:协议
扩展UIView一个方法snap--->ConstraintViewDSL
ConstraintViewDSL
父类
1:target: 一般对uiview就是view2:setlabel设置标签3.lable获取标签
ConstraintView+Extensions
ConstraintMaker(核心)
ConstraintAttributesDSL
makeConstraints->maker执行init(view)持有view(view)
ConstraintDSL
ConstraintBasicAttributesDSL