[webkit-dev] Position on emerging standard: video.requestAnimationFrame()

[Simon Fraser]

> On Jan 21, 2020, at 5:27 PM, Thomas Guilbert <tguilbert at google.com> wrote:
> 
> The idea was to reuse an API name that developers are already familiar with, in a similar context. The name is also being used in XRSession (https://developer.mozilla.org/en-US/docs/Web/API/XRSession/requestAnimationFrame <https://developer.mozilla.org/en-US/docs/Web/API/XRSession/requestAnimationFrame>), and in OffscreenCanvas (or technically DedicatedWorkerGlobalScope). The AnimationFrameProvider mixin <https://html.spec.whatwg.org/multipage/imagebitmap-and-animations.html#animationframeprovider> could also be updated so HTMLVideoElement can extend it.
> 
> Yes, this isn't formally spec'ed out, but it will be. For now, they are added to the task queue and run like any other task. So, going off the spec you linked, I think this would be "5) Perform oldestTask's step"  and not "10) Rendering: [...] 11. Foreach document run animation frame callbacks for that Document".

I would expect something that's called "requestAnimationFrame" to only fire in the "update the rendering" steps; requestAnimationFrame is a "before rendering" callback. So firing a callback with the same name at other times seems like it will lead to author confusion.

The author's expectation should be that any content/style changes they make inside a requestAnimationFrame callback will appear on-screen in the same frame as other changes in the same event loop cycle, and that requestAnimationFrame won't be called more often than is necessary to update the screen at the appropriate frame rate.

Simon

> 
> On Tue, Jan 21, 2020 at 1:01 PM Simon Fraser <simon.fraser at apple.com <mailto:simon.fraser at apple.com>> wrote:
>> On Jan 21, 2020, at 12:37 PM, Thomas Guilbert <tguilbert at google.com <mailto:tguilbert at google.com>> wrote:
>> 
>> Hello,
>> I'm reaching out to see if webkit would like to weigh in on the following proposal:
>> https://discourse.wicg.io/t/proposal-video-requestanimationframe/3691 <https://discourse.wicg.io/t/proposal-video-requestanimationframe/3691>
>> The HTMLVideoElement.requestAnimationFrame() API allows web developers to be notified when a video frame has been presented for composition, and provides metadata for that frame.
>> If you want to try it out, a prototype is available in Chromium Dev, behind the enable-experimental-web-platform-features flag.
> 
> This is not official feedback, but I have some issues with the proposal.
> 
> First, the name is confusing. It sounds like you're requesting a frame from the video, but it's really a "frame available" callback. Why not call it onFrameAvailable()?
> 
> Second, its interaction with normal requestAnimationFrame() and the HTML event loop needs to be better defined. Where in in the https://html.spec.whatwg.org/multipage/webappapis.html#event-loop-processing-model <https://html.spec.whatwg.org/multipage/webappapis.html#event-loop-processing-model> do these callbacks fire?
> 
> Simon
> 
> 

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