On Oct 1, 2013, at 2:48 AM, Benjamin Dupont (bedupont) <[email protected]> wrote:
> Hi all,
>
> I am currently working on the InbandTextTracks in webkit and I am trying to
> understand how the memory is released.
>
> When we launch the track-in-band.html layout test, two in-band text tracks
> have been created and added, the corresponding RefPtr has a refCount equals
> to three.
> 1. Why are there 3 owners for each in-band text track? Is there an hidden
> cache mechanism?
The tracks are being referenced by the JavaScript running in the test page.
> After this test, if we load another page, the player is destroyed and the
> clearTextTracks method is called.
> In my understanding, the player should be the only owner of in-band text
> tracks and thus after the clearTextTracks method is called, the ref count
> should be decreased to 0 and the in-band text track object should be deleted.
Your understanding is incorrect. Each track is also referenced by the
JSTextTrack wrapper created when referencing videoElement.textTracks[].
Furthermore, those wrappers are stored off as variables inbandTrack{1,2,3,4} in
the global context, so they won't be destroyed by GC until the window object is
destroyed
> In fact, after the clearTextTracks method the ref count isn’t equals to 0
> thus the in-band text track object isn’t deleted.
> This text track object is deleted when the clear memory cache is called.
>
> 2. Is it a normal behavior? If yes, what is the interest to use smart pointer?
Yes.
> 3. How does the clear memory cache know that this ref pointer (with a ref
> count != 0) can be released?
This is precisely the point of using a smart pointer; since the track is still
being referenced, it won't be deleted until that refcount drops to 0.
-Jer
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