Reduce texture uploads during scrollbar invalidations.

cc/ScrollbarLayerChromium.cpp

Index: cc/ScrollbarLayerChromium.cpp
diff --git a/cc/ScrollbarLayerChromium.cpp b/cc/ScrollbarLayerChromium.cpp
index 4f3a09fa5e47e6f2b4ff938adb24fb727fbff76d..5e3f7824ba86197441f2fcf5372eb18db7ac2954 100644
--- a/cc/ScrollbarLayerChromium.cpp
+++ b/cc/ScrollbarLayerChromium.cpp
@@ -10,6 +10,7 @@
 
 #include "base/basictypes.h"
 #include "BitmapCanvasLayerTextureUpdater.h"
+#include "caching_bitmap_canvas_layer_texture_updater.h"
 #include "CCLayerTreeHost.h"
 #include "CCScrollbarLayerImpl.h"
 #include "CCTextureUpdateQueue.h"
@@ -193,20 +194,20 @@ void ScrollbarLayerChromium::createTextureUpdaterIfNeeded()
     m_textureFormat = layerTreeHost()->rendererCapabilities().bestTextureFormat;
 
     if (!m_backTrackUpdater)
-        m_backTrackUpdater = BitmapCanvasLayerTextureUpdater::create(ScrollbarBackgroundPainter::create(m_scrollbar.get(), m_painter, m_geometry.get(), WebKit::WebScrollbar::BackTrackPart));
+        m_backTrackUpdater = CachingBitmapCanvasLayerTextureUpdater::Create(ScrollbarBackgroundPainter::create(m_scrollbar.get(), m_painter, m_geometry.get(), WebKit::WebScrollbar::BackTrackPart));
     if (!m_backTrack)
         m_backTrack = m_backTrackUpdater->createTexture(layerTreeHost()->contentsTextureManager());
 
     // Only create two-part track if we think the two parts could be different in appearance.
     if (m_scrollbar->isCustomScrollbar()) {
         if (!m_foreTrackUpdater)
-            m_foreTrackUpdater = BitmapCanvasLayerTextureUpdater::create(ScrollbarBackgroundPainter::create(m_scrollbar.get(), m_painter, m_geometry.get(), WebKit::WebScrollbar::ForwardTrackPart));
+            m_foreTrackUpdater = CachingBitmapCanvasLayerTextureUpdater::Create(ScrollbarBackgroundPainter::create(m_scrollbar.get(), m_painter, m_geometry.get(), WebKit::WebScrollbar::ForwardTrackPart));
         if (!m_foreTrack)
             m_foreTrack = m_foreTrackUpdater->createTexture(layerTreeHost()->contentsTextureManager());
     }
 
     if (!m_thumbUpdater)
-        m_thumbUpdater = BitmapCanvasLayerTextureUpdater::create(ScrollbarThumbPainter::create(m_scrollbar.get(), m_painter, m_geometry.get()));
+        m_thumbUpdater = CachingBitmapCanvasLayerTextureUpdater::Create(ScrollbarThumbPainter::create(m_scrollbar.get(), m_painter, m_geometry.get()));
     if (!m_thumb)
         m_thumb = m_thumbUpdater->createTexture(layerTreeHost()->contentsTextureManager());
 }
@@ -230,6 +231,9 @@ void ScrollbarLayerChromium::updatePart(LayerTextureUpdater* painter, LayerTextu
     float heightScale = static_cast<float>(contentBounds().height()) / bounds().height();
     IntRect paintedOpaqueRect;
     painter->prepareToUpdate(rect, rect.size(), widthScale, heightScale, paintedOpaqueRect, stats);
+    if (!static_cast<CachingBitmapCanvasLayerTextureUpdater*>(painter)->PixelsDidChange())

[enne (OOO), 2012/10/03 18:05:26]

Please change the signature on updatePart to have the fully derived type rather
than adding this static_cast.

I think the way to handle evicted textures is to check haveBackingTexture()
before you call canAcquireBackingTexture().  If the answer is false, then ignore
this PixelsDidChange() early-out.

[wjmaclean, 2012/10/03 19:14:59]

Hmm, no, I don't think so. I'd have to change the member variable declarations
in ScrollbarLayerChromium to be of the derived type to avoid the cast, or am I
misunderstanding what you wanted here?

Perhaps I can add a virtual to LayerTextureUpdater?
OK. I thought by early-outing in update part I wouldn't need to worry about
evicted textures, but I guess that doesn't make sense. Will do this.

+        return;
+
     texture->prepareRect(rect, stats);
 
     IntSize destOffset(0, 0);