title: How to control instance in another view controller
body: <p>I'm new to Swift and OS X programming.</p>
  
  <p>I am creating a desktop application with two view controllers, which are in a split view controller. How can I get access to the instances in another controller and change its attributes? </p>
  
  <p>An example:</p>
  
  <p>We have a drawing application, one view is canvas, the other is tools. There is a 'clear' button in tool view, how to set the canvas clear when click on this button?</p>