Refactor the way PaintArtifactCompositor is attached and used.

third_party/WebKit/Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 88 matching lines @@
 #include "platform/ScriptForbiddenScope.h"
 #include "platform/TraceEvent.h"
 #include "platform/TracedValue.h"
 #include "platform/fonts/FontCache.h"
 #include "platform/geometry/DoubleRect.h"
 #include "platform/geometry/FloatRect.h"
 #include "platform/geometry/LayoutRect.h"
 #include "platform/graphics/GraphicsContext.h"
 #include "platform/graphics/GraphicsLayer.h"
 #include "platform/graphics/GraphicsLayerDebugInfo.h"
+#include "platform/graphics/compositing/PaintArtifactCompositor.h"
 #include "platform/graphics/paint/CullRect.h"
 #include "platform/graphics/paint/PaintController.h"
 #include "platform/graphics/paint/ScopedPaintChunkProperties.h"
 #include "platform/json/JSONValues.h"
 #include "platform/scheduler/CancellableTaskFactory.h"
 #include "platform/scroll/ScrollAnimatorBase.h"
 #include "platform/scroll/ScrollbarTheme.h"
 #include "platform/text/TextStream.h"
 #include "public/platform/WebDisplayItemList.h"
 #include "public/platform/WebFrameScheduler.h"
@@ skipping 2636 matching lines @@
     for (auto& child : graphicsLayer->children())
         synchronizedPaintRecursively(child);
 }
 
 void FrameView::pushPaintArtifactToCompositor()
 {
     TRACE_EVENT0("blink", "FrameView::pushPaintArtifactToCompositor");
 
     ASSERT(RuntimeEnabledFeatures::slimmingPaintV2Enabled());
 
-    const PaintArtifact& paintArtifact = m_paintController->paintArtifact();
+    if (!m_paintArtifactCompositor) {
+        m_paintArtifactCompositor = PaintArtifactCompositor::create();
+        Page* page = frame().page();
+        if (!page)

[pdr., 2016/09/15 20:53:07]

Can this ever happen? If it does, wouldn't we forget to attach the root layer?

[chrishtr, 2016/09/15 21:05:39]

I don't know if it can happen. What I can say is that there are several places
that defensively return on null page() already in this class, 
including jbroman's here (which was probably also cargo-culted).

Yes it means we won't attach, but that's ok because there is no page to attach
to anyway. :)

[pdr., 2016/09/15 21:08:17]

Why don't we just bail then, instead of creating and updating the paint artifact
compositor?

[chrishtr, 2016/09/15 21:11:04]

Done.

+            return;
 
-    Page* page = frame().page();
-    if (!page)
-        return;
-    page->chromeClient().didPaint(paintArtifact);
+        page->chromeClient().attachRootLayer(m_paintArtifactCompositor->getWebLayer(), &frame());
+    }
+    m_paintArtifactCompositor->update(m_paintController->paintArtifact());
 }
 
 void FrameView::updateStyleAndLayoutIfNeededRecursive()
 {
     SCOPED_BLINK_UMA_HISTOGRAM_TIMER("Blink.StyleAndLayout.UpdateTime");
     updateStyleAndLayoutIfNeededRecursiveInternal();
 }
 
 void FrameView::updateStyleAndLayoutIfNeededRecursiveInternal()
 {
@@ skipping 1579 matching lines @@
 }
 
 bool FrameView::canThrottleRendering() const
 {
     if (!RuntimeEnabledFeatures::renderingPipelineThrottlingEnabled())
         return false;
     return m_subtreeThrottled || (m_hiddenForThrottling && m_crossOriginForThrottling);
 }
 
 } // namespace blink