如何在大于或等于约束的超级视图中居中两个视图

Question

我制作了一个带有两个标签的ViewController示例，以突出显示我的问题。目标是将标签垂直分隔10，然后使用大于或等于约束将标签垂直居中。我使用的是视觉格式，但是如果我设置了view.topAnchor.constraint(greaterThan...这样的约束，则该格式将适用。我也有两个约束来水平布局标签

我的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]))
    }

}

此结果为。根据我的理解，这应该将视图分开10个点，并将标签垂直居中，因为在格式"V:|-(<=50)-[titleLabel]-(10)-[descLabel]-(<=50)-|"中，我说标题标签的顶部和superView的顶部之间的距离应至少为（greaterThanOrEqualTo）50，并且描述标签底部和superView底部之间的距离应至少为50。如果我想垂直放置两个标签，我的顶部和底部约束应该是什么样？

是的，我意识到我可以设置垂直和水平中心，但这是我为无法解决的问题而制作的示例。我需要能够使View的中心大于（或小于）约束。

Answer 1

使用VFL将元素居中非常困难。

除非将两个元素嵌入UIView或UIStackView中，否则也很难将它们居中。

这里是通过将标签嵌入“容器” UIView中的一种选择：

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),

        ])

    }

}

结果：

Answer 2

这可以通过使用stackview轻松实现。将两个标签都添加到stackview中，并在顶部，前导，底部和尾随约束中将它们居中。这是您的用例的视图控制器的示例代码。

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)

    }
}

上面的代码将生成此视图，并继续占据顶部和底部的空间，直到达到safeArea为止。此外，您可以设置垂直内容拥抱和抗压优先级，以控制要扩展或收缩的标签。