cc: Allow the main thread to cancel commits

cc/trees/layer_tree_host.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/trees/layer_tree_host.h"
 
 #include <algorithm>
 #include <stack>
 
 #include "base/bind.h"
@@ skipping 447 matching lines @@
 
 const RendererCapabilities& LayerTreeHost::GetRendererCapabilities() const {
   return proxy_->GetRendererCapabilities();
 }
 
 void LayerTreeHost::SetNeedsAnimate() {
   DCHECK(proxy_->HasImplThread());
   proxy_->SetNeedsAnimate();
 }
 
+void LayerTreeHost::SetNeedsUpdateLayers() {
+  proxy_->SetNeedsUpdateLayers();
+}
+
 void LayerTreeHost::SetNeedsCommit() {
   if (!prepaint_callback_.IsCancelled()) {
     TRACE_EVENT_INSTANT0("cc",
                          "LayerTreeHost::SetNeedsCommit::cancel prepaint",
                          TRACE_EVENT_SCOPE_THREAD);
     prepaint_callback_.Cancel();
   }
   proxy_->SetNeedsCommit();
 }
 
@@ skipping 95 matching lines @@
   if (overdraw_bottom_height_ == overdraw_bottom_height)
     return;
 
   overdraw_bottom_height_ = overdraw_bottom_height;
   SetNeedsCommit();
 }
 
 void LayerTreeHost::SetPageScaleFactorAndLimits(float page_scale_factor,
                                                 float min_page_scale_factor,
                                                 float max_page_scale_factor) {
-  if (page_scale_factor == page_scale_factor_ &&
+  bool same_page_scale_factor = page_scale_factor == page_scale_factor_;
+  bool same_page_scale_limits =
       min_page_scale_factor == min_page_scale_factor_ &&
-      max_page_scale_factor == max_page_scale_factor_)
+      max_page_scale_factor == max_page_scale_factor_;
+  if (same_page_scale_factor && same_page_scale_limits)
     return;
 
   page_scale_factor_ = page_scale_factor;
   min_page_scale_factor_ = min_page_scale_factor;
   max_page_scale_factor_ = max_page_scale_factor;
-  SetNeedsCommit();
+
+  if (same_page_scale_limits)
+    SetNeedsUpdateLayers();
+  else
+    SetNeedsCommit();
 }
 
 void LayerTreeHost::SetVisible(bool visible) {
   if (visible_ == visible)
     return;
   visible_ = visible;
   if (!visible)
     ReduceMemoryUsage();
   proxy_->SetVisible(visible);
 }
@@ skipping 339 matching lines @@
 
   in_paint_layer_contents_ = false;
 
   occlusion_tracker.overdraw_metrics()->RecordMetrics(this);
 }
 
 void LayerTreeHost::ApplyScrollAndScale(const ScrollAndScaleSet& info) {
   if (!root_layer_.get())
     return;
 
+  gfx::Vector2d root_scroll_delta;
   Layer* root_scroll_layer = FindFirstScrollableLayer(root_layer_.get());
-  gfx::Vector2d root_scroll_delta;
 
   for (size_t i = 0; i < info.scrolls.size(); ++i) {
     Layer* layer =
         LayerTreeHostCommon::FindLayerInSubtree(root_layer_.get(),
                                                 info.scrolls[i].layer_id);
     if (!layer)
       continue;
     if (layer == root_scroll_layer) {
       root_scroll_delta += info.scrolls[i].scroll_delta;
     } else {
       layer->SetScrollOffsetFromImplSide(layer->scroll_offset() +
                                          info.scrolls[i].scroll_delta);
     }
   }
-  if (!root_scroll_delta.IsZero() || info.page_scale_delta != 1.f)
+
+  if (!root_scroll_delta.IsZero() || info.page_scale_delta != 1.f) {
+    // SetScrollOffsetFromImplSide above could have destroyed the tree,
+    // so re-get this layer before doing anything to it.
+    root_scroll_layer = FindFirstScrollableLayer(root_layer_.get());
+
+    // Preemptively apply the scroll offset here before sending it to
+    // the client.  If the client comes back and sets it to the same value,
+    // then the layer can early out without needing a full commit.
+    if (root_scroll_layer) {
+      root_scroll_layer->SetScrollOffsetFromImplSide(
+          root_scroll_layer->scroll_offset() + root_scroll_delta);
+    }
     client_->ApplyScrollAndScale(root_scroll_delta, info.page_scale_delta);
+  }
 }
 
 void LayerTreeHost::StartRateLimiter(WebKit::WebGraphicsContext3D* context3d) {
   if (animating_)
     return;
 
   DCHECK(context3d);
   RateLimiterMap::iterator it = rate_limiters_.find(context3d);
   if (it != rate_limiters_.end()) {
     it->second->Start();
@@ skipping 83 matching lines @@
     bool start_ready_animations = true;
     (*iter).second->UpdateState(start_ready_animations, NULL);
   }
 }
 
 skia::RefPtr<SkPicture> LayerTreeHost::CapturePicture() {
   return proxy_->CapturePicture();
 }
 
 }  // namespace cc