cc: Allow the main thread to cancel commits

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index ff4d0079fe3983e8096afe381a0cafaa7035ad7f..68fdf792f45b566764c7d804cf48b1de46b24b84 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -465,6 +465,10 @@ void LayerTreeHost::SetNeedsAnimate() {
   proxy_->SetNeedsAnimate();
 }
 
+void LayerTreeHost::SetNeedsUpdateLayers() {
+  proxy_->SetNeedsUpdateLayers();
+}
+
 void LayerTreeHost::SetNeedsCommit() {
   if (!prepaint_callback_.IsCancelled()) {
     TRACE_EVENT_INSTANT0("cc",
@@ -580,15 +584,21 @@ void LayerTreeHost::SetOverdrawBottomHeight(float overdraw_bottom_height) {
 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)

[danakj, 2013/07/17 20:53:24]

So I guess the main thread is not able to set the PSF any more?

Maybe we should split this function up and name things more clearly to express
that? Like SetPageScaleFactorFromImplThread() similar to the scroll offset
stuff?

[enne (OOO), 2013/07/18 17:36:37]

Done.

+    SetNeedsUpdateLayers();
+  else
+    SetNeedsCommit();
 }
 
 void LayerTreeHost::SetVisible(bool visible) {
@@ -948,8 +958,8 @@ void LayerTreeHost::ApplyScrollAndScale(const ScrollAndScaleSet& info) {
   if (!root_layer_.get())
     return;
 
-  Layer* root_scroll_layer = FindFirstScrollableLayer(root_layer_.get());
   gfx::Vector2d root_scroll_delta;
+  Layer* root_scroll_layer = FindFirstScrollableLayer(root_layer_.get());
 
   for (size_t i = 0; i < info.scrolls.size(); ++i) {
     Layer* layer =
@@ -964,8 +974,21 @@ void LayerTreeHost::ApplyScrollAndScale(const ScrollAndScaleSet& info) {
                                          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) {