[System-Keyboard-Lock] Add KeyboardLockHost and KeyEventInterceptor

components/keyboard_lock/key_event_interceptor.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 COMPONENTS_KEYBOARD_LOCK_KEY_EVENT_INTERCEPTOR_H_
+#define COMPONENTS_KEYBOARD_LOCK_KEY_EVENT_INTERCEPTOR_H_
+
+#include "ui/events/keycodes/keyboard_codes.h"
+
+namespace keyboard_lock {
+
+// An interface to receive and process a key event.
+class KeyEventInterceptor {
+ public:
+  KeyEventInterceptor() = default;
+  virtual ~KeyEventInterceptor() = default;
+
+  virtual bool OnKeyDown(ui::KeyboardCode code) = 0;
+  virtual bool OnKeyUp(ui::KeyboardCode code) = 0;
+
+  // An interface to retrieve a KeyEventInterceptor instance. This class is used
+  // to avoid the dependency of the singleton object of KeyboardLockHost, and to
+  // ensure late installed KeyEventInterceptor can be retrieved.
+  class Retriever {
+   public:
+    Retriever() = default;
+    virtual ~Retriever() = default;
+
+    // May return nullptr.
+    virtual KeyEventInterceptor* Get() = 0;
+  };
+};
+
+}  // namespace keyboard_lock
+
+#endif  // COMPONENTS_KEYBOARD_LOCK_KEY_EVENT_INTERCEPTOR_H_