UIKit: Relative autolayout constraints programmatically-CodePudding

I'm using swift UIKit to autolayout a view programatically...

private func autoLayout(for child: UIView, in parent: UIView, width inset: CGFloat = 0) {
    parent.addSubview(child)
    
    child.translatesAutoresizingMaskIntoConstraints = false
    child.topAnchor     .constraint(equalTo: parent.safeAreaLayoutGuide.topAnchor,      constant: inset) .isActive = true
    child.bottomAnchor  .constraint(equalTo: parent.safeAreaLayoutGuide.bottomAnchor,   constant: -inset).isActive = true
    child.leadingAnchor .constraint(equalTo: parent.safeAreaLayoutGuide.leadingAnchor,  constant: inset) .isActive = true
    child.trailingAnchor.constraint(equalTo: parent.safeAreaLayoutGuide.trailingAnchor, constant: -inset).isActive = true
}

In one instance I'm calling the method on a UILabel (with a set font size: 24), but I'd like to add a height constraint that adjusts the parent view to be 2.4x the child's height. Something like this inside the method...

if let label = child as? UILabel {
    let value = child.frame.height * 2.4
    parent.heightAnchor.constraint(equalToConstant: value).isActive = true
}

If I use the equalToConstant: parameter, then the size will be calculated when the method is called, which happens before the views are added to a UIStackView and their sizes are adjusted (essentially if I set the parent's constant equal to say child.frame.height * 2.4 it will calculate when the height is still zero).

How can I achieve this dynamically?

CodePudding user response：

All you need to do is create a height constraint on the parent that is 2.4 times the height constraint of the child (label).

Here's an updated version of your code:

private func autoLayout(for child: UIView, in parent: UIView, width inset: CGFloat = 0) {
    parent.addSubview(child)

    child.translatesAutoresizingMaskIntoConstraints = false
    child.leadingAnchor .constraint(equalTo: parent.safeAreaLayoutGuide.leadingAnchor,  constant: inset) .isActive = true
    child.trailingAnchor.constraint(equalTo: parent.safeAreaLayoutGuide.trailingAnchor, constant: -inset).isActive = true

    if child is UILabel {
        // This alternate syntax for creating a constraint allows you to set a multiplier
        NSLayoutConstraint(item: parent, attribute: .height, relatedBy: .equal, toItem: child, attribute: .height, multiplier: 2.4, constant: 0).isActive = true
        child.centerYAnchor.constraint(equalTo: parent.safeAreaLayoutGuide.centerYAnchor).isActive = true
    } else {
        child.topAnchor   .constraint(equalTo: parent.safeAreaLayoutGuide.topAnchor,    constant: inset) .isActive = true
        child.bottomAnchor.constraint(equalTo: parent.safeAreaLayoutGuide.bottomAnchor, constant: -inset).isActive = true
    }
}

I made an assumption that for the label you just want the label to be centered in the parent in addition to the parent's height being 2.4 times the child height. Adjust as needed.