UIView内部约束

Question

我创建了一个简单的UIView，其中心包含一个红色框（UIImage）。当所有约束都是常量时，代码工作正常。但是，如果我将高度约束替换为使得框的高度为视图高度的一半，则框会消失。

我认为这是因为我做错了（显然）或者我需要做更多的事情来强制约束来实现UIView高度大于零。

如何设置redBox高度约束，使其始终是BoxView高度的一半？

import UIKit

class BoxView: UIView {

    public var redBox: UIImageView

    public override init(frame: CGRect) {
        redBox = UIImageView(frame: .zero)
        redBox.backgroundColor = .red

        super.init(frame: frame)
        self.backgroundColor = .yellow

        addSubview(redBox)
        redBox.translatesAutoresizingMaskIntoConstraints = false
        let margins = layoutMarginsGuide
        redBox.centerXAnchor.constraint(equalTo: self.centerXAnchor).isActive = true
        redBox.centerYAnchor.constraint(equalTo: self.centerYAnchor).isActive = true
        redBox.widthAnchor.constraint(equalToConstant: 100).isActive = true
      //redBox.heightAnchor.constraint(equalToConstant: 100).isActive = true
        redBox.heightAnchor.constraint(equalTo: self.heightAnchor, constant: 0.5)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        view = BoxView()
    }
}

Answer 1

更换

redBox.heightAnchor.constraint(equalTo: self.heightAnchor, constant: 0.5)

同

redBox.heightAnchor.constraint(equalTo: self.heightAnchor, multiplier: 0.5).isActive = true

---

NSLayoutConstraint.activate([
    redBox.centerXAnchor.constraint(equalTo: self.centerXAnchor),
    redBox.centerYAnchor.constraint(equalTo: self.centerYAnchor),
    redBox.widthAnchor.constraint(equalToConstant: 100),
    redBox.heightAnchor.constraint(equalTo: self.heightAnchor, multiplier: 0.5)
])

在你当前的代码中，首先你会错过.isActive = true，它具有与行不存在时相同的效果，如果指定这将使框高度等于视图的高度+常量（= 0.5）

box height = view height * multiplier + constant

并且由于默认乘数= 1并且您设置常数= 0.5，这将是

box height = view height * 1.0 + 0.5

但你需要

box height = view height * 0.5 + 0 //在约束中省略常量，它将为零

---

class BoxView: UIView { 
    public var redBox: UIImageView
    public override init(frame: CGRect) {
        super.init(frame: frame)
        redBox = UIImageView(frame: .zero)
        redBox.backgroundColor = .red
        self.backgroundColor = .yellow
        addSubview(redBox)
        redBox.translatesAutoresizingMaskIntoConstraints = false
        let margins = layoutMarginsGuide

        NSLayoutConstraint.activate([
            redBox.centerXAnchor.constraint(equalTo: self.centerXAnchor),
            redBox.centerYAnchor.constraint(equalTo: self.centerYAnchor),
            redBox.widthAnchor.constraint(equalToConstant: 100),
            redBox.heightAnchor.constraint(equalTo: self.heightAnchor, multiplier: 0.5)
        ])
    }
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}