Re: [Gamepad] Liveness of Gamepad objects

On Tue Apr 29 2014 at 10:24:48 AM, Mark S. Miller <erights@google.com>
wrote:

> I don't know anything about Gamepad. Could someone provide enough context
> that I can understand the question? Thanks.
>

The Gamepad API returns an array of Gamepad state objects when you call
getGamepads(), like so:

requestAnimationFrame(function() {
    var gamepads = navigator.getGamepads();
    var playerGamepad = gamepads[0];
    if (playerGamepad.buttons[0].pressed) {
        player.jump();
    }
}

That code currently would work in both Firefox and Chrome. The "Liveness"
question concerns the following code:

var gamepads = navigator.getGamepads();
var playerGamepad = gamepads[0];
requestAnimationFrame(function() {
    if (playerGamepad.buttons[0].pressed) {
        player.jump();
    }
}

In Firefox this code will be functionally equivalent to the first code
snippet, since it continues to update the values Gamepad state object after
you've acquired it. AKA: The object is "live".

In Chrome, however, the gamepad state returned is a snapshot and as such
the second code snippet will not observe changes to the gamepad input.

The spec is currently ambiguous on the matter, so we want to clarify
whether or not the second code snippet is valid or not and update the
browser implementations accordingly.

--Brandon


> On Tue, Apr 29, 2014 at 10:16 AM, Brendan Eich <brendan@secure.meer.net>wrote:
>
>> Boris Zbarsky wrote:
>>
>>> For what it's worth, the way Gecko implements this is by setting up that
>>> lifetime guarantee only when an expando is added to the object (or some
>>> other things, like use as a WeakMap key, happen).  Until then we allow it
>>> to be GCed.
>>>
>>
>> What do other engines do in general? Unobservable GC is a requirement, if
>> you ask me and Mark Miller.
>>
>> /be
>>
>
>
>
> --
>     Cheers,
>     --MarkM
>