iOS UI（布局）约束是什么？view1.attr1 = view2.attr2 * multiplier + constant

/* 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