ios - 如何在iOS11中正确停止和删除audioTapProcessor

Question

我正在使用与audioTap example from apple类似的audioTapProcessor。

基本上，将创建audioTapProcessor并将其添加到audioMix:

MTAudioProcessingTapRef audioProcessingTap;
if (noErr == MTAudioProcessingTapCreate(kCFAllocatorDefault, &callbacks,     kMTAudioProcessingTapCreationFlag_PreEffects, &audioProcessingTap))
{
    audioMixInputParameters.audioTapProcessor = audioProcessingTap;

    CFRelease(audioProcessingTap);

    audioMix.inputParameters = @[audioMixInputParameters];

    _audioMix = audioMix;
}

创建audioMix后，会将其添加到avPlayer的currentItem中:

self.avPlayer.currentItem.audioMix = audioMix;

当我用完audioTapProcessor后，我会清除对audioTapProcessor和audioMix的引用:

if (self.avPlayer.currentItem.audioMix){

    for (AVMutableAudioMixInputParameters *inputParameter in player.currentItem.audioMix.inputParameters) {
        inputParameter.audioTapProcessor = nil;

    }

    player.currentItem.audioMix = nil;
}

这将触发tap_UnprepareCallback和tap_FinalizeCallback函数，并且audioTapProcessor对象将被释放。

直到几天前iOS11出来，这一切都没有问题。

在iOS11设备上，有时我在ClientProcessingTapManager线程上收到EXC_BAD_ACCESS错误。该错误发生在AVFoundation的一类中，因此无法调试。该错误发生在ClientProcessingTap::PerformTap中。

我已将日志记录添加到audioTap回调中，并且看起来只有tap_ProcessCallBack在ClientProcessingTapManager线程上执行。

我认为有时在此线程上触发audioTapProcessor的tap_ProcessCallback方法，而实际的audioTapProcessor已经在另一个线程上释放。

我的问题是:如何正确停止audioTapProcessor并将其从内存中删除？

这是一些输出日志记录示例:

2017-09-22 12:49:26:171 xx[1261:780894] cleaning up audioMix of AvPlayer
2017-09-22 12:49:26:213 xx[1261:781173] into tap_ProcessCallback. thread: ClientProcessingTapManager
2017-09-22 12:49:26:213 xx[1261:781173] end of tap_ProcessCallback
2017-09-22 12:49:26:217 xx[1261:781127] into tap_UnprepareCallback. 
2017-09-22 12:49:26:218 xx[1261:781127] end of tap_UnprepareCallback
2017-09-22 12:49:26:218 xx[1261:781127] into tap_FinalizeCallback. 
2017-09-22 12:49:26:218 Tunify[1261:781127] end of tap_FinalizeCallback
2017-09-22 12:49:26:218 Tunify[1261:781127] into MYAudioTapProcessor dealloc.
(lldb) 

很高兴知道:

我正在使用avPlayers池，所以我正在重用这些对象。当我不清除avPlayers的audioMix时，就不会遇到此错误。如果不清理audioMix对象，则仅当我重用avPlayer并将新的audioMix分配给avPlayer时，才会重新分配audioTapProcessor。

我发现了 related question，但是我已经在触发清理功能。那里的解决方案在iOS11设备上给出了相同的错误

Answer 1

这是iOS 11下AVFoundation中的错误，已在iOS 12中修复。
问题rdar://34977000的雷达

可能的解决方法是在释放AVPlayer / AVPlayerItem之前调用prepareReleaseAudioMix:

+ (void)prepareReleaseAudioMix:(AVAudioMix*)audioMix {
    if ( @available(iOS 11.0, *) ) {
        // Starting with iOS 11, it is not required to manually nil audioTapProcessor,
        // but we need to retain the audioMix for a bit to make sure the processing callback
        // will not be called after we release (this is due to a bug in iOS 11 which calls the release
        // callback but still calls the processing callback afterwards - it also releases internal data
        // on release, so simply checking for release in the processing block is not enough)
        // rdar://34977000
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            [self _prepareReleaseAudioMixNow:audioMix];
        });
    } else {
        // Prior to iOS 11 we need to manually nil the audioTapProcessor
        [self _prepareReleaseAudioMixNow:audioMix];
    }
}

+ (void)_prepareReleaseAudioMixNow:(AVAudioMix*)audioMix {
    for ( AVMutableAudioMixInputParameters *parameters in audioMix.inputParameters ) {
        if ( ![parameters isKindOfClass:[AVMutableAudioMixInputParameters class]] )
            continue;
        parameters.audioTapProcessor = nil;
    }
}