[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 {

[Wez, 2017/04/28 00:22:44]

So this class installs itself as the current KeyEventInterceptor, and then
removes itself in its destructor?

[Hzj_jie, 2017/04/28 20:02:03]

Yes, but technically speaking, it won't be destructed during the lifetime of the
process. The destructor works only in test cases.

+ public:
+  ~KeyEventInterceptorInstaller() override;
+
+ protected:
+  // |host| should outlive this instance. The ownership of an instance of
+  // KeyEventInterceptorInstaller should be taken by KeyboardLockHost but not
+  // consumers. A typical derived class should provide a Create() function,
+  // calls Installer() function and returns a raw pointer from it.

[Wez, 2017/04/28 00:22:44]

Why do we need this complexity?  Couldn't the implementor of the
KeyEventInterceptor just call to the Host directly to set/clear the current
interceptor instance?

[Hzj_jie, 2017/04/28 20:02:03]

That's too dangerous, the setter is not accessible out of
KeyEventInterceptorInstaller because we do not like to return a deleting object
from KeyboardLockHost. KeyEventInterceptor works cross threads, and the
consumers have only a weak reference of it from KeyboardLockHost.

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