为什么UIButton需要约束才能出现在UIStackView中

Question

鉴于以下Playground代码，我希望我的两个按钮可以填满整个StackView。

l = [(x, data.copy()) for x in range(20, 24)]

但是没有一个出现！但是，如果我向任一按钮添加1px的宽度约束，则两个按钮都按预期布局。

import UIKit

let button = UIButton(type: .custom)
button.setTitle("My Button", for: .normal)
button.backgroundColor = .purple

let anotherButton = UIButton(type: .custom)
anotherButton.setTitle("Another Button", for: .normal)
anotherButton.backgroundColor = .orange

var stackView: UIStackView = UIStackView(frame: CGRect(x: 0.0, y: 0.0, width: 300, height: 300))
stackView.axis = .vertical
stackView.spacing = 10.0
stackView.distribution = .fillEqually
stackView.alignment = .fill

stackView.addArrangedSubview(button)
stackView.addArrangedSubview(anotherButton)
stackView

我很困惑！为什么这个单一约束会产生影响？我应该做一些其他事情，而不是添加“虚拟约束”，以强制按钮正确布局吗？

Answer 1

它可能是游乐场中的一个错误，但这在app中与我合作，没有任何 widthAnchor 约束

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.

    let button = UIButton(type: .custom)
    button.setTitle("My Button", for: .normal)
    button.backgroundColor = .purple



    let anotherButton = UIButton(type: .custom)
    anotherButton.setTitle("Another Button", for: .normal)
    anotherButton.backgroundColor = .orange

    var stackView: UIStackView = UIStackView(frame: CGRect(x: 0.0, y: 100.0, width: 300, height: 300))
    stackView.axis = .vertical
    stackView.spacing = 10.0
    stackView.distribution = .fillEqually
    stackView.alignment = .fill

    stackView.addArrangedSubview(button)
    stackView.addArrangedSubview(anotherButton)

    self.view.addSubview(stackView)
}