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：約束的組件
attribute：約束的屬性
relatedBy：怎么約束锣险？
tiltem：和誰約束
attribute：和它的什么屬性約束
multiplier：倍數(shù)
converWidth.Active = true 約束生效
