Set keys to traverse accessibility tree. Using focus ring to highlight selected node.

chrome/browser/chromeos/accessibility/switch_access_event_handler.h

+// Copyright 2017 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 CHROME_BROWSER_CHROMEOS_ACCESSIBILITY_SWITCH_ACCESS_EVENT_HANDLER_H_
+#define CHROME_BROWSER_CHROMEOS_ACCESSIBILITY_SWITCH_ACCESS_EVENT_HANDLER_H_
+
+#include "base/macros.h"
+#include "ui/events/event_handler.h"
+
+namespace chromeos {
+
+// Intercepts key events.

[dmazzoni, 2017/02/24 23:06:54]

Maybe expand on this a little bit?

When Switch Access is enabled, intercepts a few keys before
the rest of the OS has a chance to handle them, as a method
for controlling the entire OS with a small number of switches.

This class just intercepts the keys and forwards them to
the component extension.

[elichtenberg, 2017/02/28 00:28:42]

Done.

+class SwitchAccessEventHandler : public ui::EventHandler {
+ public:
+  SwitchAccessEventHandler();
+  ~SwitchAccessEventHandler() override;
+
+ private:
+  // EventHandler:
+  void OnKeyEvent(ui::KeyEvent* event) override;
+
+  void CancelEvent(ui::Event* event);
+  bool DispatchKeyEventToSwitchAccess(const ui::KeyEvent& event);
+
+  enum State {
+    // No key from 1 to 5 is pressed.
+    INACTIVE,
+
+    // The 1 key is down.
+    ONE_KEY_DOWN,
+
+    // The 2 key is down.
+    TWO_KEY_DOWN,
+
+    // The 3 key is down.
+    THREE_KEY_DOWN,
+
+    // The 4 key is down.
+    FOUR_KEY_DOWN,
+
+    // The 5 key is down.
+    FIVE_KEY_DOWN,
+  };
+
+  State state_ = INACTIVE;
+
+  DISALLOW_COPY_AND_ASSIGN(SwitchAccessEventHandler);
+};
+
+}  // namespace chromeos
+
+#endif  // CHROME_BROWSER_CHROMEOS_ACCESSIBILITY_SWITCH_ACCESS_EVENT_HANDLER_H_