为什么我的多声道混音器不再在iOS 8中播放？

Question

我写了一些代码，在iOS上播放通用的多乐器MIDI文件。它在iOS 7上运行良好，但在iOS 8上停止工作。

我已经在这里把它剥离到它的本质。我没有为我的多通道混音器创建16个通道，而是只创建了一个采样器节点，并将所有轨道映射到该通道。它仍然表现出与多采样器版本相同的问题。在iOS 7或iOS 8中，所有音频工具箱调用都没有返回错误代码(它们都返回0)。在iOS 7中，无论是在模拟器上还是在iPhone/iPad设备上，该序列都会通过扬声器播放。在iOS 8模拟器或iPhone/iPad设备上运行完全相同的代码，不会发出任何声音。

如果注释掉对[self initGraphFromMIDISequence]的调用，它将在iOS 8上播放默认正弦波声音。

@implementation MyMusicPlayer {
    MusicPlayer _musicPlayer;
    MusicSequence _musicSequence;
    AUGraph _processingGraph;
}

- (void)playMidi:(NSURL*)midiFileURL {
    NewMusicSequence(&_musicSequence);
    MusicSequenceFileLoad(_musicSequence, CFBridgingRetain(midiFileURL), 0, 0);

    NewMusicPlayer(&_musicPlayer);
    MusicPlayerSetSequence(_musicPlayer, _musicSequence);

    [self initGraphFromMIDISequence];

    MusicPlayerPreroll(_musicPlayer);
    MusicPlayerStart(_musicPlayer);
}

// Sets up an AUGraph with one channel whose instrument is loaded from a sound bank.
// Maps all the tracks of the MIDI sequence onto that channel.  Basically this is a
// way to replace the default sine-wave sound with another (single) instrument.
- (void)initGraphFromMIDISequence {
    NewAUGraph(&_processingGraph);

    // Add one sampler unit to the graph.
    AUNode samplerNode;
    AudioComponentDescription cd = {};
    cd.componentManufacturer = kAudioUnitManufacturer_Apple;
    cd.componentType = kAudioUnitType_MusicDevice;
    cd.componentSubType = kAudioUnitSubType_Sampler;
    AUGraphAddNode(_processingGraph, &cd, &samplerNode);

    // Add a Mixer unit node to the graph
    cd.componentType = kAudioUnitType_Mixer;
    cd.componentSubType = kAudioUnitSubType_MultiChannelMixer;
    AUNode mixerNode;
    AUGraphAddNode(_processingGraph, &cd, &mixerNode);

    // Add the Output unit node to the graph
    cd.componentType = kAudioUnitType_Output;
    cd.componentSubType = kAudioUnitSubType_RemoteIO; // Output to speakers.
    AUNode ioNode;
    AUGraphAddNode(_processingGraph, &cd, &ioNode);

    AUGraphOpen(_processingGraph);

    // Obtain the mixer unit instance from its corresponding node, and set the bus count to 1.
    AudioUnit mixerUnit;
    AUGraphNodeInfo(_processingGraph, mixerNode, NULL, &mixerUnit);
    UInt32 const numChannels = 1;
    AudioUnitSetProperty(mixerUnit,
                         kAudioUnitProperty_ElementCount,
                         kAudioUnitScope_Input,
                         0,
                         &numChannels,
                         sizeof(numChannels));

    // Connect the sampler node's output 0 to mixer node output 0.
    AUGraphConnectNodeInput(_processingGraph, samplerNode, 0, mixerNode, 0);

    // Connect the mixer unit to the output unit.
    AUGraphConnectNodeInput(_processingGraph, mixerNode, 0, ioNode, 0);

    // Obtain reference to the audio unit from its node.
    AudioUnit samplerUnit;
    AUGraphNodeInfo(_processingGraph, samplerNode, 0, &samplerUnit);
    MusicSequenceSetAUGraph(_musicSequence, _processingGraph);

    // Set the destination for each track to our single sampler node.
    UInt32 trackCount;
    MusicSequenceGetTrackCount(_musicSequence, &trackCount);
    MusicTrack track;
    for (int i = 0; i < trackCount; i++) {
      MusicSequenceGetIndTrack(_musicSequence, i, &track);
      MusicTrackSetDestNode(track, samplerNode);
    }

    // You can use either a DLS or an SF2 file bundled with your app; both work in iOS 7.
    //NSString *soundBankPath = [[NSBundle mainBundle] pathForResource:@"GeneralUserv1.44" ofType:@"sf2"];
    NSString *soundBankPath = [[NSBundle mainBundle] pathForResource:@"gs_instruments" ofType:@"dls"];
    NSURL *bankURL = [NSURL fileURLWithPath:soundBankPath];
    AUSamplerBankPresetData bpdata;
    bpdata.bankURL  = (__bridge CFURLRef) bankURL;
    bpdata.bankMSB  = kAUSampler_DefaultMelodicBankMSB;
    bpdata.bankLSB  = kAUSampler_DefaultBankLSB;
    bpdata.presetID = 0;
    UInt8 instrumentNumber = 46;  // pick any GM instrument 0-127
    bpdata.presetID = instrumentNumber;
    AudioUnitSetProperty(samplerUnit,
                         kAUSamplerProperty_LoadPresetFromBank,
                         kAudioUnitScope_Global,
                         0,
                         &bpdata,
                         sizeof(bpdata));
}

我有一些代码，没有包含在这里，它通过调用MusicPlayer实例上的MusicPlayerGetTime来轮询序列，以查看序列是否仍在播放。在iOS 7中，每次调用的结果是自开始播放以来经过的秒数。在iOS 8中，调用总是返回0，这可能意味着MusicPlayer不会开始播放调用MusicPlayerStart的序列。

上面的代码是高度依赖顺序的--您必须在其他调用之前进行某些调用；例如，在节点上调用getInfo之前打开图形，并且在将曲目分配给通道之前不加载仪器。我遵循了其他StackOverflow线程中的所有建议，并验证了获得正确的顺序会使错误代码消失。

有没有iOS MIDI专家知道iOS 7和iOS 8之间可能发生了什么变化，从而使此代码停止工作？

Answer 1

在iOS 8中，苹果引入了一个巧妙的Obj-C核心音频应用编程接口抽象-- AVAudioEngine。你也许应该去看看。https://developer.apple.com/videos/wwdc/2014/#502