Activate LayerImpl tree with sync+push instead of pointer swap

cc/layer_tree_host_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/layer_tree_host_impl.h"
 
 #include <algorithm>
 
 #include "base/basictypes.h"
 #include "base/debug/trace_event.h"
@@ skipping 21 matching lines @@
 #include "cc/render_pass_draw_quad.h"
 #include "cc/rendering_stats.h"
 #include "cc/scrollbar_animation_controller.h"
 #include "cc/scrollbar_layer_impl.h"
 #include "cc/shared_quad_state.h"
 #include "cc/single_thread_proxy.h"
 #include "cc/software_renderer.h"
 #include "cc/solid_color_draw_quad.h"
 #include "cc/texture_uploader.h"
 #include "cc/top_controls_manager.h"
+#include "cc/tree_synchronizer.h"
 #include "cc/util.h"
 #include "ui/gfx/size_conversions.h"
 #include "ui/gfx/vector2d_conversions.h"
 
 namespace {
 
 void didVisibilityChange(cc::LayerTreeHostImpl* id, bool visible)
 {
     if (visible) {
         TRACE_EVENT_ASYNC_BEGIN1("webkit", "LayerTreeHostImpl::setVisible", id, "LayerTreeHostImpl", id);
@@ skipping 117 matching lines @@
 {
     DCHECK(m_proxy->isImplThread());
     TRACE_EVENT0("cc", "LayerTreeHostImpl::~LayerTreeHostImpl()");
 
     if (rootLayer()) {
         clearRenderSurfaces();
         // The layer trees must be destroyed before the layer tree host. We've
         // made a contract with our animation controllers that the registrar
         // will outlive them, and we must make good.
         m_activeTree.reset();
+        m_recycleTree.reset();

[enne (OOO), 2013/01/17 23:34:55]

There are a couple of other cleanup functions in LTHI, like initializeRenderer
calling sendDidLoseOutputSurfaceRecursive and clearRenderSurfaces.  Should those
get called on the recycle tree as well?

[jamesr, 2013/01/18 01:16:42]

Added code to clear the render surfaces on the recycle tree after activation.

Lost context is less clear, because logically the recycle tree shouldn't have
ownership of any resources. However it will have resource IDs and I think it'll
be really confusing to have bogus resource IDs on the recycle tree the next time
we try to sync to it, so added sendDidLose...() calls for the recycle tree as
well

         m_pendingTree.reset();
     }
 }
 
 void LayerTreeHostImpl::beginCommit()
 {
 }
 
 void LayerTreeHostImpl::commitComplete()
 {
@@ skipping 723 matching lines @@
 }
 
 LayerTreeImpl* LayerTreeHostImpl::activeTree()
 {
     return m_activeTree.get();
 }
 
 void LayerTreeHostImpl::createPendingTree()
 {
     CHECK(!m_pendingTree);
-    m_pendingTree = LayerTreeImpl::create(this);
+    if (m_recycleTree)
+        m_recycleTree.swap(m_pendingTree);
+    else
+        m_pendingTree = LayerTreeImpl::create(this);
     m_client->onCanDrawStateChanged(canDraw());
     m_client->onHasPendingTreeStateChanged(pendingTree());
 }
 
 void LayerTreeHostImpl::checkForCompletedSetPixels()
 {
     if (m_tileManager)
         m_tileManager->CheckForCompletedSetPixels();
 }
 
@@ skipping 11 matching lines @@
       return;
 
     activatePendingTree();
 }
 
 void LayerTreeHostImpl::activatePendingTree()
 {
     CHECK(m_pendingTree);
 
     m_activeTree->PushPersistedState(m_pendingTree.get());
-    m_activeTree.swap(m_pendingTree);
-    // TODO(enne): consider recycling this tree to prevent layer churn
-    m_pendingTree.reset();
+    m_activeTree->SetRootLayer(TreeSynchronizer::synchronizeTrees(m_pendingTree->RootLayer(), m_activeTree->DetachLayerTree(), m_activeTree.get()));
+    TreeSynchronizer::pushProperties(m_pendingTree->RootLayer(), m_activeTree->RootLayer());
+    DCHECK(!m_recycleTree);
+    m_activeTree->set_source_frame_number(m_pendingTree->source_frame_number());

[enne (OOO), 2013/01/17 23:34:55]

I wish there were some better way to establish the correspondence between this
block of code pushing tree values and the one in LayerTreeHost.  Maybe some
comment saying "new values pushed to the tree here should also be added to this
other function"?

[jamesr, 2013/01/18 01:16:42]

Added a comment.  While adding this I also thought it would be nicer to move
this code in LayerTreeHost over to this class so they can live side-by-side (or
even possibly share code), but that seemed like a bit too much to pile into this
patch.

+    m_activeTree->set_background_color(m_pendingTree->background_color());
+    m_activeTree->set_has_transparent_background(m_pendingTree->has_transparent_background());
+    m_pendingTree.swap(m_recycleTree);

[jamesr, 2013/01/18 01:16:42]

added another comment here, for kicks

     m_activeTree->DidBecomeActive();
 
     m_client->onCanDrawStateChanged(canDraw());
     m_client->onHasPendingTreeStateChanged(pendingTree());
     m_client->setNeedsRedrawOnImplThread();
 }
 
 void LayerTreeHostImpl::setVisible(bool visible)
 {
     DCHECK(m_proxy->isImplThread());
@@ skipping 707 matching lines @@
     LayerImpl* layer = getNonCompositedContentLayerRecursive(tree->RootLayer());
     return layer ? layer->getPicture() : skia::RefPtr<SkPicture>();
 }
 
 void LayerTreeHostImpl::savePaintTime(const base::TimeDelta& totalPaintTime)
 {
     m_paintTimeCounter->SavePaintTime(totalPaintTime);
 }
 
 }  // namespace cc