如何在不崩溃的情况下向 UIView 实例的子视图添加约束？

Question

我在 Swift 中有一个 UIView 实例，我在其中添加了一个 UIImageView 实例的子视图，但我无法在我的应用程序崩溃的情况下向 UIImageView 实例添加约束。

我曾尝试在 Xcode 10.2 和 iOS 12.2 中编写 Swift 5.0 代码来设置 UIImageView 实例的前沿距 UIView 实例的左边缘 50 的约束。

override func tableView(_ tableView: UITableView, viewForHeaderInSection section: Int) -> UIView? {

    let sectionHeaderView : UIView = UIView()

    let sectionHeaderImage: UIImage = UIImage(named: "Flag - ENGLAND.png")!
    let sectionHeaderImageView : UIImageView = UIImageView(image: sectionHeaderImage)
    sectionHeaderImageView.translatesAutoresizingMaskIntoConstraints = false

    let imageViewWidthConstraint : NSLayoutConstraint = NSLayoutConstraint(item: sectionHeaderImageView, attribute: .width, relatedBy: .equal, toItem: sectionHeaderImageView, attribute: .width, multiplier: 0.0, constant: 40)
    imageViewWidthConstraint.isActive = true
    sectionHeaderImageView.addConstraint(imageViewWidthConstraint)

    let imageViewHeightConstraint : NSLayoutConstraint = NSLayoutConstraint(item: sectionHeaderImageView, attribute: .height, relatedBy: .equal, toItem: sectionHeaderImageView, attribute: .height, multiplier: 0.0, constant: 40)
    imageViewHeightConstraint.isActive = true
    sectionHeaderImageView.addConstraint(imageViewHeightConstraint)

    let imageViewLeadingConstraint : NSLayoutConstraint = NSLayoutConstraint(item: sectionHeaderImageView, attribute: .leading, relatedBy: .equal, toItem: sectionHeaderView, attribute: .left, multiplier: 1.0, constant: 50)
    imageViewLeadingConstraint.isActive = true
    sectionHeaderImageView.addConstraint(imageViewLeadingConstraint)

    return sectionHeaderView
}

我希望包含 UIImageView 实例的 UIView 实例位于 UIView 实例右侧 50 处。相反，应用程序在 Xcode 中崩溃，并在文件 AppDelegate.swift 中显示错误消息“线程 1：信号 SIGABRT”。

Answer 1

这行得通吗？

 override func tableView(_ tableView: UITableView, viewForHeaderInSection section: Int) -> UIView? {
    let sectionHeaderView : UIView = UIView()

    let sectionHeaderImage: UIImage = UIImage(named: "Flag - ENGLAND.png")!
    let sectionHeaderImageView : UIImageView = UIImageView(image: sectionHeaderImage)
    sectionHeaderImageView.translatesAutoresizingMaskIntoConstraints = false

    let imageViewWidthConstraint : NSLayoutConstraint = NSLayoutConstraint(item: sectionHeaderImageView, attribute: .width, relatedBy: .equal, toItem: sectionHeaderImageView, attribute: .width, multiplier: 0.0, constant: 40)
    imageViewWidthConstraint.isActive = true
    sectionHeaderImageView.addConstraint(imageViewWidthConstraint)

    let imageViewHeightConstraint : NSLayoutConstraint = NSLayoutConstraint(item: sectionHeaderImageView, attribute: .height, relatedBy: .equal, toItem: sectionHeaderImageView, attribute: .height, multiplier: 0.0, constant: 40)
    imageViewHeightConstraint.isActive = true
    sectionHeaderImageView.addConstraint(imageViewHeightConstraint)

    let imageViewLeadingConstraint : NSLayoutConstraint = NSLayoutConstraint(item: sectionHeaderImageView, attribute: .leading, relatedBy: .equal, toItem: sectionHeaderView, attribute: .left, multiplier: 1.0, constant: 50)
    imageViewLeadingConstraint.isActive = true

    sectionHeaderView.addSubview(sectionHeaderImageView) // add subview first
    sectionHeaderImageView.addConstraint(imageViewLeadingConstraint)
    return sectionHeaderView
}

Answer 2

这段代码应该可以工作：

override func tableView(_ tableView: UITableView, viewForHeaderInSection section: Int) -> UIView? {

    let sectionHeaderView = UIView()
    let sectionHeaderImage = UIImage(named: "Flag - ENGLAND.png")!
    let sectionHeaderImageView = UIImageView(image: sectionHeaderImage)
    sectionHeaderImageView.translatesAutoresizingMaskIntoConstraints = false
    sectionHeaderView.addSubview(sectionHeaderImageView)

    NSLayoutConstraint.activate([
        sectionHeaderImageView.widthAnchor.constraint(equalToConstant: 40),
        sectionHeaderImageView.heightAnchor.constraint(equalToConstant: 40),
        sectionHeaderImageView.leadingAnchor.constraint(equalTo: sectionHeaderView.leadingAnchor, constant: 50),
        sectionHeaderImageView.topAnchor.constraint(equalTo: sectionHeaderView.topAnchor, constant: 50)
    ])
    return sectionHeaderView
}

Answer 3

首先你必须将图像添加到父视图

 override func tableView(_ tableView: UITableView, viewForHeaderInSection section: Int) -> UIView? {
    let sectionHeaderView: UIView = UIView()

    let sectionHeaderImage: UIImage = UIImage(named: "Flag - ENGLAND.png")!
    let sectionHeaderImageView: UIImageView = UIImageView(image: sectionHeaderImage)

        sectionHeaderImageView.translatesAutoresizingMaskIntoConstraints = false

        sectionHeaderView.addSubview(sectionHeaderImageView)

        let imageViewHeightConstraint = NSLayoutConstraint(
            item: sectionHeaderImageView,
            attribute: .height,
            relatedBy: .equal,
            toItem: nil,
            attribute: .notAnAttribute,
            multiplier: 1.0,
            constant: 40
        )
        let imageViewWidthConstraint = NSLayoutConstraint(
            item: sectionHeaderImageView,
            attribute: .width,
            relatedBy: .equal,
            toItem: nil,
            attribute: .notAnAttribute,
            multiplier: 1.0,
            constant: 40
        )
        let imageViewLeadingConstraint = NSLayoutConstraint(
            item: sectionHeaderImageView,
            attribute: .left,
            relatedBy: .equal,
            toItem: sectionHeaderView,
            attribute: .left,
            multiplier: 1.0,
            constant: 50
        )
        sectionHeaderView.addConstraints([
            imageViewHeightConstraint,
            imageViewWidthConstraint,
            imageViewLeadingConstraint
            ])
    }
    return sectionHeaderView
}