title: Mac: Check for keyboard events
body: <p>I am making a simple piano game for Mac. And when user pressed the keyboard,the key of piano </p>
  
  <p>can be pressed simultaneously.</p>
  
  <p>However, I am clueless about how to check when "D,F....." key is pressed on the Mac's keyboard. </p>
  
  <p>Objective-C</p>