iOS音频系统。 开始和停止还是只是开始？

Question

I have an app, where audio recording is the main and the most important part. However user can switch to table view controller where all records are displayed and no recording is performed.

The question is what approach is better: "start & stop audio system or just start it". It may seem obvious that the first one is more correct, like "allocate when you need it, deallocate when used it". I will show my thoughts on this question and I hope to find approval or disapproval with arguments among skilled people.

When I constructed AudioController.m the first time I implemented methods to open/close audio session and to start/stop audio unit. I wanted to stop audio system when recording is not active. I used the following code:

- (BOOL)startAudioSystem {

    // open audio session
    AVAudioSession *audioSession = [AVAudioSession sharedInstance];
    NSError *err = nil;
    if (![audioSession setActive:YES error:&err] ) {
        NSLog(@"Couldn't activate audio session: %@", err);
    }
    // start audio unit
    OSStatus status;
    status = AudioOutputUnitStart([self audioUnit]);
    BOOL noErrors = err == nil && status == noErr;    
    return noErrors;
}

and

- (BOOL)stopAudioSystem {
    // stop audio unit
    BOOL result;
    result = AudioOutputUnitStop([self audioUnit]) == noErr;
    HANDLE_RESULT(result);
    // close audio session
    NSError *err;
    HANDLE_RESULT([[AVAudioSession sharedInstance] setActive:NO withOptions:AVAudioSessionSetActiveOptionNotifyOthersOnDeactivation error:&err]);
    HANDLE_ERROR(err);
    BOOL noErrors = err == nil && result;
    return noErrors;
}

I found this approach problematic because of the following reasons:

1. Audio system starts with delay. That means, recording_callback() not called for some time. I suspect it is AudioOutputUnitStart, which is responsible for that. I tried to comment out the line with this function call and move it to initialization. the delay was gone.
2. If user performs switching between recording view and table view very very fast (audio system's starts and stops are very fast too), it cause the death of media service (I know that observing AVAudioSessionMediaServicesWereResetNotification could help here, but it is not the point).

To resolve these issues I modified AudioController.m with other approach which I managed to discover: start audio system when application becomes active and do not stop it before the app is terminated In this case there are also several issues:

1. CPU usage
2. If audio category is set to recording only, then no other audio could be played when user explores table view controller.

The first one surprisingly is not a big deal, if cancel any kind of processing in recording_callback() like this:

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

    AudioController *input = (__bridge AudioController*)inRefCon;

    if(!input->shouldPerformProcessing)
        return noErr;

    // processing
    // ...
    //

    return noErr;
}

By doing this CPU usage equals to 0% on real device, when no recording is needed and no other actions are performed.

And the second issue can be solved by switching audio category to RecordAndPlay and enable mixing or just ignore the problem. For example in my case app requires mini Jack to be used by external device, so no headphones can be used in parallel.

Despite all this, the first approach is more close to me since I like to close/clean every stream/resource when it is no longer needed. And I want to be sure that there is indeed no other option than just start audio system. Please make me sure that I'm not the only one who came to this solution and it is the correct one.

Answer 1

The key to solving this problem is to note that the audio system actually runs in another (real-time) thread. And you can't really stop and deallocate something running in another thread exactly when you (or the app's main UI thread) "don't need it", but have to delay in order to allow the other thread to realize it needs to do something and then finish and clean up itself. This can potentially take up to many 100's of milliseconds for audio.

Given that, strategy 2 (just start) is safer and more realistic.

Or perhaps set a delay of many many seconds of non-use before attempting to stop audio, and possibly another short delay after that before attempting any restart.