void StartProgressTimer(TimeSpan duration)
{
if (_progressTimer == null) {
#if __UNIFIED__
_progressTimer = NSTimer.CreateRepeatingTimer (duration, timer => UpdateProgress ());
#else
_progressTimer = NSTimer.CreateRepeatingTimer(duration, UpdateProgress);
#endif
NSRunLoop.Current.AddTimer (_progressTimer, NSRunLoopMode.Common);
}
}