迅捷：如何在代码中使用约束与乘数

Question

I have a custom view and I want to add an image such that it's bottom anchor is at 3/4 of the view. In the storyboard I would just set a constraint with first item imgview.bottom, second item view.bottom, constant 0 and multiplier 0.75

In code I tried:

let imgview = UIImageView(image: img)
imgview.translatesAutoresizingMaskIntoConstraints = false
addSubview(imgview)
NSLayoutConstraint.activate([
    imgview.widthAnchor.constraint(equalTo: widthAnchor, multiplier: 0.4),
    imgview.heightAnchor.constraint(equalTo: heightAnchor, multiplier: 0.4),
    imgview.centerXAnchor.constraint(equalTo: centerXAnchor),
    imgview.bottomAnchor.constraint(equalToSystemSpacingBelow: bottomAnchor, multiplier: 0.75)
    ])

but the bottom of the image is even below the bottom of the view. If I use bottomAnchor.constraint(equalToSystemSpacingBelow: imgview.bottomAnchor, multiplier: 0.75)

It is above it but still too low. How do I do this?

Answer 1

I would have done it using equalTo:,constant: .

let frameHeight = frame.height
let padding = frameHeight * 0.25
imgview.bottomAnchor.constraint(equalTo: bottomAnchor, constant: -padding)

Frame value might return 0 if the view is not loaded yet.

override func layoutSubviews() {
super.layoutSubviews()
if self.frame.height > 0 {
//Add constraints here
}
}

Answer 2

Use constraint creation for that last one the old-fashion way:

let imgview = UIImageView(image: img)
imgview.translatesAutoresizingMaskIntoConstraints = false
addSubview(imgview)
NSLayoutConstraint.activate([
    imgview.widthAnchor.constraint(equalTo: widthAnchor, multiplier: 0.4),
    imgview.heightAnchor.constraint(equalTo: heightAnchor, multiplier: 0.4),
    imgview.centerXAnchor.constraint(equalTo: centerXAnchor),
/// imgview.bottomAnchor.constraint(equalToSystemSpacingBelow: bottomAnchor, multiplier: 0.75)
    ])

let constraint = NSLayoutConstraint(item: imgview, attribute: .bottom, 
                                    relatedBy: .equal, 
                                    toItem: view, attribute: .bottom, 
                                    multiplier: 0.75, constant: 0)
constraint.isActive = true