Observe AirPlay button's alpha

AirPlayButtonAlpha.mm

//setting the observer
-(void)observeAirPlayFromView:(MPVolumeView *)voluemView {
  for (UIButton *button in volumeView.subviews) {
      if([button isKindOfClass:[UIButton class]]) {
          [button addObserver:self 
                   forKeyPath:@"alpha" 
                      options:NSKeyValueObservingOptionNew 
                      context:nil];
      }
  }
}

//observing
- (void)observeValueForKeyPath:(NSString *)keyPath 
                      ofObject:(id)object 
                        change:(NSDictionary *)change 
                       context:(void *)context {
    if ([object isKindOfClass:[UIButton class]] && 
        [[change valueForKey:NSKeyValueChangeNewKey] intValue] == 1) {
        //do something
    }
}

If your read this at now, this solution is not relevant. Read documentation link below.
https://developer.apple.com/documentation/foundation/nsnotification/name/1620065-mpvolumeviewwirelessroutesavaila