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

device/usb/usb_service_impl.cc

Index: device/usb/usb_service_impl.cc
diff --git a/device/usb/usb_service_impl.cc b/device/usb/usb_service_impl.cc
index a23a4a1548b913efe763a5b140951f056b6d424c..66244d742e4c0479c5dcd10dca27f540fb47f79c 100644
--- a/device/usb/usb_service_impl.cc
+++ b/device/usb/usb_service_impl.cc
@@ -173,21 +173,22 @@ void UsbServiceImpl::RefreshDevices() {
   }
 
   // Find disconnected devices.
-  for (PlatformDeviceMap::iterator it = platform_devices_.begin();
-       it != platform_devices_.end();
-       ++it) {
-    if (!ContainsKey(connected_devices, it->second.get())) {
-      disconnected_devices.push_back(it->first);
-      devices_.erase(it->second->unique_id());
-      it->second->OnDisconnect();
+  for (const auto& map_entry : platform_devices_) {
+    PlatformUsbDevice platform_device = map_entry.first;
+    scoped_refptr<UsbDeviceImpl> device = map_entry.second;
+    if (!ContainsKey(connected_devices, device.get())) {
+      disconnected_devices.push_back(platform_device);
+      devices_.erase(device->unique_id());
+      NotifyDeviceRemoved(device);
+      device->OnDisconnect();

[asargent_no_longer_on_chrome, 2015/01/05 22:01:58]

I assume you've already thought about the ramifications of notifying observers
of the UsbDevice removal before calling OnDisconnect on the device itself. Would
there ever be a case where an implementor of the Observer interface might need
to know or care about this ordering? If so it might be worth a mention in a
comment above the Observer interface. 

[Reilly Grant (use Gerrit), 2015/01/05 22:10:34]

I'm actually about to remove the observer list from UsbDevice for other reasons
so I'll be removing this call. I'll update the patch with that change.

     }
   }
 
   // Remove disconnected devices from platform_devices_.
-  for (size_t i = 0; i < disconnected_devices.size(); ++i) {
+  for (const PlatformUsbDevice& platform_device : disconnected_devices) {
     // UsbDevice will be destroyed after this. The corresponding
     // PlatformUsbDevice will be unref'ed during this process.
-    platform_devices_.erase(disconnected_devices[i]);
+    platform_devices_.erase(platform_device);
   }
 
   libusb_free_device_list(platform_devices, true);
@@ -208,6 +209,7 @@ scoped_refptr<UsbDeviceImpl> UsbServiceImpl::AddDevice(
         descriptor.idProduct, unique_id));
     platform_devices_[platform_device] = new_device;
     devices_[unique_id] = new_device;
+    NotifyDeviceAdded(new_device);
     return new_device;
   } else {
     VLOG(1) << "Failed to get device descriptor: "
@@ -274,6 +276,7 @@ void UsbServiceImpl::OnDeviceRemoved(PlatformUsbDevice platform_device) {
     } else {
       NOTREACHED();
     }
+    NotifyDeviceRemoved(device);
     device->OnDisconnect();
     platform_devices_.erase(it);
   } else {
@@ -310,4 +313,26 @@ void UsbService::SetInstanceForTest(UsbService* instance) {
   g_usb_service_instance.Get().reset(instance);
 }
 
+UsbService::UsbService() {
+}
+
+UsbService::~UsbService() {

[asargent_no_longer_on_chrome, 2015/01/05 22:01:58]

Consider adding a DCHECK in the destructor here that observer_list_ is empty, to
catch cases where someone is accidentally holding on to a pointer to this object
for later calling RemoveObserver (eg via a ScopedObserver). 

[Reilly Grant (use Gerrit), 2015/01/05 22:10:34]

There's a template flag I can pass to ObserverList that adds this check.

+}
+
+void UsbService::AddObserver(Observer* observer) {
+  observer_list_.AddObserver(observer);
+}
+
+void UsbService::RemoveObserver(Observer* observer) {
+  observer_list_.RemoveObserver(observer);
+}
+
+void UsbService::NotifyDeviceAdded(scoped_refptr<UsbDevice> device) {
+  FOR_EACH_OBSERVER(Observer, observer_list_, OnDeviceAdded(device));
+}
+
+void UsbService::NotifyDeviceRemoved(scoped_refptr<UsbDevice> device) {
+  FOR_EACH_OBSERVER(Observer, observer_list_, OnDeviceRemoved(device));
+}
+
 }  // namespace device