AVAudioSession - How to switch between speaker and headphones output

I'm trying to mimic behaviour as in Phone app during calling. You can easily switch output sources from/to speaker or headphones. I know I can force speaker as an output when headphones are connected by calling:

try! audioSession.setCategory(AVAudioSessionCategoryPlayAndRecord)  
try! audioSession.overrideOutputAudioPort(.speaker)  

However, when I do that, I don't see any way to detect if headphones are still connected to the device.

I initially thought outputDataSources on AVAudioSession would return all posible outputs but it always returns nil.

Is there something I'm missing

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