AudioBufferList contents in remoteIO audio unit pl

问题:

I'd like to "intercept" audio data on its way to the iOS device's speaker. I believe this can be done using remoteIO audio units and callbacks. In the playbackCallback below does ioData actually contain any audio data?

static OSStatus playbackCallback(void *inRefCon, 
                                 AudioUnitRenderActionFlags *ioActionFlags, 
                                 const AudioTimeStamp *inTimeStamp, 
                                 UInt32 inBusNumber, 
                                 UInt32 inNumberFrames, 
                                 AudioBufferList *ioData) { ... } 

I'm confused because logging info about ioData seems to imply it contains audio data...

// if (ioData->mNumberBuffers > 0)
AudioBuffer buffer = ioData->mBuffers[0];
NSLog(@"buffer.mNumberChannels: %ld", buffer.mNumberChannels); // prints 2
NSLog(@"buffer.mDataByteSize  : %ld", buffer.mDataByteSize); // prints 4096

However creating a CMSampleBufferRef from the contents of ioData and writing it to a CoreAudioFile using an AVAssetWriter yields a silent file. The length of the output file seems fine (a few seconds) but opening the file in Audacity for example shows a flatline.

After reading numerous SO posts and experimenting with lots of remoteIO audio unit sample code I'm starting to wonder if ioData above contains pre-sized but empty buffers that should be populated in the playbackCallback.

回答1:

The ioData buffers in a playCallback are where the callback should put the audio samples you want to play. The buffers do not contain other audio intercepted on its way to the speaker.