iPhone - Capture device button push

Question

I know you cannot control device volume from within your application, but I would like the device volume to be able to affect the UIScrollBar I have in my application to control volume.

I know this is possible because the Last.fm application does it, I would like to implement this behaviour.

I can find very little information on the interwebs. Anyone here can help me maybe? :)

Answer 1

It's easy with a listener callback

    void audioVolumeChangeListenerCallback (
          void                      *inUserData,
          AudioSessionPropertyID    inID,
          UInt32                    inDataSize,
          const void                *inData)
{
    RootViewController *controller = (RootViewController *) inUserData;
    Float32 newGain = *(Float32 *)inData;
    [controller setGainManual:newGain]; 
}

which gets initialized in my view controller's viewDidLoad like this

    AudioSessionAddPropertyListener (
         kAudioSessionProperty_CurrentHardwareOutputVolume ,
         audioVolumeChangeListenerCallback,
         self
         );

This is all SDK/App Store friendly too.