swift纯代码实现自动布局

早期的IOS设备都是固定屏幕的尺寸，所以大家写iOS应用几乎都是用frame布局，直接写死宽高和坐标。
但是iPhone6开始，iOS设备出现了多种尺寸，于是苹果推出了一种新的布局方式：`autolayout`

autolaout又名自动布局，或者是是相对布局。使用autolayout可以轻易写出目前主流的所有页面布局。

关闭autoresizing布局方式

在 iOS中，默认使用aotoresizing的布局方式来写页面，autoresizing相对来说比较简单。只做了相对于父控件的大小的伸缩。使用起来不是特别顺手。
控件的translatesAutoresizingMaskIntoConstraints为false，可以启用自动布局

NSLayoutConstraint简单了解

顾名思义，这是创建一个布局约束

NSLayoutConstraint(item: 当前的控件, attribute: 约束属性, relatedBy: 前后之间的关系, toItem: 相对的控件, attribute: 相对控件的某个属性, multiplier: 比例, constant: 常量)

比如

NSLayoutConstraint(item:myview , attribute: .left, relatedBy: .equal, toItem: self.view, attribute: .left, multiplier: 1, constant: 0)

意思就是myview左边相对于self.view的左边，距离为0

创建一个与ViewController一样大的View

 let myview = UIView();
        self.view.addSubview(myview)
        myview.translatesAutoresizingMaskIntoConstraints = false
        
        myview.backgroundColor = UIColor.systemBlue
        let left = NSLayoutConstraint(item:myview , attribute: .left, relatedBy: .equal, toItem: self.view, attribute: .left, multiplier: 1, constant: 0)
        let right = NSLayoutConstraint(item: myview
                                       , attribute: .right, relatedBy: .equal, toItem: self.view
                                       , attribute: .right, multiplier: 1, constant: 0)
        let top = NSLayoutConstraint(item: myview
                                     , attribute: .top, relatedBy: .equal, toItem: self.view, attribute: .top, multiplier: 1, constant: 0)
        let bottom = NSLayoutConstraint(item: myview
                                        , attribute: .bottom, relatedBy: .equal, toItem: self.view, attribute: .bottom, multiplier: 1, constant: 0)
        myview.superview?.addConstraints([left,right,top,bottom])

约束某个某个属性为固定值

比如我们需要设置当前view的宽度为100.高度为200

let myview = UIView();
        self.view.addSubview(myview)
        myview.translatesAutoresizingMaskIntoConstraints = false
        
        myview.backgroundColor = UIColor.systemBlue
        let left = NSLayoutConstraint(item:myview , attribute: .left, relatedBy: .equal, toItem: self.view, attribute: .left, multiplier: 1, constant: 0)
        let width = NSLayoutConstraint(item: myview
                                       , attribute: .width, relatedBy: .equal, toItem: nil
                                       , attribute: .notAnAttribute, multiplier: 1, constant: 100)
        let top = NSLayoutConstraint(item: myview
                                     , attribute: .top, relatedBy: .equal, toItem: self.view, attribute: .top, multiplier: 1, constant: 0)
        let height = NSLayoutConstraint(item: myview
                                        , attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 200)
        myview.superview?.addConstraints([left,width,top,height])

View居中

 let myview = UIView();
        self.view.addSubview(myview)
        myview.translatesAutoresizingMaskIntoConstraints = false
        
        myview.backgroundColor = UIColor.systemBlue
        let centerX = NSLayoutConstraint(item:myview , attribute: .centerX, relatedBy: .equal, toItem: self.view, attribute: .centerX, multiplier: 1, constant: 0)
        let width = NSLayoutConstraint(item: myview
                                       , attribute: .width, relatedBy: .equal, toItem: nil
                                       , attribute: .notAnAttribute, multiplier: 1, constant: 100)
        let centerY = NSLayoutConstraint(item: myview
                                     , attribute: .centerY, relatedBy: .equal, toItem: self.view, attribute: .centerY, multiplier: 1, constant: 0)
        let height = NSLayoutConstraint(item: myview
                                        , attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 200)
        myview.superview?.addConstraints([centerX,width,centerY,height])

View动画