Make compositingState explicit

Source/core/page/scrolling/ScrollingCoordinator.cpp

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 140 matching lines @@
 }
 
 static void clearPositionConstraintExceptForLayer(GraphicsLayer* layer, GraphicsLayer* except)
 {
     if (layer && layer != except && scrollingWebLayerForGraphicsLayer(layer))
         scrollingWebLayerForGraphicsLayer(layer)->setPositionConstraint(WebLayerPositionConstraint());
 }
 
 static WebLayerPositionConstraint computePositionConstraint(const RenderLayer* layer)
 {
-    ASSERT(layer->isComposited());
+    ASSERT(layer->backing());
     do {
         if (layer->renderer()->style()->position() == FixedPosition) {
             const RenderObject* fixedPositionObject = layer->renderer();
             bool fixedToRight = !fixedPositionObject->style()->right().isAuto();
             bool fixedToBottom = !fixedPositionObject->style()->bottom().isAuto();
             return WebLayerPositionConstraint::fixedPosition(fixedToRight, fixedToBottom);
         }
 
         layer = layer->parent();
-    } while (layer && !layer->isComposited());
+
+        // This walk ends once we found the first layer that has its own backing.
+    } while (layer && (layer->compositingState() == PaintsIntoOwnBacking || layer->compositingState() == HasOwnBackingButPaintsIntoAncestor));

[Julien - ping for review, 2013/10/03 20:08:36]

Couldn't we use backing() here instead to be consistent with the ASSERT above?

[shawnsingh, 2013/10/07 11:45:04]

Actually yes, I am reverting this back to using backing()  (actually
compositedLayerMapping(), the new accessor).  The reason, however, is not just
to be consistent with the ASSERT.  I've actually convinced myself that, for now,
in cases where compositingState is queried for the sake of computing
positioning, usually they care whether the compositedLayerMapping() exists or
not.  They don't care whether the layer paints into ancestor or paints into own
backing.

I'm looking forward to fixing this over time.  As you've suggested, eventually
no code should use the compositedLayerBacking() accessor as a boolean condition
unless they really are dealing with allocating/deallocating it.  For now, a lot
of usage still exists for that.

     return WebLayerPositionConstraint();
 }
 
 void ScrollingCoordinator::updateLayerPositionConstraint(RenderLayer* layer)
 {
     ASSERT(layer->backing());
     RenderLayerBacking* backing = layer->backing();
     GraphicsLayer* mainLayer = backing->childForSuperlayers();
 
     // Avoid unnecessary commits
@@ skipping 616 matching lines @@
     const FrameView::ViewportConstrainedObjectSet* viewportConstrainedObjects = frameView->viewportConstrainedObjects();
     if (!viewportConstrainedObjects)
         return false;
 
     for (FrameView::ViewportConstrainedObjectSet::const_iterator it = viewportConstrainedObjects->begin(), end = viewportConstrainedObjects->end(); it != end; ++it) {
         RenderObject* viewportConstrainedObject = *it;
         if (!viewportConstrainedObject->isBoxModelObject() || !viewportConstrainedObject->hasLayer())
             return true;
         RenderLayer* layer = toRenderBoxModelObject(viewportConstrainedObject)->layer();
         // Any explicit reason that a fixed position element is not composited shouldn't cause slow scrolling.
-        if (!layer->isComposited() && layer->viewportConstrainedNotCompositedReason() == RenderLayer::NoNotCompositedReason)
+        if (layer->compositingState() != PaintsIntoOwnBacking && layer->viewportConstrainedNotCompositedReason() == RenderLayer::NoNotCompositedReason)
             return true;
 
         // Composited layers that actually paint into their enclosing ancestor
         // must also force main thread scrolling.
-        if (layer->isComposited() && layer->backing()->paintsIntoCompositedAncestor())
+        if (layer->compositingState() == HasOwnBackingButPaintsIntoAncestor)
             return true;
     }
     return false;
 }
 
 MainThreadScrollingReasons ScrollingCoordinator::mainThreadScrollingReasons() const
 {
     // The main thread scrolling reasons are applicable to scrolls of the main
     // frame. If it does not exist or if it is not scrollable, there is no
     // reason to force main thread scrolling.
@@ skipping 31 matching lines @@
         stringBuilder.resize(stringBuilder.length() - 2);
     return stringBuilder.toString();
 }
 
 String ScrollingCoordinator::mainThreadScrollingReasonsAsText() const
 {
     return mainThreadScrollingReasonsAsText(mainThreadScrollingReasons());
 }
 
 } // namespace WebCore