title: How to update DetailView
body: <p>I have a swift app based on <code>Master-Detail</code> template. Every row in <code>MasterView</code> table is based on custom cell received from a nib. Every cell includes <code>UIlabel</code> and <code>UIbutton</code>. The logic of the app is following. If user taps on a row <code>DetailView</code> shows some details depending on selected row. The button on the row does not call <code>tableView(_, didSelectRowAtIndexPath)</code>. If user taps on the button inside a row only an image belongs to <code>DetailView</code> should be changed (other elements on <code>DetailView</code> remain the same) but it isn't. If I select another row and than select previous row back, changed image is shown on the <code>DetailView</code> as it was foreseen. The question is how to redraw the image in the <code>DetailView</code> just by tapping on the button.
  I've tried to do following but with no success:</p>
  
  <pre><code>class MasterViewCell: UITableViewCell {
     weak var detailViewController: DetailViewController?
  
     @IBAction func buttonTap(sender: AnyObject) {
         //method to set new image
         detailViewController!.setNewImage()
         detailViewController!.view.setNeedsDisplay()    
     }
  }
  
  class MasterViewController: UITableViewController {
      override func viewDidLoad() {
          super.viewDidLoad()
          let nib = UINib(nibName: "itemCell", bundle: nil)
          tableView.registerNib(nib, forCellReuseIdentifier: "Cell")
          if let split = self.splitViewController {
              let controllers = split.viewControllers
              self.detailViewController = (controllers[controllers.count-1] as! UINavigationController).topViewController as? DetailViewController
          }
      }
  
     override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -&gt; UITableViewCell {
          let cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath) as? MasterViewCell
          cell?.detailView = self.detailViewController
          return cell!
  }
  </code></pre>