Add basic observer support to devices::HidService.

Add an observer list to HidService that is notified when a device is
added or removed from the enumeration. This currently works as expected
on Linux and OS X while on Windows devices are only enumerated on request
and so these are not as useful.

BUG=376719

Review URL: https://codereview.chromium.org/779353003

Cr-Commit-Position: refs/heads/master@{#307105}

device/hid/hid_service.cc

@@ -81,6 +81,14 @@ void HidService::GetDevices(std::vector<HidDeviceInfo>* devices) {
   }
 }
 
+void HidService::AddObserver(HidService::Observer* observer) {
+  observer_list_.AddObserver(observer);
+}
+
+void HidService::RemoveObserver(HidService::Observer* observer) {
+  observer_list_.RemoveObserver(observer);
+}
+
 // Fills in the device info struct of the given device_id.
 bool HidService::GetDeviceInfo(const HidDeviceId& device_id,
                                HidDeviceInfo* info) const {
@@ -99,14 +107,17 @@ void HidService::AddDevice(const HidDeviceInfo& info) {
   DCHECK(thread_checker_.CalledOnValidThread());
   if (!ContainsKey(devices_, info.device_id)) {
     devices_[info.device_id] = info;
+    FOR_EACH_OBSERVER(Observer, observer_list_, OnDeviceAdded(info));
   }
 }
 
 void HidService::RemoveDevice(const HidDeviceId& device_id) {
   DCHECK(thread_checker_.CalledOnValidThread());
   DeviceMap::iterator it = devices_.find(device_id);
-  if (it != devices_.end())
+  if (it != devices_.end()) {
     devices_.erase(it);
+    FOR_EACH_OBSERVER(Observer, observer_list_, OnDeviceRemoved(device_id));
+  }
 }
 
 }  // namespace device

device/hid/hid_service.h

@@ -10,6 +10,7 @@
 #include <vector>
 
 #include "base/memory/ref_counted.h"
+#include "base/observer_list.h"
 #include "base/single_thread_task_runner.h"
 #include "base/threading/thread_checker.h"
 #include "device/hid/hid_device_info.h"
@@ -20,6 +21,12 @@ class HidConnection;
 
 class HidService {
  public:
+  class Observer {
+   public:
+    virtual void OnDeviceAdded(const HidDeviceInfo& info) {}
+    virtual void OnDeviceRemoved(const HidDeviceId& device_id) {}
+  };
+
   typedef base::Callback<void(scoped_refptr<HidConnection> connection)>
       ConnectCallback;
 
@@ -31,6 +38,9 @@ class HidService {
   // Enumerates and returns a list of device identifiers.
   virtual void GetDevices(std::vector<HidDeviceInfo>* devices);
 
+  void AddObserver(Observer* observer);
+  void RemoveObserver(Observer* observer);
+
   // Fills in a DeviceInfo struct with info for the given device_id.
   // Returns |true| if successful or |false| if |device_id| is invalid.
   bool GetDeviceInfo(const HidDeviceId& device_id, HidDeviceInfo* info) const;
@@ -59,6 +69,7 @@ class HidService {
   class Destroyer;
 
   DeviceMap devices_;
+  ObserverList<Observer> observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(HidService);
 };