[Webkit-unassigned] [Bug 78872] [chromium] ScrollbarLayerChromium/CCScrollbarLayerImpl for CC-side scrollbar painting

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=78872





--- Comment #23 from James Robinson <jamesr at chromium.org>  2012-03-01 18:23:30 PST ---
(In reply to comment #22)
> (In reply to comment #20)
> > (In reply to comment #19)
> > > > Source/WebCore/platform/graphics/chromium/ScrollbarLayerChromium.h:48
> > > > +    RefPtr<Scrollbar> m_scrollbar;
> > > 
> > > this seems very wrong. the layer shouldn't have any ownership concern with the scrollbar.
> > 
> > the alternative would be copying everything when creating ScrollbarLayerChromium.
> 
> I find this won't work. m_visibleSize and m_totalSize is something very dynamic, we need to update their value on every commits.
> I think we might be able to get these information from the layer data, any suggestion?

These are asking about the visible / total size of the scrollable area, not the scrollbar itself.  I think you should get those from the CCLayerImpl that's actually scrolling by giving the CCScrollbar (or CCScrollbarLayerImpl) a weak pointer to the CCLayerImpl that scrolls.  For example, totalSize() is some combination of the CCLayerImpl's maxScrollPosition, bounds, etc.

Keep in mind that these values can change on the thread without us going through a commit - for example, if the page scale changes due to a pinch gesture that changes the max scroll position and thus the totalSize() value. I think the only way to stay sane with these updates is to always query the value from the scrolling layer and not try to proactively sync them to other objects.

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