如何在大于或等于约束的超级视图中居中两个视图

Question

I made an example ViewController with two Labels to highlight my issue. The goal is to vertically separate the labels by 10, and then center them vertically using greater than or equal to constraints. I'm using visual format, but this should apply if I setup my constraints like view.topAnchor.constraint(greaterThan... . I also have two constraints to horizontally layout the labels

My ViewController:

class myVC: UIViewController {
    lazy var titleLabel: UILabel = {
        let l = UILabel(frame: .zero)
        l.translatesAutoresizingMaskIntoConstraints = false
        l.text = "Hello World"
        l.font = .systemFont(ofSize: 50)
        l.textColor = .black
        return l
    }()

    lazy var descLabel: UILabel = {
        let l = UILabel(frame: .zero)
        l.translatesAutoresizingMaskIntoConstraints = false
        l.text = "description"
        l.font = .systemFont(ofSize: 35)
        l.textColor = .gray
        return l
    }()

    override func viewDidLoad() {
        super.viewDidLoad()

        view.backgroundColor = .yellow
        view.addSubview(titleLabel)
        view.addSubview(descLabel)
        titleLabel.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
        descLabel.leadingAnchor.constraint(equalTo: titleLabel.leadingAnchor).isActive = true
        NSLayoutConstraint.activate(NSLayoutConstraint.constraints(withVisualFormat: "V:|-(<=50)-[titleLabel]-(10)-[descLabel]-(<=50)-|", options: .init(), metrics: nil, views: ["titleLabel": titleLabel, "descLabel": descLabel]))
    }

}

This results in . From my understanding, this SHOULD separate the views by 10 pts, and center the labels vertically because in the format "V:|-(<=50)-[titleLabel]-(10)-[descLabel]-(<=50)-|" I say that the distance between the Title Label's top and the superView's top should be at least (greaterThanOrEqualTo) 50, and the distance between the description Label's bottom and the superView's bottom should be at least 50. What should my top and bottom constraints look like if I want to center the two labels vertically?

Yes, I realize I can just set vertical and horizontal centers, but this is an example I made for a problem I can't use those for. I need to be able to center the View with greater(or less) than or equal to constraints.

Answer 1

It's very difficult to center elements using VFL.

It's also difficult to center two elements unless they are embedded in a UIView or a UIStackView .

Here is one option by embedding the labels in a "container" UIView :

class MyVC: UIViewController {
    lazy var titleLabel: UILabel = {
        let l = UILabel(frame: .zero)
        l.translatesAutoresizingMaskIntoConstraints = false
        l.text = "Hello World"
        l.font = .systemFont(ofSize: 50)
        l.textColor = .black

        // center the text in the label - change to .left if desired
        l.textAlignment = .center

        return l
    }()

    lazy var descLabel: UILabel = {
        let l = UILabel(frame: .zero)
        l.translatesAutoresizingMaskIntoConstraints = false
        l.text = "description"
        l.font = .systemFont(ofSize: 35)
        l.textColor = .gray

        // center the text in the label - change to .left if desired
        l.textAlignment = .center

        return l
    }()

    lazy var containerView: UIView = {
        let v = UIView()
        v.translatesAutoresizingMaskIntoConstraints = false
        return v
    }()

    override func viewDidLoad() {
        super.viewDidLoad()

        view.backgroundColor = .yellow

        // give the labels and containerView background colors to make it easy to see the layout
        titleLabel.backgroundColor = .green
        descLabel.backgroundColor = .cyan
        containerView.backgroundColor = .blue

        // add containerView to view
        view.addSubview(containerView)

        // add labels to containerView
        containerView.addSubview(titleLabel)
        containerView.addSubview(descLabel)

        NSLayoutConstraint.activate([

            // constrain titleLabel Top to containerView Top
            titleLabel.topAnchor.constraint(equalTo: containerView.topAnchor),

            // constrain titleLabel Leading and Trailing to containerView Leading and Trailing
            titleLabel.leadingAnchor.constraint(equalTo: containerView.leadingAnchor),
            titleLabel.trailingAnchor.constraint(equalTo: containerView.trailingAnchor),

            // constrain descLabel Leading and Trailing to containerView Leading and Trailing
            descLabel.leadingAnchor.constraint(equalTo: containerView.leadingAnchor),
            descLabel.trailingAnchor.constraint(equalTo: containerView.trailingAnchor),

            // constrain descLabel Bottom to containerView Bottom
            descLabel.bottomAnchor.constraint(equalTo: containerView.bottomAnchor),

            // constrain descLabel Top 10-pts from titleLabel Bottom
            descLabel.topAnchor.constraint(equalTo: titleLabel.bottomAnchor, constant: 10.0),

            // constrain containerView centered horizontally and vertically
            containerView.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            containerView.centerYAnchor.constraint(equalTo: view.centerYAnchor),

        ])

    }

}

Result:

Answer 2

This can be achieved easily by using stackview. Add both the labels in stackview and center it vertically in the superview with all other constraints(top, leading, bottom, trailing). Here is the sample code of view controller for your use-case.

class ViewController: UIViewController {

    lazy var titleLabel: UILabel = {
        let label = UILabel()
        label.translatesAutoresizingMaskIntoConstraints = false
        label.text = "Hello \nWorld"
        label.font = .systemFont(ofSize: 50)
        label.backgroundColor = .orange
        label.numberOfLines = 0
        label.textColor = .black
        return label
    }()

    lazy var descLabel: UILabel = {
        let label = UILabel()
        label.translatesAutoresizingMaskIntoConstraints = false
        label.text = "a\n b\n c\n"
        label.font = .systemFont(ofSize: 35)
        label.backgroundColor = .green
        label.numberOfLines = 0
        label.textColor = .gray
        return label
    }()

    lazy var contentView: UIStackView = {
        let stackView = UIStackView()
        stackView.axis = .vertical
        stackView.translatesAutoresizingMaskIntoConstraints = false
        stackView.spacing = 10
        stackView.distribution = .fill
        return stackView
    }()

    override func viewDidLoad() {
        super.viewDidLoad()

        self.view.backgroundColor = UIColor.white

        contentView.addArrangedSubview(titleLabel)
        contentView.addArrangedSubview(descLabel)
        self.view.addSubview(contentView)

        let constraints = [
            contentView.topAnchor.constraint(greaterThanOrEqualTo: view.safeAreaLayoutGuide.topAnchor),
            contentView.centerYAnchor.constraint(equalTo: view.centerYAnchor),
            contentView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            contentView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            contentView.bottomAnchor.constraint(lessThanOrEqualTo: view.safeAreaLayoutGuide.bottomAnchor)
        ]
        NSLayoutConstraint.activate(constraints)

    }
}

The above code will result this view and it goes on to take the top and buttom space until it meets the safeArea. Moreover you can set the vertical content hugging and compression resistance priority to control which label to expand or shrink.