我的工作图看起来像这样：

(My Audio render callback) <- [Format converter] <- [DefaultOutput]

另外,我能够成功插入这样的EQ单元：

(My Audio render callback) <- [Format converter] <- [GraphicEQ] <- [DefaultOutput]

我的目标是插入一个混音器单元,但没有EQ,所以我可以独立于iOS上的系统音量控制我的音量.但是,当我设置图表时,它看起来似乎没问题,但我得到的只是沉默.

这是图表的样子：

(My Audio render callback) <- [Format converter] <- [MultiChannelMixer] <- [DefaultOutput]

……以及CAShow的输出：

AudioUnitGraph 0x397000:
  Member Nodes:
    node 1: 'auou' 'def ' 'appl',instance 0x8039704d O I
    node 2: 'aumx' 'mcmx' 'appl',instance 0x8039704e O I
    node 3: 'aufc' 'conv' 'appl',instance 0x8039704f O I
  Connections:
    node   3 bus   0 => node   2 bus   0  [ 2 ch,44100 Hz,'lpcm' (0x00000029) 32-bit little-endian float,deinterleaved]
    node   2 bus   0 => node   1 bus   0  [ 2 ch,deinterleaved]
  Input Callbacks:
    {0x100035d90,0x10037cd60} => node   3 bus   0  [2 ch,44100 Hz]
  CurrentState:
    mLastUpdateError=0,eventsToProcess=F,isRunning=T (1)

最后,设置图表的代码包含错误检查和iOS特定代码：

// A description of the output device we're looking for.
AudioComponentDescription outputDescription;
outputDescription.componentType = kAudioUnitType_Output;
outputDescription.componentSubType = kAudioUnitSubType_DefaultOutput;
outputDescription.componentManufacturer = kAudioUnitManufacturer_Apple;
outputDescription.componentFlags = 0;
outputDescription.componentFlagsMask = 0;

// A description of the mixer unit
AudioComponentDescription mixerDescription;
mixerDescription.componentType = kAudioUnitType_Mixer;
mixerDescription.componentSubType = kAudioUnitSubType_MultiChannelMixer;
mixerDescription.componentManufacturer = kAudioUnitManufacturer_Apple;
mixerDescription.componentFlags = 0;
mixerDescription.componentFlagsMask = 0;

// A description for the libspotify -> standard PCM device
AudioComponentDescription converterDescription;
converterDescription.componentType = kAudioUnitType_FormatConverter;
converterDescription.componentSubType = kAudioUnitSubType_AUConverter;
converterDescription.componentManufacturer = kAudioUnitManufacturer_Apple;
converterDescription.componentFlags = 0;
converterDescription.componentFlagsMask = 0;    

// Create an AUGraph
OSErr status = NewAUGraph(&audioProcessingGraph);

// Open the graph. AudioUnits are open but not initialized (no resource allocation occurs here)
AUGraphOpen(audioProcessingGraph);

// Add audio output...
status = AUGraphAddNode(audioProcessingGraph,&outputDescription,&outputNode);

// Add mixer
status = AUGraphAddNode(audioProcessingGraph,&mixerDescription,&mixerNode);

// Get mixer unit so we can change volume etc
status = AUGraphNodeInfo(audioProcessingGraph,mixerNode,NULL,&mixerUnit);

// Set mixer bus count
UInt32 busCount = 1;
status = AudioUnitSetProperty(mixerUnit,kAudioUnitProperty_ElementCount,kAudioUnitScope_Input,&busCount,sizeof(busCount));

// Create PCM converter
status = AUGraphAddNode(audioProcessingGraph,&converterDescription,&inputConverterNode);

// Connect converter to mixer
status = AUGraphConnectNodeInput(audioProcessingGraph,inputConverterNode,0);

// Connect mixer to output
status = AUGraphConnectNodeInput(audioProcessingGraph,outputNode,0);

// Set render callback
AURenderCallbackStruct rcbs;
rcbs.inputProc = AudioUnitRenderDelegateCallback;
rcbs.inputProcRefCon = (__bridge void *)(self);

status = AUGraphSetNodeInputCallback(audioProcessingGraph,&rcbs);

// Init Queue
status = AUGraphInitialize(audioProcessingGraph);

AUGraphUpdate(audioProcessingGraph,NULL);
AUGraphStart(audioProcessingGraph);