AudioUnit渲染回调问题（未调用回调）

Question

此时我有一个简单的要求，一个iOS应用程序，它从音频文件读取并使用AudioUnits输出到扬声器。不使用高级API的原因是，在某些时候，我需要处理来自音频文件的样本，并最终通过网络发送它。

我有一个有效的代码，读取音频文件并播放到扬声器。这里唯一的问题是，渲染回调不起作用。回调永远不会被调用，注册时也不会收到任何错误。非常感谢帮助（我是Core Audio的初学者，这是我关于stackoverflow的第一个问题，所以请原谅任何基本错误/忽视）。我附上了用于初始化图表的代码段。

void createMyAUGraph (MyAUGraphPlayerST *player) {
    // Create a new AUGraph

    CheckError(NewAUGraph(&player->graph), "New AUGraph failed");

    // Generate description for output

    AudioComponentDescription outputcd = {0};
    outputcd.componentType = kAudioUnitType_Output;
    outputcd.componentSubType = kAudioUnitSubType_RemoteIO;
    outputcd.componentManufacturer = kAudioUnitManufacturer_Apple;
    outputcd.componentFlags = 0;
    outputcd.componentFlagsMask = 0;

    // Add new node

    AUNode outputNode;
    CheckError(AUGraphAddNode(player->graph, &outputcd, &outputNode), "Add output node failed");

    // Node for file player

    AudioComponentDescription fileplayercd = {0};
    fileplayercd.componentType = kAudioUnitType_Generator;
    fileplayercd.componentSubType = kAudioUnitSubType_AudioFilePlayer;
    fileplayercd.componentManufacturer = kAudioUnitManufacturer_Apple;

    // Add new node

    AUNode fileNode;
    CheckError(AUGraphAddNode(player->graph, &fileplayercd, &fileNode), "Add file node failed");

    // Open graph

    CheckError(AUGraphOpen(player->graph), "Graph open failed");

    // Retrive AudioUnit

    CheckError(AUGraphNodeInfo(player->graph, outputNode, NULL, &player->outputAU), "file unit retrive failed");
    CheckError(AUGraphNodeInfo(player->graph, fileNode, NULL, &player->fileAU), "file unit retrive failed");

    // connect nodes

    CheckError(AUGraphConnectNodeInput(player->graph, fileNode, 0, outputNode, 0), "failed to connect nodes");

    // some other setup

    UInt32 flag = 1;
    CheckError(AudioUnitSetProperty(player->outputAU,
                                      kAudioOutputUnitProperty_EnableIO,
                                      kAudioUnitScope_Output,
                                      0,
                                      &flag,
                                      sizeof (flag)), "Set io property failed");

    // Register render callback

    AURenderCallbackStruct output_cb;
    output_cb.inputProc = recording_cb;
    output_cb.inputProcRefCon = player;
    CheckError(AudioUnitSetProperty(player->outputAU, kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Global, 0, &output_cb, sizeof (output_cb)), "callback register failed");

    // initialize graph

    CheckError(AUGraphInitialize(player->graph), "graph initialization failed");
}