public void TearDown()
 {
     _sink = null;
     _source = null;
 }
 public DigitalClock(Clock clock)
 {
     clock.RegisterObserver(this);
     _clock = clock;
 }
 public void SetUp()
 {
     _source = new Clock();
     _sink = new DigitalClock(_source);
     _source.RegisterObserver(_sink);
 }