On Mon, Apr 13, 2009 at 8:43 AM, Avi Drissman <a...@chromium.org> wrote:
> I'm still a little lost in this discussion. I'm repeatedly reading the code
> and the patch, and I'll get back to you when I fully understand it. But what
> I wanted to say is that there is a significant difference in the paint
> pipeline as it currently exists on the Mac compared to Win/Lin.
>
> On Win, DidPaintRect() calls Redraw() which calls
> RedrawWindow(...RDW_UPDATENOW...) which (AFAIK) does a synchronous draw via
> Win32 callback to OnPaint().
>
> On Lin, DidPaintRect() calls Paint() which blits via
> BackingStore::ShowRect(). Again, synchronously.
>
> On Mac, DidPaintRect() calls Redraw() which calls -[NSView
> setNeedsDisplayInRect:], setting the damaged rect in the view. The call
> returns, and sometime later, during the next pump of the message loop, Cocoa
> wakes up, decides to repaint all damaged windows, calls our -drawRect:,
> which then calls RenderWidgetHost::GetBackingStore().
The Mac implementation is wrong. On a single core machine, the renderer
could starve the UI thread of the browser, preventing any pixels from ever
being updated on the screen. You need to change the Mac implementation to
flush to the screen when DidPaintRect is not called withing drawRect /
GetBackingStore.
Prior to Peter's change it was not possible for DidPaintRect to be called
from within GetBackingStore. Now that is possible, so the consumer needs to
take care.
-Darin
>
>
> That gap prevents recursion. If for some reason GetBackingStore ends up
> causing a DidPaintRect() message, what will happen is:
>
> - Original DidPaintRect(), adds damage
> ... message loop ...
> - -drawRect:
> -- GetBackingStore()
> --- New DidPaintRect(), adds more damage
> -- blitting
> ... Cocoa clears the damage region, all of it
>
> Avi
>
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