Creates AXPosition to uniquely identify a position in the accessibility tree

ui/accessibility/ax_position.h

Index: ui/accessibility/ax_position.h
diff --git a/ui/accessibility/ax_position.h b/ui/accessibility/ax_position.h
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+// Copyright 2016 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_POSITION_H_
+#define UI_ACCESSIBILITY_AX_POSITION_H_
+
+#include <stdint.h>
+
+#include "ui/accessibility/ax_node.h"
+
+namespace ui {
+
+// A position in the |AXTree|.
+// It could either indicate a non-textual node in the accessibility tree, or a
+// text node and a character offset.
+//
+// This class is copiable.
+class AX_EXPORT AXPosition {
+ public:
+  static AXPosition CreateNullPosition();
+  static AXPosition CreateTreePosition(int tree_id, int32_t anchor_id, int child_index = 0);
+  static AXPosition CreateTextPosition(int tree_id, int32_t anchor_id, int text_offset = 0);
+
+  virtual ~AXPosition();
+
+  static AXPosition CommonAncestor(const AXPosition& first, const AXPosition& second);
+
+  int get_tree_id() const { return tree_id_; }
+  int32_t get_anchor_id() const { return anchor_id_; }
+  AXNode* GetAnchor() const;
+  int get_child_index() const { return child_index_; }
+  int get_text_offset() const { return text_offset_; }
+
+  bool IsNullPosition() const { return !GetAnchor(); }
+  bool IsTreePosition() const { return GetAnchor() && type_ == AXPositionType::TreePosition; }
+  bool IsTextPosition() const { return GetAnchor() && type_ == AXPositionType::TextPosition; }
+
+  bool AtStartOfAnchor() const;
+  bool AtEndOfAnchor() const;
+
+  bool operator<(const AXPosition& position) const;
+  bool operator<=(const AXPosition& position) const;
+  bool operator>(const AXPosition& position) const;
+  bool operator>=(const AXPosition& position) const;
+
+  AXPosition GetPositionAtStartOfAnchor() const;
+  AXPosition GetPositionAtEndOfAnchor() const;
+
+  // The following methods work across anchors.
+
+  AXPosition GetNextCharacterPosition() const;
+  AXPosition GetPreviousCharacterPosition() const;
+  // TODO(nektar): Add word, line and paragraph navigation methods.
+
+ protected:
+  AXPosition(int tree_id, int32_t anchor_id, int offset, AXPositionType type);
+
+  virtual int AnchorChildCount() const { return GetAnchor() ? GetAnchor()->child_count() : -1; }
+  virtual AXPosition GetChildPositionAt(int child_index) const;
+  virtual AXPosition GetParentPosition() const;
+  virtual int AnchorIndexInParent() const { return GetAnchor() ? GetAnchor()->index_in_parent() : -1; }
+  // Uses depth-first pre-order traversal.
+  virtual AXPosition GetNextAnchorPosition() const;
+  // Uses depth-first pre-order traversal.
+  virtual AXPosition GetPreviousAnchorPosition() const;
+
+  virtual AXNode* GetNodeInTree(int tree_id, int32-t node_id) const = 0;
+  virtual int MaxTextOffset() const = 0;
+
+  // Defines the type of position in the accessibility tree.
+  enum class AXPositionType {
+    TreePosition,
+    TextPosition
+  };
+
+ private:
+  int tree_id_;
+  int32_t anchor_id_;
+
+  // For text positions, |child_index_| is initially set to |-1| and only computed on demand. The same with tree positions and |text_offset_|.
+  int child_index_;
+  int text_offset_;
+
+  AXPositionType type_;
+};
+
+bool operator==(const AXPosition& first, const AXPosition& second);
+bool operator!=(const AXPosition& first, const AXPosition& second);
+
+}  // namespace ui
+
+#endif  // UI_ACCESSIBILITY_AX_POSITION_H_