Guidance on events and promises

[marcoscaceres]

An issue came up recently in for the Screen Orientation lock API where by we have a promise being resolved and event being dispatched due to some action (the screen locking to a new orientation, which can trigger a "change" event).

The question that came up is:

• should an event be dispatched before a related promise is resolved? Or should the promise be resolved first, then the event dispatched? See Resolving the promise after the change event is fired w3c/screen-orientation#147

The closest the Promise Guide comes to providing guidance is in the "one time events" section. My reading of that section is that the event should fire fist, and then the promise is resolved based on that action (e.g., "the image loaded, .then(...)", "the orientation locked, .then(...)" and so on).

[annevk]

Yeah, queue a task to

1. Dispatch the event.
2. Resolve the promise.

[marcoscaceres]

Worth amending The Guide? Might avoid a situation like happened with Screen Orientation (now causing issues downstream).

[annevk]

https://www.w3.org/2001/tag/doc/promises-guide#queue-tasks goes into it, though is also partially wrong in suggesting you can avoid queuing tasks.

I'd add advice like this to https://w3ctag.github.io/design-principles/#event-design somewhere as this promises guide needs to have its useful bits folded into IDL and then go away...

[marcoscaceres]

@kenchris, or someone on the TAG with sufficient privileges, could you please transfer this bug to the design guide?