您需要确保UIStackView的distribution属性设置为.fill或.fillProportionally。
我创建了下面的swift playground，看起来我可以在运行时将setCustomSpacing与随机值一起使用，并看到效果。

import UIKit
import PlaygroundSupport

public class VC: UIViewController {

    let view1 = UIView()
    let view2 = UIView()
    let view3 = UIView()
    var stackView: UIStackView!

    public init() {
        super.init(nibName: nil, bundle: nil)
    }

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

    public override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .white
        view1.backgroundColor = .red
        view2.backgroundColor = .green
        view3.backgroundColor = .blue

        view2.isHidden = true

        stackView = UIStackView(arrangedSubviews: [view1, view2, view3])
        stackView.spacing = 10
        stackView.axis = .horizontal
        stackView.distribution = .fillProportionally

        let uiSwitch = UISwitch()
        uiSwitch.addTarget(self, action: #selector(onSwitch), for: .valueChanged)

        view1.addSubview(uiSwitch)
        uiSwitch.translatesAutoresizingMaskIntoConstraints = false

        NSLayoutConstraint.activate([
            uiSwitch.centerXAnchor.constraint(equalTo: view1.centerXAnchor),
            uiSwitch.centerYAnchor.constraint(equalTo: view1.centerYAnchor)
        ])

        view.addSubview(stackView)
        stackView.translatesAutoresizingMaskIntoConstraints = false

        NSLayoutConstraint.activate([
            stackView.heightAnchor.constraint(equalToConstant: 50),
            stackView.centerYAnchor.constraint(equalTo: view.centerYAnchor),
            stackView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 50),
            stackView.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -50)
        ])
    }

    @objc public func onSwitch(sender: Any) {
        view2.isHidden = !view2.isHidden
        if !view2.isHidden {
            stackView.setCustomSpacing(CGFloat(arc4random_uniform(40)), after: view2)
        }
    }
}

PlaygroundPage.current.liveView = VC()
PlaygroundPage.current.needsIndefiniteExecution = true

setCustomSpacing可能失败的另一个原因是，如果将其称为“在添加要应用间距的已排列子视图之前”。

行不通：

headerStackView.setCustomSpacing(50, after: myLabel)
headerStackView.addArrangedSubview(myLabel)

将工作：

headerStackView.addArrangedSubview(myLabel)
headerStackView.setCustomSpacing(50, after: myLabel)

我还注意到自定义间距值在隐藏/取消隐藏子视图后会被重置。我能够覆盖父视图的updateConstraints()并根据需要设置自定义间距。视图然后保持它们预期的间距。

override func updateConstraints() {
  super.updateConstraints()
  stackView.setCustomSpacing(10, after: childView)
}