/* Create constraints explicitly. Constraints are of the form "view1.attr1 = view2.attr2 * multiplier + constant"
If your equation does not have a second view and attribute, use nil and NSLayoutAttributeNotAnAttribute.
*/
自动布局核心公式
view1.attr1 = view2.attr2*multiplier+constant
自动布局构造函数
NSLayoutConstraint( item: 视图,
attribute: 约束属性,
relatedBy: 约束关系,
toItem: 参照视图,
attribute: 参照属性,
multiplier: 乘积,
constant: 约束数值)所有衍生的布局工具都以此为基础
iOS UI（布局）约束是什么？view1.attr1 = view2.attr2 * multiplier + constant