使用未声明的标识符'audioRouteChangeListenerCallback'

Question

Trying to initialize audiosession so that app audio plays in background and shows remote control events to control audio when screen locks but getting error use of undeclared identifier 'audioRouteChangeListenerCallback'

- (void) setupAudioSession {

AVAudioSession *mySession = [AVAudioSession sharedInstance];

// Specify that this object is the delegate of the audio session, so that
//    this object's endInterruption method will be invoked when needed.
[mySession setDelegate: self];

// Assign the Playback category to the audio session.
NSError *audioSessionError = nil;
[mySession setCategory: AVAudioSessionCategoryPlayback
                 error: &audioSessionError];

if (audioSessionError != nil) {

    NSLog (@"Error setting audio session category.");
    return;
}

// Activate the audio session
[mySession setActive: YES
               error: &audioSessionError];

if (audioSessionError != nil) {

    NSLog (@"Error activating audio session during initial setup.");
    return;
}

// Increase the maximum frames per slice allows the mixer unit to accommodate the
//    larger slice size used when the screen is locked.
UInt32 maximumFramesPerSlice = 4096;

 AudioUnitSetProperty (
                               mixerUnit,
                               kAudioUnitProperty_MaximumFramesPerSlice,
                               kAudioUnitScope_Global,
                               0,
                               &maximumFramesPerSlice,
                               sizeof (maximumFramesPerSlice)
                               );

// Register the audio route change listener callback function with the audio session.
AudioSessionAddPropertyListener (
kAudioSessionProperty_AudioRouteChange,
audioRouteChangeListenerCallback, self );
 }

- (void)viewDidAppear:(BOOL)animated {
[super viewDidAppear:animated];
[[UIApplication sharedApplication] beginReceivingRemoteControlEvents];
[self becomeFirstResponder];
 }

- (void)viewWillDisappear:(BOOL)animated {
[super viewWillDisappear:animated];
[[UIApplication sharedApplication] endReceivingRemoteControlEvents];
[self resignFirstResponder];
}

- (BOOL)canBecomeFirstResponder {
return YES;
}

- (void)remoteControlReceivedWithEvent:(UIEvent *)event {
//if it is a remote control event handle it correctly
if (event.type == UIEventTypeRemoteControl) {
    if (event.subtype == UIEventSubtypeRemoteControlTogglePlayPause) {

        [self playAction];

    }  else if (event.subtype == UIEventSubtypeRemoteControlPreviousTrack) {
        [self rewButtonPressed];

    } else if (event.subtype == UIEventSubtypeRemoteControlNextTrack)
        [self ffwButtonPressed:nil];
}}

Any help will be really appreciated.

Thanks

Answer 1

You really have an undeclared identifier called audioRouteChangeListenerCallback. This isn't a symbol that is defined in the CoreAudio framework AFAIK, however, some of Apple's sample code sources define it, eg MixerHost , in MixerHostAudio.m .

The comment in that source file is instructive:

// Audio session callback function for responding to audio route changes. If playing back audio and
//   the user unplugs a headset or headphones, or removes the device from a dock connector for hardware
//   that supports audio playback, this callback detects that and stops playback.
//
// Refer to AudioSessionPropertyListener in Audio Session Services Reference.