如何在导航栏Swift右侧添加自定义视图？

//创建uiview并添加三个自定义按钮

 func addRightButton(){    let viewFN = UIView(frame: CGRectMake(0, 0, 180,40))        viewFN.backgroundColor = UIColor.yellowColor()    let button1 = UIButton(frame: CGRectMake(0,8, 40, 20))    button1.setImage(UIImage(named: "notification"), forState: UIControlState.Normal)    button1.setTitle("one", forState: .Normal)    button1.addTarget(self, action: #selector(self.didTapOnRightButton), forControlEvents: UIControlEvents.TouchUpInside)    let button2 = UIButton(frame: CGRectMake(40, 8, 60, 20))    button2.setImage(UIImage(named: "notification"), forState: UIControlState.Normal)    button2.setTitle("tow", forState: .Normal)    let button3 = UIButton(frame: CGRectMake(80, 8, 60, 20))     button3.setImage(UIImage(named: "notification"), forState: UIControlState.Normal)    button3.setTitle("three", forState: .Normal)    button3.addTarget(self, action: #selector(self.didTapOnRightButton), forControlEvents: UIControlEvents.TouchUpInside)    viewFN.addSubview(button1)    viewFN.addSubview(button2)    viewFN.addSubview(button3)    let rightBarButton = UIBarButtonItem(customView: viewFN)    self.navigationItem.rightBarButtonItem = rightBarButton}