First step to move common accessibility code out of content.

content/public/common/ax_tree_update.h

Index: content/public/common/ax_tree_update.h
diff --git a/content/public/common/ax_tree_update.h b/content/public/common/ax_tree_update.h
new file mode 100644
index 0000000000000000000000000000000000000000..57beeeec5e19ccf9fd143098f749d9cdaa8da023
--- /dev/null
+++ b/content/public/common/ax_tree_update.h
@@ -0,0 +1,45 @@
+// 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 CONTENT_PUBLIC_COMMON_AX_TREE_UPDATE_H_
+#define CONTENT_PUBLIC_COMMON_AX_TREE_UPDATE_H_
+
+#include <vector>
+
+#include "content/public/common/ax_node_data.h"
+
+namespace content {
+
+// An AXTreeUpdate is a serialized representation of an atomic change
+// to an AXTree. The sender and receiver must be in sync; the update
+// is only meant to bring the tree from a specific previous state into
+// its next state. Trying to apply it to the wrong tree should immediately
+// die with a fatal assertion. An AXTreeUpdate is just an ordered vector
+// of AXNodeData structures to be applied to the tree in order.
+//
+// Suppose that the next AXNodeData to be applied is |node|. The following
+// invariants must hold:
+// 1. Either |node.id| is already in the tree, or else |node| is the new
+//        root of the tree and |node.role| == WebAXRoleRootWebArea.
+// 2. Every child id in |node.child_ids| must either be already a child
+//        of this node, or a new id not previously in the tree. It is not
+//        allowed to "reparent" a child to this node without first removing
+//        that child from its previous parent.
+// 3. When a new id appears in |node.child_ids|, the tree should create a
+//        new uninitialized placeholder node for it immediately. That
+//        placeholder must be updated within the same AXTreeUpdate, otherwise
+//        it's a fatal error. This guarantees the tree is always complete
+//        before or after an AXTreeUpdate.
+struct CONTENT_EXPORT AXTreeUpdate {

[David Tseng, 2013/11/13 18:00:07]

Btw, what do you see the purpose of this class considering that we will need
another structure to be defined as an IPC message?

[dmazzoni, 2013/11/13 19:35:59]

I expect this to be contained in the IPC message. It allows
testing serialization and unserialization of an
accessibility tree without any IPC infrastructure. It also
allows us to send updates in more than one IPC message -
for example I expect the new extension code will use a
different message.

+  AXTreeUpdate();
+  ~AXTreeUpdate();
+
+  std::vector<AXNodeData> nodes;
+
+  // TODO(dmazzoni): location changes
+};
+
+}  // namespace content
+
+#endif  // CONTENT_PUBLIC_COMMON_AX_TREE_UPDATE_H_