Connecting Avaudiosourcenode to Avaudiosinknode Does Not Work

Connecting AVAudioSourceNode to AVAudioSinkNode does not work

outputNode drives the audio processing graph when AVAudioEngine is configured normally ("online"). outputNode pulls audio from its input node, which pulls audio from its input node(s), etc. When you connect sineNode and sink to each other without making a connection to outputNode, there is nothing attached to an output bus of sink or an input bus of outputNode, and therefore when the hardware asks for audio from outputNode it has nowhere to get it.

If I understand correctly I think you can accomplish what you'd like to do by getting rid of sink, connecting sineNode to outputNode, and running AVAudioEngine in manual rendering mode. In manual rendering mode you pass a manual render block to receive audio (similar to AVAudioSinkNode) and drive the graph manually by calling renderOffline(_:to:).

Can I send sound buffers received using AVAudioSinkNode to be rendered in real-time using AVAudioSourceNode?

The way to do this in real time is to use Audio Unit callbacks (the buffers of which can be as small as a few milliseconds). They will almost always be the same size (except maybe at device power state changes), so just save each one, process it as needed, and have it ready for the next output, a few mS later. Or use a circular/ring fifo/buffer. The RemoteIO Audio Unit in iOS has synchronized I/O.

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