Add an Observer interface to UsbService for device add/remove.

device/usb/usb_service.h

 // Copyright 2014 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 DEVICE_USB_USB_SERVICE_H_
 #define DEVICE_USB_USB_SERVICE_H_
 
 #include <vector>
 
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/observer_list.h"
 #include "base/threading/non_thread_safe.h"
 
 namespace base {
 class SingleThreadTaskRunner;
 }
 
 namespace device {
 
 class UsbDevice;
 
 // The USB service handles creating and managing an event handler thread that is
 // used to manage and dispatch USB events. It is also responsible for device
 // discovery on the system, which allows it to re-use device handles to prevent
 // competition for the same USB device.
 class UsbService : public base::NonThreadSafe {
  public:
+  class Observer {
+   public:
+    virtual void OnDeviceAdded(scoped_refptr<UsbDevice> device);
+    virtual void OnDeviceRemoved(scoped_refptr<UsbDevice> device);
+  };
+
   // Must be called on a thread with a MessageLoopForIO (for example
   // BrowserThread::FILE). The UI task runner reference is used to talk to the
   // PermissionBrokerClient on ChromeOS (UI thread). Returns NULL when
   // initialization fails.
   static UsbService* GetInstance(
       scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner);
 
   static void SetInstanceForTest(UsbService* instance);
 
   virtual scoped_refptr<UsbDevice> GetDeviceById(uint32 unique_id) = 0;
 
   // Get all of the devices attached to the system, inserting them into
   // |devices|. Clears |devices| before use. The result will be sorted by id
   // in increasing order. Must be called on FILE thread.
   virtual void GetDevices(std::vector<scoped_refptr<UsbDevice> >* devices) = 0;
 
+  void AddObserver(Observer* observer);
+  void RemoveObserver(Observer* observer);
+
  protected:
   friend struct base::DefaultDeleter<UsbService>;
-  UsbService() {}
-  virtual ~UsbService() {}
+
+  UsbService();
+  virtual ~UsbService();
+
+  void NotifyDeviceAdded(scoped_refptr<UsbDevice> device);
+  void NotifyDeviceRemoved(scoped_refptr<UsbDevice> device);
+
+  ObserverList<Observer, true> observer_list_;
+
   DISALLOW_COPY_AND_ASSIGN(UsbService);
 };
 
 }  // namespace device
 
 #endif  // DEVICE_USB_USB_SERVICE_H_