ios - 添加到 View 时，约束项必须是该 View (或 View 本身)的后代。

Question

下面是我的代码:

    func tableView(_ tableView: UITableView, viewForHeaderInSection section: Int) -> UIView? {
    let headerView = UIView()
    headerView.backgroundColor = UIColor.white
    let headerLabel = UILabel(frame: CGRect(x: 30, y: 0, width:
      tableView.bounds.size.width, height: tableView.bounds.size.height))
    headerLabel.font = UIFont().robotoMedium(withFontSize: 10)
    headerLabel.textColor = CustomColor.lightGrey.color
    headerLabel.text = "Travel Shops"
    headerLabel.sizeToFit()
    headerView.addSubview(headerLabel)
    headerLabel.translatesAutoresizingMaskIntoConstraints = false

    headerLabel.addConstraint(NSLayoutConstraint(item: headerLabel, attribute: .trailing, relatedBy: .equal, toItem: tableView, attribute: .trailing, multiplier: 1, constant: 0))
    headerLabel.addConstraint(NSLayoutConstraint(item: headerLabel, attribute: .leading, relatedBy: .equal, toItem: tableView, attribute: .leading, multiplier: 1, constant: 0))

    headerLabel.addConstraint(NSLayoutConstraint(item: headerLabel, attribute: .top, relatedBy: .equal, toItem: self.topLayoutGuide, attribute: .bottom, multiplier: 1, constant: 0))
    headerLabel.addConstraint(NSLayoutConstraint(item: headerLabel, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 131))
    return headerView
  }

但是应用程序崩溃了:

添加到 View 时，约束项必须是该 View (或 View 本身)的后代。如果在组装 View 层次结构之前需要解决约束，这将崩溃。中断 -[UIView(UIConstraintBasedLayout) _viewHierarchyUnpreparedForConstraint:] 进行调试。

Answer 1

headerLabel 是 headerView 的 subview ，headerView 是层次结构中 TableView 的 subview ，这就是为什么显示消息时，您需要将约束添加到 headerView 这是 headerLabel.superView 而不是 UITableView 这是 headerLabel.superView.superView

更改代码

    headerLabel.addConstraint(NSLayoutConstraint(item: headerLabel, attribute: .trailing, relatedBy: .equal, toItem: headerView, attribute: .trailing, multiplier: 1, constant: 0))
    headerLabel.addConstraint(NSLayoutConstraint(item: headerLabel, attribute: .leading, relatedBy: .equal, toItem: headerView, attribute: .leading, multiplier: 1, constant: 0))

    headerLabel.addConstraint(NSLayoutConstraint(item: headerLabel, attribute: .top, relatedBy: .equal, toItem: self.topLayoutGuide, attribute: .bottom, multiplier: 1, constant: 0))
    headerLabel.addConstraint(NSLayoutConstraint(item: headerLabel, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 131))

由这个

    headerLabel.addConstraint(NSLayoutConstraint(item: headerLabel, attribute: .trailing, relatedBy: .equal, toItem: tableView, attribute: .trailing, multiplier: 1, constant: 0))
    headerLabel.addConstraint(NSLayoutConstraint(item: headerLabel, attribute: .leading, relatedBy: .equal, toItem: headerView, attribute: .leading, multiplier: 1, constant: 0))

    headerLabel.addConstraint(NSLayoutConstraint(item: headerLabel, attribute: .top, relatedBy: .equal, toItem: headerView, attribute: .bottom, multiplier: 1, constant: 0))
    headerLabel.addConstraint(NSLayoutConstraint(item: headerLabel, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 131))

希望对你有帮助