拖动一个 UIPanGestureRecognizer 到该 View 上：
界面搭建完成。
属性绑定
切换到双向视图，分别右键拖动 UIPanGestureRecognizer 和该 View 的 Top Space 的 Auto Layout 属性到 ViewController 中绑定：
然后将 UIPanGestureRecognizer 右键拖动绑定：
编写代码
复制代码class ViewController: UIViewController {
var middleViewTopSpaceLayoutConstant: CGFloat!
var middleViewOriginY: CGFloat!
@IBOutlet weak var middleView: UIView!
@IBOutlet weak var middleViewTopSpaceLayout: NSLayoutConstraint!
@IBOutlet var panGesture: UIPanGestureRecognizer!
override func viewDidLoad() {
super.viewDidLoad()
panGesture.addTarget(self, action: Selector("pan"))
middleViewTopSpaceLayoutConstant = middleViewTopSpaceLayout.constant
middleViewOriginY = middleView.frame.origin.y
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
