Specify if/when new Gamepad objects are created

[ddorwin]

The WebXR spec is not explicit about when to create new Gamepad objects. This was a source of interop problems for the Gamepad API, so we should try to avoid that.

Gamepad API implementations create a new Gamepad object when the state changes. If we decide not to do that for WebXR, we should have a good reason for the inconsistency in the platform and consider the impact on developers and use cases.

[ddorwin]

The Gamepad API spec issue is w3c/gamepad#8.

/cc @nondebug

[Manishearth]

Ah, I stumbled across this in my implementation too -- it's not clear to me if the Gamepad is dynamic or not.

[toji]

The spec has been updated recently to specify that XRInputSources are live, and the gamepad attribute on XRInputSource has the [SameObject] extended attribute. Is that sufficient in this case? We can certainly add a line of text to indicate that the same Gamepad instance must be returned from the gamepad attribute each time, but that's literally the entire purpose of [SameObject] so it would feel strange to call this one specific attribute out further.

[ddorwin]

That sounds fine. This is different behavior from (some?) Gamepad API implementations, though, so it might be worth calling that out in a non-normative note.

Context: IIUC some developers were comparing Gamepad objects between frames, which is not a supported pattern for XRInputSource/WebXR.

[toji]

Ah, thanks for the context! That does sound worthy of a non-normative note.