AudioUnit(一) 播放音频文件

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黄色部分是APP可以控制的部分
因此，采集麦克风数据时设置格式时，总是element1与output scope 相结合，
使用扬声器播放设置格式时，总是element0和intput scope 相结合，

由于kAudioOutputUnitProperty_EnableIO 是设置的kAudioUnitScope_Output,表明IO是与扬声器相连接（输出），因此总线标识AudioUnitElement应该设置为0，所以后面与输出相关的AudioUnitElement都是0

代码中使用的是 OUTPUT_BUS
总线：是计算机各种功能部件之间传送信息的公共通信干线

1. 创建kAudioUnitSubType_RemoteIO类型的AudieUnit

         AudioComponentDescription acd;
        acd.componentType = kAudioUnitType_Output;
        acd.componentSubType = kAudioUnitSubType_RemoteIO;
        acd.componentManufacturer = kAudioUnitManufacturer_Apple;
        acd.componentFlags = 0;
        acd.componentFlagsMask = 0;
  
     AudioComponent ioUnitRef = AudioComponentFindNext(NULL, &acd);
     status = AudioComponentInstanceNew(ioUnitRef, &_ioUnit);

2.连接扬声器，注意AudioUnitScope设置为kAudioUnitScope_Output类型，

UInt32   OUTPUT_BUS = 0；
 UInt32 flag = 1;
        if (flag) {
            status = AudioUnitSetProperty(_ioUnit, kAudioOutputUnitProperty_EnableIO, kAudioUnitScope_Output, OUTPUT_BUS, &flag, sizeof(flag));
        }

3. 设置输入格式 ,此处是播放文件，所以需要设置输入格式

          AudioStreamBasicDescription asbd;
          asbd.mSampleRate = 44100;
           asbd.mFormatID         = kAudioFormatLinearPCM; // PCM格式
           asbd.mFormatFlags      = kLinearPCMFormatFlagIsSignedInteger; // 整形
           asbd.mFramesPerPacket  = 1; // 每帧只有1个packet
           asbd.mChannelsPerFrame = 1; // 声道数
           asbd.mBytesPerFrame    = 2; // 每帧只有2个byte 声道*位深*Packet数
           asbd.mBytesPerPacket   = 2; // 每个Packet只有2个byte
           asbd.mBitsPerChannel   = 16; // 位深

UInt32   OUTPUT_BUS = 0；
  status = AudioUnitSetProperty(_ioUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, OUTPUT_BUS, &asbd, sizeof(asbd));

4.设置回调

AURenderCallbackStruct callBack;
 callBack.inputProc = PlayCallBack;
callBack.inputProcRefCon = (__bridge void *)self;
status = AudioUnitSetProperty(_ioUnit, kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Output, OUTPUT_BUS, &callBack, sizeof(callBack));


OSStatus PlayCallBack(    void *                            inRefCon,
                        AudioUnitRenderActionFlags *    ioActionFlags,
                        const AudioTimeStamp *            inTimeStamp,
                        UInt32                            inBusNumber,
                        UInt32                            inNumberFrames,
                      AudioBufferList * __nullable    ioData) {
    
    RGAudioBasePlay *player = (__bridge RGAudioBasePlay *)inRefCon;
      
      ioData->mBuffers[0].mDataByteSize = (UInt32)[player->_inputSteam read:ioData->mBuffers[0].mData maxLength:(NSInteger)ioData->mBuffers[0].mDataByteSize];;
      NSLog(@"out size: %d", ioData->mBuffers[0].mDataByteSize);
      
      if (ioData->mBuffers[0].mDataByteSize <= 0) {
          dispatch_async(dispatch_get_main_queue(), ^{
//              [player stop];
          });
      }
      return noErr;

}

4.播放

AudioOutputUnitStart(_ioUnit);

播放时，需要知道PCM原文件的格式是怎么样的，否走播放很容易就出错
例如一个PMC原文件格式是, 播放格式需要设置对应的格式才容易正确的播放

File format:
Sample Rate:              44100
Format ID:                 lpcm
Format Flags:                 kAudioFormatFlagIsSignedInteger
Bytes per Packet:             4
Frames per Packet:            1
Bytes per Frame:              4
Channels per Frame:           2
Bits per Channel:            16

设置输出格式的AudioStreamBasicDescription原则
mBitsPerChannel 每个声道几位表示， 一般为 16位
mSampleRate采样率一般为44100
mFormatFlags为量化的类型，一般的数据都是整型
mFramesPerPacket每个包多少帧，一般音频为1帧
mChannelsPerFrame每帧的声道数(可以为1，也可以为2)
mBytesPerFrame 每帧多少个字节，帧是最小单位，所以最小值可以设置为
n * mChannelsPerFrame * mBitsPerChannel / 8 (n为大于等于1的整数)
mBytesPerPacket 等于mBytesPerFrame

注意mBytesPerFrame设置越大，渲染回调时，所需要读取的字节数越大ioData->mBuffers[0].mDataByteSize值越大，ioData->mBuffers[0].mDataByteSize只跟mBytesPerFrame有关，单位时间里面ioData->mBuffers[0].填充的数据越多，音频播放的越快

OSStatus PlayCallBack(    void *                            inRefCon,
                        AudioUnitRenderActionFlags *    ioActionFlags,
                        const AudioTimeStamp *            inTimeStamp,
                        UInt32                            inBusNumber,
                        UInt32                            inNumberFrames,
                      AudioBufferList * __nullable    ioData) {

 ioData->mBuffers[0].mDataByteSize = (UInt32)[player->_inputSteam read:player->_buffer maxLength:(NSInteger)ioData->mBuffers[0].mDataByteSize] ;
      NSLog(@"out size: %d", ioData->mBuffers[0].mDataByteSize);

      return noErr;

}

    AudioStreamBasicDescription asbd;
        asbd.mSampleRate = 44100;
        asbd.mFormatID = kAudioFormatLinearPCM; //编码格式
        asbd.mFormatFlags = kAudioFormatFlagIsSignedInteger ; //每个样本量化的类型(这里是float)
        asbd.mBytesPerPacket = 8;
        asbd.mFramesPerPacket = 1;
        asbd.mBytesPerFrame = 8;
        asbd.mChannelsPerFrame = 1;
        asbd.mBitsPerChannel = 2 * 8;