NSLayoutConstraint 详解

self.view.translatesAutoresizingMaskIntoConstraints = false;
        let leftView = UILabel();
        leftView.backgroundColor = UIColor.redColor();
        leftView.translatesAutoresizingMaskIntoConstraints = false;
        self.view.addSubview(leftView);
        leftView.text = "layout contraint";
        

        
        
        
        let leftConstraint = NSLayoutConstraint(item: leftView, attribute: .Top, relatedBy: .Equal, toItem: self.view, attribute: .Top, multiplier: 1, constant: 40);

        let updaConstraint = NSLayoutConstraint(item: leftView, attribute: .Leading, relatedBy: .Equal, toItem: self.view, attribute: .Leading, multiplier: 1, constant: 40);
        let widthL = NSLayoutConstraint(item: leftView, attribute: .Width, relatedBy: .Equal, toItem: self.view, attribute: .Width, multiplier: 0.2, constant: -20);
        
        let heithL = NSLayoutConstraint(item: leftView, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier:1, constant: 100);

        
        self.view.addConstraints([heithL,widthL,updaConstraint,leftConstraint]);
        
        
        
        let buttom = UILabel();
        buttom.text = "button";
        buttom.backgroundColor = UIColor.greenColor();
        buttom.translatesAutoresizingMaskIntoConstraints = false;
        self.view.addSubview(buttom);
        
        
        let blfetb = NSLayoutConstraint(item: buttom, attribute: .Leading, relatedBy: .Equal, toItem: leftView, attribute: .Trailing, multiplier: 1, constant: 0);
        
        let upbele = NSLayoutConstraint(item: buttom, attribute: .Top, relatedBy: .Equal, toItem: leftView, attribute: .Bottom, multiplier: 1, constant: 0);
        
        let downbel = NSLayoutConstraint(item: buttom, attribute: .Width, relatedBy: .Equal, toItem: leftView, attribute: .Width, multiplier: 1, constant: 0);
        
        let heigjs = NSLayoutConstraint(item: buttom, attribute: .Height, relatedBy: .Equal, toItem: leftView, attribute: .Height, multiplier: 1, constant: 20);
        
        
        self.view.addConstraints([heigjs,downbel,upbele,blfetb]);

 这是一个布局,很简单.那么现在就分析这个例子:

 1.如果使用 NSLayoutConstraint 必须设置 translatesAutoresizingMaskIntoConstraints为false

 2.初始化控件的时候可以先不用设置frame属性的.

 3.使用translatesAutoresizingMaskIntoConstraints之前必须先添加到父View中(也就是如果要对leftView使用自动布局,那么就必须先把leftview添加的它的父view里面,也就是leftView.superView,并且对leftView的约束条件添加到superView上,让superView约束leftView,)

 4.第一个约束的意思是leftView的顶部等于父view的顶部的1倍加40

 这个需要注意几点:1.这是个二元约束,toItem: 的参数不能为空,并且multiplier也不能为0,否则会报错,错误信息是:

 大意是说:乘数为0或者第二个item(也就是view)为空,对第一个item(view)的约束是不合法的.约束条件必须是成对出现.

 细心的同学可能注意到了第三个约束,哦....不,第四个.怎么第二个参数是nil啊

let heithL = NSLayoutConstraint(item: leftView, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier:1, constant: 100);

这一行代码的意识是啥呢.仔细看看,toItem是nil的时候说明是一元约束,attbute必须为NotAnAttribute,这个约束的是自身,比如width,height.这中情况中可以为nil.

这句代码翻译成"中文"意识就是leftView的高等于100;

 好了,剩下的几个自己琢磨一下他们的意思.其实这里是有个公式的的

 如果英文比较好的同学建议还是看看官方文档

  实在不想看的话,那么就简单看一下下面这个公式: