Optimize BlinkAXTreeSource by adding freeze/thaw for things like root, focus.

content/renderer/accessibility/blink_ax_tree_source.h

Index: content/renderer/accessibility/blink_ax_tree_source.h
diff --git a/content/renderer/accessibility/blink_ax_tree_source.h b/content/renderer/accessibility/blink_ax_tree_source.h
index 90f7f152224750dd9b2545a09f512e6dc5880d9f..2f4001c558015deef532eb487f86d43037451c0c 100644
--- a/content/renderer/accessibility/blink_ax_tree_source.h
+++ b/content/renderer/accessibility/blink_ax_tree_source.h
@@ -9,13 +9,28 @@
 
 #include "content/common/ax_content_node_data.h"
 #include "third_party/WebKit/public/web/WebAXObject.h"
+#include "third_party/WebKit/public/web/WebDocument.h"
 #include "ui/accessibility/ax_node_data.h"
 #include "ui/accessibility/ax_tree_source.h"
 
 namespace content {
 
+class BlinkAXTreeSource;
 class RenderFrameImpl;
 
+// Create this on the stack to freeze BlinkAXTreeSource and automatically
+// un-freeze it when it goes out of scope.
+class ScopedFreezeBlinkAXTreeSource {
+ public:
+  explicit ScopedFreezeBlinkAXTreeSource(BlinkAXTreeSource* tree_source);
+  ~ScopedFreezeBlinkAXTreeSource();
+
+ private:
+  BlinkAXTreeSource* tree_source_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedFreezeBlinkAXTreeSource);
+};
+
 class BlinkAXTreeSource
     : public ui::AXTreeSource<blink::WebAXObject,
                               AXContentNodeData,
@@ -24,6 +39,13 @@ class BlinkAXTreeSource
   BlinkAXTreeSource(RenderFrameImpl* render_frame);
   ~BlinkAXTreeSource() override;
 
+  // Freeze caches the document, accessibility root, and current focused
+  // object for fast retrieval during a batch of operations. Use
+  // ScopedFreezeBlinkAXTreeSource on the stack rather than calling
+  // these directly.
+  void Freeze();
+  void Thaw();
+
   // It may be necessary to call SetRoot if you're using a WebScopedAXContext,
   // because BlinkAXTreeSource can't get the root of the tree from the
   // WebDocument if accessibility isn't enabled globally.
@@ -57,10 +79,34 @@ class BlinkAXTreeSource
   blink::WebDocument GetMainDocument() const;
 
  private:
+  const blink::WebDocument& document() const {
+    DCHECK(frozen_);
+    return document_;
+  }
+  const blink::WebAXObject& root() const {
+    DCHECK(frozen_);
+    return root_;
+  }
+  const blink::WebAXObject& focus() const {
+    DCHECK(frozen_);
+    return focus_;
+  }
+
+  blink::WebAXObject ComputeRoot() const;
+
   RenderFrameImpl* render_frame_;
-  blink::WebAXObject root_;
 
+  // An explicit root to use, otherwise it's taken from the WebDocument.
+  blink::WebAXObject explicit_root_;
+
+  // The id of the object with accessibility focus.
   int accessibility_focus_id_;
+
+  // These are updated when calling |Freeze|.
+  bool frozen_;
+  blink::WebDocument document_;
+  blink::WebAXObject root_;
+  blink::WebAXObject focus_;
 };
 
 }  // namespace content