iOS9.0 NSLayoutAnchor初探

NSLayoutAnchor API是iOS9版本引入,不僅讓約束聲明更加清晰明了，而且還通過靜態類型檢查以確保約束能夠正常工作,

其實是一個工廠類,類似NSNumber這樣的設計思想.

NSLayoutAnchor用來創建NSLayoutConstraint對象,使用這些對象從而實現自動布局.

但是一般不會直接創建NSLayoutConstraint對象,而是用UIView(NSView)或者其子類,或者UILayoutGuide的某個anchor屬性(比如centerXAnchor),

這些屬性對應Auto Layout中主要的NSLayoutAttribute值(InterfaceBuilder下屬性欄可以看到),所以也可以用NSLayoutAnchor子類創建這些NSLayoutAttribute值.

   個人使用之后,感覺就是語法更加易於理解和使用了,和Masonry語法一樣親切.

    主意:UIView本身並沒有提供anchor屬性對應Auto Layout的margin屬性,但是UILayoutGuide有這樣的屬性與之對應.

 

1.使用NSLayoutConstraint創建

constraints,實現一個100*100大小,左邊和superView相距20,頂部相距100的View

2.使用LayoutAnchor實現類似的約束

對比可以看到NSLayoutAnchor類提供了更有優勢的NSLayoutConstaint API

        :1.更加整潔,優雅,易讀

        2.通過NSLayoutAnchor中的方法來約束錨點參數以及作為接收器的相同泛型類型(NSLayoutAttribute)，API 便能夠使用類型檢查以確保能夠創建出有效的約束

        主意:盡管NSLayoutAnchor類會進行類型檢測,但然不能一定確保創建的約束是有效的.比如一個View的leadingAnchor和另外一個View的leftAnchor,進行約束,盡管都是NSLayoutXAnchor的實例,編譯也能通過,但是,Auto Layout不允許leading和trailling的屬性和left或者right混和約束.會導致在運行時崩潰的結果.

        Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** +[NSLayoutConstraint constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:]: A constraint cannot be made between a leading/trailing attribute and a right/left attribute. Use leading/trailing for both or neither.'

        *** First throw call stack:

        */

實現的效果

 

3.NSLayoutAnchor的練習

實現2個view大小一樣,和屏幕距離為20,

let yellowView = UIView()
        yellowView.backgroundColor = UIColor.yellowColor()
        yellowView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(yellowView)
        
        let blueView = UIView()
        blueView.backgroundColor = UIColor.blueColor()
        blueView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(blueView)
        
        //LayoutAnchor約束

      let yLeftCon = yellowView.leftAnchor.constraintEqualToAnchor(view.leftAnchor, constant: 20)
      let ytopCon = yellowView.topAnchor.constraintEqualToAnchor(view.topAnchor, constant: 250)
      let yHeightCon = yellowView.heightAnchor.constraintEqualToConstant(150)
      let yWidthCon = yellowView.widthAnchor.constraintEqualToAnchor(blueView.widthAnchor)
     let yRightCon = yellowView.rightAnchor.constraintEqualToAnchor(blueView.leftAnchor,constant: -100)
        
      let bLeftCon = blueView.leftAnchor.constraintEqualToAnchor(yellowView.rightAnchor, constant: 100)
     let bTopCon = blueView.topAnchor.constraintEqualToAnchor(yellowView.topAnchor)
     let bRightCon = blueView.rightAnchor.constraintEqualToAnchor(view.rightAnchor, constant: -20)
     let bWidthCon = blueView.widthAnchor.constraintEqualToAnchor(yellowView.widthAnchor)
    let bHeightCon = blueView.heightAnchor.constraintEqualToAnchor(yellowView.heightAnchor)
        
        NSLayoutConstraint.activateConstraints([yLeftCon,ytopCon,yHeightCon,yWidthCon,yRightCon,bLeftCon,bTopCon,bRightCon,bWidthCon,bHeightCon])

　　

最終的效果

 

4.Layout動畫的實現

 

let conX =  iconView.centerXAnchor.constraintEqualToAnchor(view.centerXAnchor)
        conY =  iconView.centerYAnchor.constraintEqualToAnchor(view.centerYAnchor)
      let conW = iconView.widthAnchor.constraintEqualToConstant(100.0)
       let conH = iconView.heightAnchor.constraintEqualToConstant(100.0)
        
        NSLayoutConstraint.activateConstraints([conX,conY,conW,conH])


override func viewDidAppear(animated: Bool) {
        super.viewDidAppear(animated)
        UIView.animateWithDuration(1.0, delay: 2.0, usingSpringWithDamping: 0.4, initialSpringVelocity: 0.0, options: [], animations: { () -> Void in
            self.conY.constant -= 100
            self.view.layoutIfNeeded()
            }, completion: nil)

        
    }

　　會看到進入界面2秒后,iconView在Y方向上有一個位移彈性動畫

關於更多的NSLayout Anchor可以查閱文檔,更深入了解,實現自己想要的效果