ios - 添加 subview 并以编程方式使用约束定位它

Question

我在 View Controller 上使用 IB 添加了一个 UIView。我们称它为 redView。

然后我在代码中使用自动布局约束固定了它的所有四个边，当我运行它时，它看起来像预期的那样。

现在我想以编程方式向此 View 添加一个 UILabel 并使用自动布局约束将其定位在中心。

下面是我到目前为止的代码。

import UIKit

class ViewController: UIViewController {

    @IBOutlet private var redView: UIView!

    override func viewDidLoad() {
        super.viewDidLoad()

        redView.setTranslatesAutoresizingMaskIntoConstraints(false)

        let leadingConstraint = NSLayoutConstraint(item: redView, attribute: .Leading, relatedBy: .Equal, toItem: view, attribute: .Leading, multiplier: 1, constant: 0)
        let trailingConstraint = NSLayoutConstraint(item: redView, attribute: .Trailing, relatedBy: .Equal, toItem: view, attribute: .Trailing, multiplier: 1, constant: 0)
        let topConstraint = NSLayoutConstraint(item: redView, attribute: .Top, relatedBy: .Equal, toItem: view, attribute: .Top, multiplier: 1, constant: 0)
        let bottomConstraint = NSLayoutConstraint(item: redView, attribute: .Bottom, relatedBy: .Equal, toItem: view, attribute: .Bottom, multiplier: 1, constant: 0)
        view.addConstraints([leadingConstraint, trailingConstraint, topConstraint, bottomConstraint])


        let label = UILabel()
        label.text = "Auto Layout Exercise"
        redView.addSubview(label)

        let xCenterConstraint = NSLayoutConstraint(item: label, attribute: .CenterX, relatedBy: .Equal, toItem: redView, attribute: .CenterX, multiplier: 1, constant: 0)
        let yCenterConstraint = NSLayoutConstraint(item: label, attribute: .CenterY, relatedBy: .Equal, toItem: redView, attribute: .CenterY, multiplier: 1, constant: 0)
        let leadingConstraint1 = NSLayoutConstraint(item: label, attribute: .Leading, relatedBy: .Equal, toItem: redView, attribute: .Leading, multiplier: 1, constant: 20)
        let trailingConstraint1 = NSLayoutConstraint(item: label, attribute: .Trailing, relatedBy: .Equal, toItem: redView, attribute: .Trailing, multiplier: 1, constant: -20)
        redView.addConstraints([xCenterConstraint, yCenterConstraint, leadingConstraint1, trailingConstraint1])

    }

}

问题是标签没有出现在 redView 上。谁能告诉我这里缺少什么？

谢谢。

Answer 1

我很确定你还需要设置:

label.setTranslatesAutoresizingMaskIntoConstraints(false)

否则我认为约束不会应用于标签。