ui/accessibility/ax_tree.h

// Copyright 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_H_
#define UI_ACCESSIBILITY_AX_TREE_H_

#include <set>

#include "base/containers/hash_tables.h"
#include "ui/accessibility/ax_export.h"
#include "ui/accessibility/ax_tree_update.h"

namespace ui {

class AXNode;
struct AXTreeUpdateState;

// Used when you want to be notified when changes happen to the tree.
//
// Some of the notifications are called in the middle of an update operation.
// Be careful, as the tree may be in an inconsistent state at this time;
// don't walk the parents and children at this time:
//   OnNodeWillBeDeleted
//   OnNodeCreated
//   OnNodeChanged
//
// Other notifications are called at the end of an atomic update operation.
// From these, it's safe to walk the tree and do any initialization that
// assumes the tree is in a consistent state.
//   OnNodeCreationFinished
//   OnNodeChangeFinished
//   OnRootChanged
class AX_EXPORT AXTreeDelegate {
 public:
  AXTreeDelegate();
  virtual ~AXTreeDelegate();

  // Called just before a node is deleted. Its id and data will be valid,
  // but its links to parents and children are invalid. This is called
  // in the middle of an update, the tree may be in an invalid state!
  virtual void OnNodeWillBeDeleted(AXNode* node) = 0;

  // Called immediately after a new node is created. The tree may be in
  // the middle of an update, don't walk the parents and children now.
  virtual void OnNodeCreated(AXNode* node) = 0;

  // Called when a node changes its data or children. The tree may be in
  // the middle of an update, don't walk the parents and children now.
  virtual void OnNodeChanged(AXNode* node) = 0;

  // Called for each new node at the end of an update operation,
  // when the tree is in a consistent state.
  virtual void OnNodeCreationFinished(AXNode* node) = 0;

  // Called for each existing node that changed at the end of an update
  // operation, when the tree is in a consistent state.
  virtual void OnNodeChangeFinished(AXNode* node) = 0;

  // Called at the end of an update operation when the root node changes.
  virtual void OnRootChanged(AXNode* new_root) = 0;
};

// AXTree is a live, managed tree of AXNode objects that can receive
// updates from another AXTreeSource via AXTreeUpdates, and it can be
// used as a source for sending updates to another client tree.
// It's designed to be subclassed to implement support for native
// accessibility APIs on a specific platform.
class AX_EXPORT AXTree {
 public:
  AXTree();
  explicit AXTree(const AXTreeUpdate& initial_state);
  virtual ~AXTree();

  virtual void SetDelegate(AXTreeDelegate* delegate);

  virtual AXNode* GetRoot() const;
  virtual AXNode* GetFromId(int32 id) const;

  // Returns true on success. If it returns false, it's a fatal error
  // and this tree should be destroyed, and the source of the tree update
  // should not be trusted any longer.
  virtual bool Unserialize(const AXTreeUpdate& update);

  // Return a multi-line indented string representation, for logging.
  std::string ToString() const;

  // A string describing the error from an unsuccessful Unserialize,
  // for testing and debugging.
  const std::string& error() { return error_; }

 private:
  AXNode* CreateNode(AXNode* parent, int32 id, int32 index_in_parent);

  // This is called from within Unserialize(), it returns true on success.
  bool UpdateNode(const AXNodeData& src, AXTreeUpdateState* update_state);

  void OnRootChanged();

  // Call Destroy() on |node|, and delete it from the id map, and then
  // call recursively on all nodes in its subtree.
  void DestroyNodeAndSubtree(AXNode* node);

  // Iterate over the children of |node| and for each child, destroy the
  // child and its subtree if its id is not in |new_child_ids|. Returns
  // true on success, false on fatal error.
  bool DeleteOldChildren(AXNode* node,
                         const std::vector<int32> new_child_ids);

  // Iterate over |new_child_ids| and populate |new_children| with
  // pointers to child nodes, reusing existing nodes already in the tree
  // if they exist, and creating otherwise. Reparenting is disallowed, so
  // if the id already exists as the child of another node, that's an
  // error. Returns true on success, false on fatal error.
  bool CreateNewChildVector(AXNode* node,
                            const std::vector<int32> new_child_ids,
                            std::vector<AXNode*>* new_children,
                            AXTreeUpdateState* update_state);

  AXTreeDelegate* delegate_;
  AXNode* root_;
  base::hash_map<int32, AXNode*> id_map_;
  std::string error_;
};

}  // namespace ui

#endif  // UI_ACCESSIBILITY_AX_TREE_H_