以编程方式向单元格添加约束:找不到其内容视图

Question

我正在尝试向UITableViewCell中的子视图添加约束，但收到错误Unable to interpret '|' character, because the related view doesn't have a superview

这是我的自定义UITableViewCell类：

class MessageTableViewCell: UITableViewCell {
var titleLabel: UILabel {
    get {
        let titleLabel = UILabel()
        titleLabel.translatesAutoresizingMaskIntoConstraints = false
        titleLabel.font = UIFont.boldSystemFont(ofSize: self.defaultFontSize)
        return titleLabel
    }
}
var bodyLabel: UILabel {
    get {
        let bodyLabel = UILabel()
        bodyLabel.translatesAutoresizingMaskIntoConstraints = false
        bodyLabel.font = UIFont.systemFont(ofSize: self.defaultFontSize)
        return bodyLabel
    }
}
var thumbnailView: UIImageView {
    get {
        let thumbnailView = UIImageView()
        thumbnailView.translatesAutoresizingMaskIntoConstraints = false
        thumbnailView.layer.cornerRadius = CGFloat(TableViewCellArrangements.kMessageTableViewCellAvatarHeight/2.0)
        thumbnailView.layer.masksToBounds = true
        return thumbnailView
    }
}
override init(style: UITableViewCellStyle, reuseIdentifier: String?) {
    super.init(style: style, reuseIdentifier: reuseIdentifier)
    configureSubviews()
}
private func configureSubviews() {
    self.contentView.addSubview(thumbnailView)
    self.contentView.addSubview(titleLabel)
    self.contentView.addSubview(bodyLabel)
    let views: [String : Any] = ["thumbnailView" : thumbnailView,
                                 "titleLabel" : titleLabel,
                                 "bodyLabel" : bodyLabel]
    let metrics: [String : Any] = ["thumbSize" : TableViewCellArrangements.kMessageTableViewCellAvatarHeight,
                                   "padding" : 15,
                                   "right" : 10,
                                   "left" : 5]
    //The next line throws an exception. In fact, all the `let constraint` code will throw exceptions 
    let constraint1 = NSLayoutConstraint.constraints(withVisualFormat: "H:|-left-[thumbnailView(thumbSize)]-right-[titleLabel(>=0)]-right-|", options: .alignAllLeft, metrics: metrics, views: views)
    contentView.addConstraints(constraint1)
    let constraint2 = NSLayoutConstraint.constraints(withVisualFormat: "H:|-left-[thumbnailView(thumbSize)]-right-[bodyLabel(>=0)]-right-|", options: .alignAllLeft, metrics: metrics, views: views)
    contentView.addConstraints(constraint2)
    let constraint3 = NSLayoutConstraint.constraints(withVisualFormat: "V:|-right-[thumbnailView(thumbSize)]-(>=0)-|", options: .alignAllLeft, metrics: metrics, views: views)
    contentView.addConstraints(constraint3)
    if self.reuseIdentifier == TableViewCellArrangements.MessengerCellIdentifier {
        let constraint4 = NSLayoutConstraint.constraints(withVisualFormat: "V:|-right-[titleLabel(20)]-left-[bodyLabel(>=0@999)]-left-|", options: .alignAllLeft, metrics: metrics, views: views)
        contentView.addConstraints(constraint4)
    } else {
        let constraint5 = NSLayoutConstraint.constraints(withVisualFormat: "V:|[titleLabel]|", options: .alignAllLeft, metrics: metrics, views: views)
        contentView.addConstraints(constraint5)
    }
}

如上所述，约束行是引发异常的位置。但是，在设置约束之前，我已经将子视图添加到了contentView中，因此我很困惑为什么它认为存在超级视图。

谢谢！

Answer 1

每次

引用元素时，您都会重新创建元素的新实例。像这样尝试：

var xtitleLabel: UILabel?
var titleLabel: UILabel {
    get {
        if xtitleLabel == nil {
            xtitleLabel = UILabel()
        }
        xtitleLabel?.translatesAutoresizingMaskIntoConstraints = false
        xtitleLabel?.font = UIFont.boldSystemFont(ofSize: self.defaultFontSize)
        return xtitleLabel!
    }
}

您仍然在整个代码中引用titleLabel - 这只会导致"创建一次"过程。