title: Check if current screen is on or off in iOS
body: <p>I know that it is possible to register an event listener when there is screen on/off event. What if I want to check whether currently the screen is on or off? Any method for me to check it?</p>
  
  <p>If I use notification to check, here is the event that will happen:</p>
  
  <p>When I lock the screen. It will trigger</p>
  
  <blockquote>
    <p>--- received notification: com.apple.springboard.hasBlankedScreen
    --- received notification: com.apple.springboard.lockcomplete
    --- received notification: com.apple.springboard.lockstate
    --- received notification: com.apple.iokit.hid.displayStatus</p>
  </blockquote>
  
  <p>When I unlock the screen, it will trigger</p>
  
  <blockquote>
    <p>--- received notification: com.apple.springboard.hasBlankedScreen
    --- received notification: com.apple.springboard.lockstate
    --- received notification: com.apple.iokit.hid.displayStatus</p>
  </blockquote>
  
  <p>I cannot simply detect lockcomplete to see if it is currently off, because it will also trigger lockstate and displaystatus when i tried to lock the screen. </p>