[System-Keyboard-Lock] Add KeyboardLockHost and KeyEventInterceptor

components/keyboard_lock/key_event_interceptor_installer.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_INSTALLER_H_
+#define COMPONENTS_KEYBOARD_LOCK_KEY_EVENT_INTERCEPTOR_INSTALLER_H_
+
+#include "base/macros.h"
+#include "components/keyboard_lock/key_event_interceptor.h"
+
+namespace keyboard_lock {
+
+class KeyboardLockHost;
+
+// Installs itself as the default implementation of KeyEventInterceptor in
+// KeyboardLockHost.
+class KeyEventInterceptorInstaller : public KeyEventInterceptor {

[dtapuska, 2017/04/21 14:48:33]

Can you detail how the implementation of this class is going to be thread safe?

[Hzj_jie, 2017/04/21 21:13:09]

A typical implementation does not need to take care of the multithreading, it
would be guarded by a KeyEventInterceptorThreadWrapper. i.e. The OnKeyDown() and
OnKeyUp() function calls will always ensure to be multithreading-protected.

I will update the comments of KeyEventInterceptor.

+ public:
+  ~KeyEventInterceptorInstaller() override;
+
+ protected:
+  // |host| should outlive this instance.
+  explicit KeyEventInterceptorInstaller(KeyboardLockHost* host);
+  void Install();

[dtapuska, 2017/04/21 14:48:33]

The lifecyle of the install is weird. In that you take a ptr and move yourself
as a unique_ptr.

[Hzj_jie, 2017/04/21 21:13:09]

The KeyLockHost takes owner-ship of the KeyEventInterceptorInstaller, so user
cannot create an instance of KeyEventInterceptorInstaller(), but only use
Create() function to create one and return a weak reference. But I do agree this
looks a little bit weird, do you happen to have a better solution?

+
+ private:
+  KeyboardLockHost* const host_;
+
+  DISALLOW_COPY_AND_ASSIGN(KeyEventInterceptorInstaller);
+};
+
+}  // namespace keyboard_lock
+
+#endif  // COMPONENTS_KEYBOARD_LOCK_KEY_EVENT_INTERCEPTOR_INSTALLER_H_