Sound not playing when App in background (prepareToPlay returns NO)

Question

I've got a strange problem using AVAudioPlayer to play sound files (wav files) on an iPhone in the Background. I am using the following code:

        AVAudioPlayer* audioplayer;
        NSError* error;

        audioplayer = [[AVAudioPlayer alloc] initWithData:soundfile error:&error];
        if (error) {
            NSLog(@"an error occured while init audioplayer...");
            NSLog(@"%@", [error localizedDescription]);
        }
        audioplayer.currentTime = 0;
        if (![audioplayer prepareToPlay])
            NSLog(@"could not preparetoPlay");

        audioplayer.volume = 1.0;

        [[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayback error:nil];
        [[AVAudioSession sharedInstance] setActive: YES error: &error];

        if (![audioplayer play])
            NSLog(@"could not play sound");

        audioplayer.delegate = [myApp sharedInstance];

This works fine while the app is in foreground. However, when moving the app to background [audioplayer prepareToPlay] returns NO.

This happens with AND without "App plays audio" added to the "Required background modes". Is there a way how to get a more precise error report from [audioplayer prepareToPlay]? Or do you have any hints what I am doing wrong or forgot?

Answer 1

You need to initialize your audio session before preparing the AVAudioPlayer instance. Ideally, move the audio session calls to your application delegate's didFinishLaunchingWithOptions method.

Answer 2

I'm not entirely sure this can be achieved using AVFoundation alone, you may need to use the AudioUnit framework and create a stream. Should be relatively simple to send the content of the .WAV file to the audio buffer.

This is how I've been doing it in Piti Piti Pa. The other benefit is you can better control the delay in the audio, in order to synchronize audio and video animations (more obvious when using Bluetooth).

Here's the code I'm using to initialize the audio unit:

+(BOOL)_createAudioUnitInstance
{
    // Describe audio component
    AudioComponentDescription desc;
    desc.componentType = kAudioUnitType_Output;
    desc.componentSubType = kAudioUnitSubType_RemoteIO;
    desc.componentFlags = 0;
    desc.componentFlagsMask = 0;
    desc.componentManufacturer = kAudioUnitManufacturer_Apple;
    AudioComponent inputComponent = AudioComponentFindNext(NULL, &desc);

    // Get audio units
    OSStatus status = AudioComponentInstanceNew(inputComponent, &_audioUnit);
    [self _logStatus:status step:@"instantiate"];
    return (status == noErr );
}

+(BOOL)_setupAudioUnitOutput
{
    UInt32 flag = 1;
    OSStatus status = AudioUnitSetProperty(_audioUnit,
                              kAudioOutputUnitProperty_EnableIO,
                              kAudioUnitScope_Output,
                              _outputAudioBus,
                              &flag,
                              sizeof(flag));
    [self _logStatus:status step:@"set output bus"];
    return (status == noErr );
}

+(BOOL)_setupAudioUnitFormat
{
    AudioStreamBasicDescription audioFormat = {0};
    audioFormat.mSampleRate         = 44100.00;
    audioFormat.mFormatID           = kAudioFormatLinearPCM;
    audioFormat.mFormatFlags        = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked;
    audioFormat.mFramesPerPacket    = 1;
    audioFormat.mChannelsPerFrame   = 2;
    audioFormat.mBitsPerChannel     = 16;
    audioFormat.mBytesPerPacket     = 4;
    audioFormat.mBytesPerFrame      = 4;

    OSStatus status = AudioUnitSetProperty(_audioUnit,
                                       kAudioUnitProperty_StreamFormat,
                                       kAudioUnitScope_Input,
                                       _outputAudioBus,
                                       &audioFormat,
                                       sizeof(audioFormat));
    [self _logStatus:status step:@"set audio format"];
    return (status == noErr );
}


+(BOOL)_setupAudioUnitRenderCallback
{
    AURenderCallbackStruct audioCallback;
    audioCallback.inputProc = playbackCallback;
    audioCallback.inputProcRefCon = (__bridge void *)(self);
    OSStatus status = AudioUnitSetProperty(_audioUnit,
                                       kAudioUnitProperty_SetRenderCallback,
                                       kAudioUnitScope_Global,
                                       _outputAudioBus,
                                       &audioCallback,
                                       sizeof(audioCallback));
    [self _logStatus:status step:@"set render callback"];
    return (status == noErr);
}


+(BOOL)_initializeAudioUnit
{
    OSStatus status = AudioUnitInitialize(_audioUnit);
    [self _logStatus:status step:@"initialize"];
    return (status == noErr);
}

+(void)start
{
    [self clearFeeds];
    [self _startAudioUnit];
}

+(void)stop
{
    [self _stopAudioUnit];
}

+(BOOL)_startAudioUnit
{
    OSStatus status = AudioOutputUnitStart(_audioUnit);
    [self _logStatus:status step:@"start"];
    return (status == noErr);
}

+(BOOL)_stopAudioUnit
{
    OSStatus status = AudioOutputUnitStop(_audioUnit);
    [self _logStatus:status step:@"stop"];
    return (status == noErr);
}

+(void)_logStatus:(OSStatus)status step:(NSString *)step
{
    if( status != noErr )
    {
        NSLog(@"AudioUnit failed to %@, error: %d", step, (int)status);
    }
}

#pragma mark - Mixer

static OSStatus playbackCallback(void *inRefCon,
                             AudioUnitRenderActionFlags *ioActionFlags,
                             const AudioTimeStamp *inTimeStamp,
                             UInt32 inBusNumber,
                             UInt32 inNumberFrames,
                             AudioBufferList *ioData) {

    @autoreleasepool {
        AudioBuffer *audioBuffer = ioData->mBuffers;

        _lastPushedFrame = _nextFrame;
        [SIOAudioMixer _generateAudioFrames:inNumberFrames into:audioBuffer->mData];
    }
    return noErr;
}

Now you only need to extract the content of the .Wav files (easier if you export them to RAW format) and send it out to the buffer via the callback.

I hope that helps!

Answer 3

In AppDelegate set the AVAudioSession category like this: (Swift 2)

    do {
        try AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayAndRecord, withOptions: AVAudioSessionCategoryOptions.MixWithOthers)
    }catch{
        self.fireAnAlert("Set Category Failed", theMessage: "Failed to set AVAudioSession Category")
    }

Setting the options to "Mix With Others" Is an important piece!

Then where ever you are going to play sound make sure you call beginReceivingRemoteControlEvents and then set the AVAudioSession to active like this:

    do{

        UIApplication.sharedApplication().beginReceivingRemoteControlEvents()
        try AVAudioSession.sharedInstance().setActive(true)

    }catch{

        let e = error as NSError

        self.appDelegate?.fireAnAlert("Error", theMessage: "\(e.localizedDescription)")
    }