Implement visibility API and the notion of non-visible pipeline

[paulrouget]

Spec: https://w3c.github.io/page-visibility/
MDN: https://developer.mozilla.org/en-US/docs/Web/API/Page_Visibility_API

When a document is not visible (usually a background tab), the document is in "slow mode". setTimeout and setInterval are called less often, requestAnimationFrame is not called and there's probably a lot more things that can slowed down, or stopped.

We can't just use pipeline.freeze() / pipeline.thaw() as we still need events to occur.

Related issue: #9566

[jmr0]

I could probably take a stab at this with a little guidance. Let me know if you have any other thoughts I could use as a starting point or any places in the codebase off the top of your head where you'd expect to see some major changes.

[jdm]

This has several parts:

• adding the APIs to the Document interface per the specification (Document.webidl, document.rs)
• adding the notion of visibility to Pipeline (pipeline.rs)
• making the Document APIs communicate with the constellation to update the pipeline visibility
• using the pipeline visibility to inhibit animation callbacks (compositor.rs?)
• changing the behaviour of timers based on visibility (maybe timers.rs and timer_scheduler.rs?)

[paulrouget]

And I'll add:

• add iframe visibility-related methods and events: Browser API: implement setVisible #9566

This is needed to make a pipeline non visible.

[paulrouget]

Re starting point:

I built a fake setVisible method that freeze the pipeline: paulrouget@fe29671

This is wrong because it uses thaw/freeze which is too aggressive, but it gives you an idea how to expose an iframe method that would call a pipeline method.

You need to start Servo with --pref dom.mozbrowser.enabled and use a <iframe mozbrowser="true"> to make the setVisible method callable.

Hope that helps.

[jmr0]

Thanks guys -- I'll start looking at adapting the pipeline and preparing the Document API changes and circle back with any questions. Feel free to assign.

[jdm]

@jmr0 Any progress? Any questions?

[jmr0]

Hey @jdm, I caught up with Paul over on IRC yesterday and decided I'd focus on the iframe part, since it seems we currently don't support the events necessary to hook to the Document API (e.g. OS lock screen, screen minimized). Ongoing work is here https://github.com/jmr0/servo/commits/visibility_api -- hope to have something deliverable over the weekend. Thanks!

[jmr0]

Almost there with this one. Ended up implementing set/getVisible (Browser API) and some of the timer/compositor reactions to Pipeline visibility changes. Need to take a closer look at requestAnimationFrame and add some tests but all the other parts are pretty much complete.

https://github.com/servo/servo/compare/master...jmr0:visibility_api?expand=1

[paulrouget]

@jmr0 is it possible to set the iframe visibility property in ScriptThread::handle_visible_msg? Then you won't have to do it in iframe::SetVisible.

GetVisibility is usually called after the event mozbrowservisibilitychange is received (which will happen in handle_visible_msg), so we don't really care about the intermediate stage between when SetVisible is called and when the actually visibility changed.

Does that make sense?

Also - the fact that 1) there is an event and 2) getVisibility() is async, doesn't make a lot of sense. It's a consequence of how it was built in Gecko. I think getVisibility() should not be async.

[jmr0]

@paulrouget Yeah that makes perfect sense, I had added that hack to SetVisible for now but there's no need to do that if we stick to a sync GetVisible. I'll go ahead and make that change.

[jmr0]

@paulrouget @jdm We can probably close this and #9566, and possibly open a new related issue with the caveats and parts that weren't covered in #10225 (as per my PR comment) and include anything else you folks can think of. Happy to file that if you agree!

[jdm]

That's reasonable. Please do!