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
new file mode 100644
index 0000000000000000000000000000000000000000..12ea140c153b38f2f02c7f73ce6e6cf5b7c3c7f6
--- /dev/null
+++ b/ui/accessibility/ax_position.h
@@ -0,0 +1,120 @@
+// 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_enums.h"
+
+namespace ui {
+
+// Defines the type of position in the accessibility tree.
+enum class AXPositionType { NullPosition, TreePosition, TextPosition };
+
+// 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.
+template <class AXNodeType>
+class AXPosition {
+ public:
+  virtual ~AXPosition();
+
+  static typename AXNodeType::AXPosition CreateNullPosition();
+  static typename AXNodeType::AXPosition CreateTreePosition(int tree_id,

[dmazzoni, 2016/10/05 18:15:30]

Why is it returning a typename AXNodeType::AXPosition?
That seems confusing to me. I think it should return a
AXPosition<AXNodeType>

+                                                            int32_t anchor_id,
+                                                            int child_index);
+  static typename AXNodeType::AXPosition CreateTextPosition(int tree_id,
+                                                            int32_t anchor_id,
+                                                            int text_offset);
+
+  int get_tree_id() const { return tree_id_; }
+  int32_t get_anchor_id() const { return anchor_id_; }
+  AXNodeType* GetAnchor() const;
+  int get_child_index() const { return child_index_; }
+  int get_text_offset() const { return text_offset_; }
+  ui::AXTextAffinity get_affinity() const { return affinity_; }
+  void set_affinity(ui::AXTextAffinity affinity) { affinity_ = affinity; }
+
+  bool IsNullPosition() const {
+    return type_ == AXPositionType::NullPosition || !GetAnchor();
+  }
+  bool IsTreePosition() const {
+    return GetAnchor() && type_ == AXPositionType::TreePosition;
+  }
+  bool IsTextPosition() const {
+    return GetAnchor() && type_ == AXPositionType::TextPosition;
+  }
+
+  bool AtStartOfAnchor() const;
+  bool AtEndOfAnchor() const;
+
+  AXPosition CommonAncestor(const AXPosition& second) 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 child_index,
+             int text_offset,
+             AXPositionType type);
+
+  virtual int AnchorChildCount() const {
+    return GetAnchor() ? GetAnchor()->child_count() : -1;

[dmazzoni, 2016/10/05 18:15:30]

BrowserAccessibility doesn't have a method called
child_count.

+  }
+  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 AXNodeType* GetNodeInTree(int tree_id, int32_t node_id) const = 0;
+  virtual int MaxTextOffset() const = 0;
+
+ 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_;
+
+  // TODO(nektar): Get rid of affinity and make Blink handle affinity
+  // internally since inline text objects don't span lines.
+  ui::AXTextAffinity affinity_;
+};
+
+template <class AXNodeType>
+bool operator==(const AXPosition<AXNodeType>& first,
+                const AXPosition<AXNodeType>& second);
+template <class AXNodeType>
+bool operator!=(const AXPosition<AXNodeType>& first,
+                const AXPosition<AXNodeType>& second);
+
+}  // namespace ui
+
+#endif  // UI_ACCESSIBILITY_AX_POSITION_H_