settings: Make DeviceOAuth2TokenServiceFactory::Get() async

chrome/browser/chromeos/settings/device_oauth2_token_service_factory.h

 // Copyright 2013 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_SETTINGS_DEVICE_OAUTH2_TOKEN_SERVICE_FACTORY_H_
 #define CHROME_BROWSER_CHROMEOS_SETTINGS_DEVICE_OAUTH2_TOKEN_SERVICE_FACTORY_H_
 
 #include "base/basictypes.h"
+#include "base/callback_forward.h"
 
 namespace chromeos {
 
 class DeviceOAuth2TokenService;
 
 class DeviceOAuth2TokenServiceFactory {
  public:
-  // Returns the instance of the DeviceOAuth2TokenService singleton.
-  // May return null during browser startup and shutdown.  Do not hold
-  // the pointer returned by this method; call this method every time
-  // and check for null to handle the case where this instance is destroyed
-  // during shutdown.
-  static DeviceOAuth2TokenService* Get();
+  // Callback type used for Get() function.
+  typedef base::Callback<void(DeviceOAuth2TokenService*)> GetCallback;
+
+  // Returns the instance of the DeviceOAuth2TokenService singleton via the
+  // given callback. This function is asynchronous as initializing
+  // DeviceOAuth2TokenService involves asynchronous D-Bus method calls.
+  //
+  // May return NULL during browser startup and shutdown. May also return
+  // NULL if Initialize() is not called beforehand, which can happen in unit
+  // tests.
+  //
+  // Do not hold the pointer returned by this method; call this method every
+  // time and check for NULL to handle the case where this instance is
+  // destroyed during shutdown.
+  static void Get(const GetCallback& callback);
 
   // Called by ChromeBrowserMainPartsChromeOS in order to bootstrap the
   // DeviceOAuth2TokenService instance after the required global data is
   // available (local state and request context getter).
   static void Initialize();
 
   // Called by ChromeBrowserMainPartsChromeOS in order to shutdown the
   // DeviceOAuth2TokenService instance and cancel all in-flight requests
   // before the required global data is destroyed (local state and request
   // context getter).
   static void Shutdown();
 
  private:
   DeviceOAuth2TokenServiceFactory();
+  ~DeviceOAuth2TokenServiceFactory();
+
+  DeviceOAuth2TokenService* token_service_;
 
   DISALLOW_COPY_AND_ASSIGN(DeviceOAuth2TokenServiceFactory);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_SETTINGS_DEVICE_OAUTH2_TOKEN_SERVICE_FACTORY_H_