jul 8, 2021

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domenic force-pushed the main branch from 0947339 to 1b5099f Compare July 27, 2021 17:07

new context lost - address comment

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delete a bad case

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domenic and others added 7 commits August 13, 2021 05:20

Cleanups and tweaks

e12e825

update context lost steps

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update context lost steps

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add reset step to context lost steps

64dc6e3

add reset step to context lost steps

cc80f83

update

98130bb

Typo

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domenic mentioned this pull request Aug 16, 2021

Add a way to trigger synthetic canvas context loss web-platform-tests/wpt#30039

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domenic approved these changes Aug 16, 2021

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domenic merged commit 88c994a into whatwg:main Aug 18, 2021

annevk mentioned this pull request Sep 29, 2021

contextlost follow-up #7130

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yiyix deleted the contextlost branch January 4, 2023 16:32

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Add support to Contextlost to Canvas 2d API #6797

Add support to Contextlost to Canvas 2d API #6797

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		<li><p>Reset <span data-x="concept-canvas-context-lost-flag">context lost flag</span>.</p></li>

		<li><p>Fire an event <code data-x="event-contextrestored">contextrestored</code> at <var>canvas</var>.</p></li>

		attribute <span>EventHandler</span> <span data-x="handler-oncontextmenu">oncontextmenu</span>;
		attribute <span>EventHandler</span> <span data-x="handler-oncontextrestored">oncontextrestored</span>;

Add support to Contextlost to Canvas 2d API #6797

Add support to Contextlost to Canvas 2d API #6797

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