Question

我正在使用继承自UIView class MyView: UIView {

的类来创建自己的类

这个想法是创建一个视图并在其中放置一些标签。我是通过以下方式实现的：

init() {
    super.init(frame: CGRect(x: 8.0, y: 104.0, width: UIScreen.main.bounds.width - 16.0, height: 60.0))
    initialize()
}

其中

fileprivate func initialize() {        
    label = UILabel()
    label.numberOfLines = 0
    label = UILabel(frame: CGRect(x: 16.0, y: 8.0, width: self.frame.width - 32.0, height: self.frame.height - 16.0))
    self.addSubview(label)
}

，现在一切正常。但是现在我想使此自定义视图的高度动态化，因此视图的高度将取决于其中标签的高度（文本）。

我尝试添加：

let constraints = [
        NSLayoutConstraint(item: label, attribute: .top, relatedBy: .equal, toItem: self, attribute: .top, multiplier: 1.0, constant: 8.0),
        NSLayoutConstraint(item: label, attribute: .leading, relatedBy: .equal, toItem: self, attribute: .leading, multiplier: 1.0, constant: 8.0),
        NSLayoutConstraint(item: label, attribute: .trailing, relatedBy: .equal, toItem: self, attribute: .trailing, multiplier: 1.0, constant: 8.0),
        NSLayoutConstraint(item: label, attribute: .bottom, relatedBy: .equal, toItem: self, attribute: .bottom, multiplier: 1.0, constant: 8.0)]

    label.addConstraints(constraints)

和

self.translatesAutoresizingMaskIntoConstraints = false
    self.heightAnchor.constraint(greaterThanOrEqualToConstant: 60.0).isActive = true

最终，我在日志中遇到了一个巨大的错误：

The view hierarchy is not prepared for the constraint
When added to a view, the constraint's items must be descendants of that view (or the view itself). This will crash if the constraint needs to be resolved before the view hierarchy is assembled.

有人可以帮助我解决这个问题吗？我已经用谷歌搜索了，但是找不到任何可以帮助我解决问题的东西。也许我的布局错误。感谢您的帮助！

Answer 1

问题自

label.addConstraints(constraints)

您在父标签和标签之间添加了约束，这是不正确的，您需要将其添加到父标签中

self.addConstraints(constraints)

另一个苹果推荐的方法是

NSLayoutConstraint.activate([constraints])

哪个会为您处理

let mmmview = MyView()
mmmview.translatesAutoresizingMaskIntoConstraints = false
view.addSubview(mmmview)
let constraints = [
    NSLayoutConstraint(item: mmmview , attribute: .top, relatedBy: .equal, toItem: view, attribute: .top, multiplier: 1.0, constant: 8.0),
    NSLayoutConstraint(item: mmmview , attribute: .leading, relatedBy: .equal, toItem: view, attribute: .leading, multiplier: 1.0, constant: 8.0),
    NSLayoutConstraint(item: mmmview , attribute: .trailing, relatedBy: .equal, toItem: view, attribute: .trailing, multiplier: 1.0, constant: 8.0)
 ]
 NSLayoutConstraint.activate(constraints)

如何以编程方式为视图设置动态高度？

1 个答案: