初始化AUGraph后的AVAudioSession属性

Question

To start a call, our VOIP app sets up an AVAudioSession, then builds, initializes and runs an AUGraph.

During the call, we allow the user to switch back and forth between a speakerphone mode using code such as:

avSession = [AVAudioSession sharedInstance];
AVAudioSessionCategoryOptions categoryOptions = [avSession categoryOptions];
categoryOptions |= AVAudioSessionCategoryOptionDefaultToSpeaker;
NSLog(@"AudioService:setSpeaker:setProperty:DefaultToSpeaker=1 categoryOptions = %lx", (unsigned long)categoryOptions);
BOOL success = [avSession setCategory:AVAudioSessionCategoryPlayAndRecord withOptions:categoryOptions error:&error];

Which works just fine. But if we try to certain AVAudioSession queries after the AUGraph has been initialized, for example:

    AVAudioSessionDataSourceDescription *myInputDataSource = [avSession inputDataSource];

the result is null. Running the same line of code BEFORE we execute the AUGraphInitialize gives the correct non-null result. Can anyone explain what is going on here and how to properly access AVAudioSession properties/methods while using AUGraph?

Answer 1

This is expected behavior per the developer documentation, inputDataSource should return nil if it is not possible to switch sources. So Apple is really not letting anything bad happen via a mis-config, but a nil source can also give the wrong idea. Hope this helps.

Discussion
The value of this property is nil if switching between multiple input
sources is not currently possible. This feature is supported only on
certain devices and peripherals–for example, on an iPhone equipped with
both front- and rear-facing microphones.