First step to move common accessibility code out of content.

ui/accessibility/ax_tree_source.h

Index: ui/accessibility/ax_tree_source.h
diff --git a/ui/accessibility/ax_tree_source.h b/ui/accessibility/ax_tree_source.h
new file mode 100644
index 0000000000000000000000000000000000000000..55a31102b2b1f574e57d0223a7ab62c77d81447e
--- /dev/null
+++ b/ui/accessibility/ax_tree_source.h
@@ -0,0 +1,43 @@
+// Copyright (c) 2013 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.
+
+#ifndef UI_ACCESSIBILITY_AX_TREE_SOURCE_H_
+#define UI_ACCESSIBILITY_AX_TREE_SOURCE_H_
+
+#include "ui/accessibility/ax_node_data.h"
+
+namespace ui {
+
+// An AXTreeSource is an abstract interface for a serializable
+// accessibility tree. The tree may be in some other format or
+// may be computed dynamically, but maintains the properties that
+// it's a strict tree, it has a unique id for each node, and all
+// of the accessibility information about a node can be serialized
+// as an AXNodeData. This is the primary interface to use when
+// an accessibility tree will be sent over an IPC before being
+// consumed.
+template<class AXNodeSource>
+class UI_EXPORT AXTreeSource {
+ public:
+  virtual AXNodeSource* GetFromId(int32 id) const = 0;
+  virtual int32 GetId(const AXNodeSource* node) const = 0;
+  virtual int GetChildCount(const AXNodeSource* node) const = 0;
+  virtual AXNodeSource* GetChildAtIndex(const AXNodeSource* node, int index)
+      const = 0;
+
+  // Returns the id of this node's parent, or 0 if it doesn't have a parent.
+  virtual int32 GetParentId(const AXNodeSource* node) const = 0;
+
+  // Serialize one node in the tree.
+  virtual void SerializeNode(

[David Tseng, 2013/11/15 18:05:31]

Why not just serialize the entire subtree here and eliminate the need for a
serializer class?

[dmazzoni, 2013/11/18 08:09:25]

Because the serializer needs to maintain state and stay in sync with its
destination.
It makes it possible for there to be multiple destinations all listening to the
same
source tree that aren't all in sync with each other. Or, it allows you to
temporarily get a
snapshot of the whole tree without disturbing an existing serializer.

+      const AXNodeSource* node, AXNodeData* out_data) const = 0;
+
+ protected:
+  AXTreeSource() {}
+  virtual ~AXTreeSource() {}
+};
+
+}  // namespace ui
+
+#endif  // UI_ACCESSIBILITY_AX_TREE_SOURCE_H_