如何在 iOS 11 中正确停止和删除音频处理器

我正在使用类似于的audioTapProcessor苹果的 audioTap 示例 https://developer.apple.com/library/content/samplecode/AudioTapProcessor/Introduction/Intro.html.

基本上，audioTapProcessor 被创建并添加到 audioMix 中：

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

    CFRelease(audioProcessingTap);

    audioMix.inputParameters = @[audioMixInputParameters];

    _audioMix = audioMix;
}

当创建audioMix时，它被添加到avPlayer的currentItem中：

self.avPlayer.currentItem.audioMix = audioMix;

当我完成audioTapProcessor 后，我清除了对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;
}

这会触发tap_UnprepareCallback和tap_FinalizeCallback函数，并且 audioTapProcessor 对象被释放。

在 iOS11 几天前发布之前，这一切都没有任何问题。

在 iOS11 设备上，我有时会在 ClientProcessingTapManager 线程上收到 EXC_BAD_ACCESS 错误。错误发生在AVFoundation的其中一个类中，所以我无法调试。该错误发生在 ClientProcessingTap::PerformTap 中的某个位置。

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

我认为有时audioTapProcessor 的tap_ProcessCallback 方法会在此线程上触发，而实际的audioTapProcessor 已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，因此我正在重用这些对象。当我不清除 avPlayers 的 audioMix 时，我不会遇到此错误。如果不清理audioMix对象，只有当我重用avPlayer并将新的audioMix分配给avPlayer时，audioTapProcessors才会被释放。

我找到了一个相关问题 https://stackoverflow.com/questions/19202306/how-do-you-release-an-mtaudioprocessingtap，但我已经触发了清理功能。那里的解决方案在 iOS11 设备上给出了相同的错误

这是iOS 11下AVFoundation的一个bug。在iOS 12中已修复。 针对该问题的雷达雷达://34977000 http://rdar://34977000

一个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;
    }
}