向子视图前导/尾随/顶部/底部添加约束不起作用

Question

我试图添加固定到其超级视图的前导/尾随/顶部/底部的子视图，但是当我尝试激活约束时，都没有绘制视图。

当我注释掉约束代码时，我看到蓝色视图，激活它们我只是在操场上看到一个黑色视图。

为了帮助调试此问题，我创建了一个游乐场，但仍然无法弄清楚为什么会这样：

//: A UIKit based Playground for presenting user interface


import UIKit
import PlaygroundSupport


class MyView: UIView
{
    override init(frame: CGRect)
    {
        super.init(frame: frame)

        translatesAutoresizingMaskIntoConstraints = false
        backgroundColor = .blue
    }

    required init?(coder aDecoder: NSCoder)
    {
        fatalError()
    }
}


// Present the view controller in the Live View window
let myView = MyView(frame: CGRect(x: 0, y: 0, width: 300, height: 200))

PlaygroundPage.current.liveView = myView


let subview = UIView()
subview.translatesAutoresizingMaskIntoConstraints = false
subview.backgroundColor = .red

myView.addSubview(subview)

subview.leadingAnchor.constraint(equalTo: myView.leadingAnchor).isActive = true
subview.trailingAnchor.constraint(equalTo: myView.trailingAnchor).isActive = true
subview.topAnchor.constraint(equalTo: myView.topAnchor).isActive = true
subview.bottomAnchor.constraint(equalTo: myView.bottomAnchor).isActive = true

Answer 1

因此仅设置框架是不够的，superview必须设置宽度和高度约束。