在自定义 UITableViewHeaderFooterView 中的现有 textLabel 上添加约束

Question

我想在表头中的 textLabel 旁边添加一个简单的表中对象数量的计数器。所以我创建了这个类：

import UIKit

class CounterHeaderView: UITableViewHeaderFooterView {
    static let reuseIdentifier: String = String(describing: self)

    var counterLabel: UILabel

    override init(reuseIdentifier: String?) {
        counterLabel = UILabel()
        super.init(reuseIdentifier: reuseIdentifier)

        contentView.addSubview(counterLabel)

        counterLabel.translatesAutoresizingMaskIntoConstraints = false
        counterLabel.backgroundColor = .red

        if let textLabel = self.textLabel{
            counterLabel.leadingAnchor.constraint(equalTo: textLabel.trailingAnchor, constant: 6.0).isActive = true
            counterLabel.topAnchor.constraint(equalTo: textLabel.topAnchor).isActive = true
            counterLabel.heightAnchor.constraint(equalToConstant: 24.0).isActive = true
        }

    }

    required init?(coder aDecoder: NSCoder) {
        counterLabel = UILabel()
        super.init(coder: aDecoder)
    }
}

但是运行这个会导致以下错误：

'Unable to activate constraint with anchors 
<NSLayoutXAxisAnchor:0x60000388ae00 "UILabel:0x7fb8314710a0.leading"> 
and <NSLayoutXAxisAnchor:0x60000388ae80 "_UITableViewHeaderFooterViewLabel:0x7fb8314718c0.trailing"> 
because they have no common ancestor.  
Does the constraint or its anchors reference items in different view hierarchies?  
That's illegal.'

如何根据现有的 textLabel 为我的 counterLabel 添加约束？ textLabel 不是已经是 ContentView 的子视图了吗？

Answer 1

您正在尝试使用内置的textLabel，我很确定在init 时间不可用。尝试在 layoutSubviews 方法中执行您的布局代码，就在 super 调用之后。该方法可能会被评估几次，所以你应该检查你是否已经布局了你的视图（例如couterLabel.superview != nil）

它应该是这样的：

final class CounterHeaderView: UITableViewHeaderFooterView {

    static let reuseIdentifier: String = String(describing: self)

    let counterLabel = UILabel()

    override func layoutSubviews() {
        super.layoutSubviews()

        if counterLabel.superview == nil {
            layout()
        }
    }

    func layout() {
        contentView.addSubview(counterLabel)
        counterLabel.translatesAutoresizingMaskIntoConstraints = false
        counterLabel.backgroundColor = .red
        if let textLabel = self.textLabel {
            counterLabel.leadingAnchor.constraint(equalTo: textLabel.trailingAnchor, constant: 6.0).isActive = true
            counterLabel.topAnchor.constraint(equalTo: textLabel.topAnchor).isActive = true
            counterLabel.heightAnchor.constraint(equalToConstant: 24.0).isActive = true
        }
    }
}