let coverWidth = NSLayoutConstraint(item: coverImageView, attribute: .width, relatedBy: .equal, toItem: coverImageView.superview, attribute: .width, multiplier: 1, constant: 0)
let coverHeight = NSLayoutConstraint(item: coverImageView, attribute: .height, relatedBy: .equal, toItem: coverImageView.superview, attribute: .height, multiplier: 1, constant: 0)
coverWidth.isActive = true
coverHeight.isActive = true
image.png
item：约束的组件
attribute：约束的属性
relatedBy：怎么约束？
tiltem：和谁约束
attribute：和它的什么属性约束
multiplier：倍数
converWidth.Active = true 约束生效
