title: Level Metering with AVAudioEngine
body: <p>I just watched the WWDC Video (Session 502 <code>AVAudioEngine</code> in Practice) on <code>AVAudioEngine</code> and am very excited to make an app built on this tech.</p>
  
  <p>I haven't been able to figure out how I might do level monitoring of the microphone input, or a mixer's output.</p>
  
  <p>Can anyone help?  To be clear, I'm talking about monitoring the current input signal (and displaying this in the UI), not the input/output volume setting of a channel/track.</p>
  
  <p>I know you can do this with <code>AVAudioRecorder</code>, but this is not an <code>AVAudioNode</code> which the <code>AVAudioEngine</code> requires.</p>