1. 动画
// 修改从 StoryBoard 绑定到类的约束的值
self.boxView.constant += 80 // 在动画闭包里对其父级进行 layoutIfNeeded()
UIView.animateWithDuration(0.5, animations: { () -> Void in
self.view.layoutIfNeeded()
})
2. frame的确定
viewDidLoad
viewWillAppear
viewWillLayoutSubviews
viewDidLayoutSubviews // frame 确定
viewDidAppear
viewWillDisappear
viewDidDisappear
3. why?