title: How does this iOS app communicate over the USB port?
body: <p><a href="http://www.duetdisplay.com">This app</a> promises to turn your iOS device into a second monitor and uses the <a href="http://help.duetdisplay.com/questions/119908-which-cable-do-i-use-to-connect">standard USB cable</a> to connect to the computer, as opposed to the network which all other similar apps use.</p>
  
  <p>Back when I heard about it, it wasn't yet available so I thought it was a scam. To my surprise, they released the app and it actually works as described.</p>
  
  <p>How does it work ? As far as I know there are no APIs to drive the USB port in iOS, and the computer connected via USB through the standard charging/syncing cable doesn't count as an MFI accessory.</p>
  
  <p>Note that they provide the server software (the one that installs on the Mac and streams the desktop to the device) on their download page for free, reverse-engineering it could shed some light on this but I unfortunately don't have the skills for that.</p>