Define in greater detail what effect setting playbackState has

[foolip]

Related to #175

https://wicg.github.io/mediasession/#playback-state-model says "playback state is a hint from the page and and MAY be overridden by the user agent"

The setter definition in https://wicg.github.io/mediasession/#dom-mediasession-playbackstate doesn't clearly say what to do. It would be good if as much of the normative requirements as possible were in the algorithms reachable from API entry points, not in the model description.

https://wicg.github.io/mediasession/#media-session-routing doesn't reference playback state, so it's not clear if it should have any impact at all.

Concretely, what happens when the hint doesn't match the actual state?

• If no media is playing or has been playing, does setting playbackState to "paused" do anything?
• If some media was playing but is now paused, does setting playbackState to "playing" do anything?
• If media is playing, does setting playbackState to "none" or "paused" do anything?

If the API ever has any effect, it does look like a miniature audio focus API, or have I misunderstood?

[xxyzzzq]

The playbackState has nothing to do with https://wicg.github.io/mediasession/#media-session-routing, nor audio focus. It is only used to help the UA decide whether to show a play button or pause button.

It could be useful to make the behavior consistent when different UAs have their own way of deciding whether a page is playing. For example:

• The UA considers seeking or loading media as "not playing", but the page expects it to be interrupted by pause.
• The UA does not have ability to decide whether the page is playing or not.
• The page just want to do some preparation before media plays but it expects it could be interrupted by pause.

WDYT?

[xxyzzzq]

Original issue & PR discussing about adding playbackState attribute: #139, #141

[foolip]

Thanks for that extra context, I had totally the wrong idea about this was. In addition to examples, there are a few things in the spec text I think would be useful:

• Define in more detail how actual playback state is calculated. It would presumably be "playing" or "paused" if that's set, or calculated based on media elements and their states if not. People will reach for playbackState when the default is wrong, so it's best if browsers agree on what that default is.
• Editorial: Define "playback state" as an internal concept of MediaSession alongside metadata and use that to define the playbackState attribute and the "actual playback state".