Adds metronome time sync dbus client

chromeos/dbus/metronome_client.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 CHROMEOS_DBUS_METRONOME_CLIENT_H_
+#define CHROMEOS_DBUS_METRONOME_CLIENT_H_
+
+#include "base/basictypes.h"
+#include "base/callback_forward.h"
+#include "chromeos/dbus/dbus_client.h"
+#include "chromeos/dbus/dbus_client_implementation_type.h"
+
+namespace chromeos {
+
+// A DBus client class for the org.chromium.Metronome service.
+class MetronomeClient : public DBusClient {
+ public:
+  // A callback to handle TimestampUpdated signal.
+  typedef base::Callback<void(uint64 beacon_timestamp, uint64 mac_timestamp)>
+      TimestampUpdatedHandler;
+
+  virtual ~MetronomeClient();
+
+  // Registers |timestamp_updated_handler| as a callback to be invoked when a
+  // TimestampUpdated signal is received.
+  virtual void SetTimestampUpdatedHandler(

[dtapuska, 2015/02/18 13:59:12]

I don't know if you want to use an Observer class instead of a Callback here
much like I used in the leadership and peerd..

I was asked to follow the Bluetooth client example; as you might want to become
aware that the metronome dbus service has crashed or become unavailable?

Ben might have an opinion.

[varkha, 2015/02/18 18:21:17]

Done (multiple observers part).

[Ben Chan, 2015/02/18 18:39:22]

It depends.  I generally use Delegate for situation where there should only be
one client observing the event, and use Observer for situation where there would
be multiple observers.  As we may do another DBus-to-mojo bridge, it may not
necessarily to have multiple observers here.

[varkha, 2015/02/18 18:42:24]

Yes, this is why I've added observers.

+      const TimestampUpdatedHandler& timestamp_updated_handler) = 0;
+
+  // Factory function, creates a new instance and returns ownership.
+  // For normal usage, access the singleton via DBusThreadManager::Get().
+  static MetronomeClient* Create(DBusClientImplementationType type);
+
+ protected:
+  // Create() should be used instead.
+  MetronomeClient();
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(MetronomeClient);
+};
+
+}  // namespace chromeos
+
+#endif  // CHROMEOS_DBUS_METRONOME_CLIENT_H_