iOS Private APIs(2): Observing Volume Keys' Press Events


Observing Volume Keys' Press Events in iOS

If you've used iOS devices, you might have observed that certain apps enable you to capture photos by pressing the volume buttons. This functionality mimics the experience of using a DSLR camera and improves the overall photography experience. But how is this feature actually implemented? One popular method involves monitoring changes in the audio session using AVAudioSession to detect volume button presses. However, it's worth noting that this approach is not completely reliable and necessitates handling specific edge cases. Interestingly, iOS has had private NSNotifications available since early versions for tracking volume button presses.

Below are four events that can be utilized by NSNotificationCenter to monitor volume key press events on the iPhone. These events have been extracted from iOS header dump files:

  • _UIApplicationVolumeDownButtonDownNotification
  • _UIApplicationVolumeDownButtonUpNotification
  • _UIApplicationVolumeUpButtonDownNotification
  • _UIApplicationVolumeUpButtonUpNotification

Additionally, it is necessary to invoke the private method [UIApplication setWantsVolumeButtonEvents:YES] in order to enable the aforementioned notifications before observing them.

Here's an example implementation:

- (void)viewDidLoad {
    [super viewDidLoad];

    [[UIApplication sharedApplication] performSelector:@selector(setWantsVolumeButtonEvents:) withObject:(@YES)];

    // Subscribe to the volumedown button press notification
    [[NSNotificationCenter defaultCenter] addObserver:self 

// Method that gets called when the volume down button is pressed
- (void)volumeDownButtonPressed:(NSNotification *)notification {
    NSLog(@"Volume Down Button was pressed.");

Make sure to call [UIApplication setWantsVolumeButtonEvents:YES] to enable the notifications before observing them.

Please keep in mind that using private APIs can lead to your app being rejected from the App Store. Hence, it's advised to always use public APIs that Apple has documented for developers.