带有swift 1.2错误的NSlayoutConstraint

Question

我的代码曾经在 swift 1.1 中工作，但现在在 1.2 中它会因此错误而崩溃。

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** +[NSLayoutConstraint constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:]: A multiplier of 0 or a nil second item together with a location for the first attribute creates an illegal constraint of a location equal to a constant. Location attributes must be specified in pairs'

 /* Create View with label top header  TABLE */
    let header:UIView = UIView(frame: CGRectMake(0,0, UIScreen.mainScreen().bounds.width, 40))

    var noteBodyLabel = UILabel()
    noteBodyLabel.text = "Aquí te presentamos las últimas tendencias en Alacarta.do"
    noteBodyLabel.numberOfLines = 2
    noteBodyLabel.textColor = UIColor.lightGrayColor()
      noteBodyLabel.textAlignment = .Center
    noteBodyLabel.font = UIFont(name:noteBodyLabel.font.fontName, size: 12)
    noteBodyLabel.setTranslatesAutoresizingMaskIntoConstraints(false)


    var widthCons = NSLayoutConstraint(
        item: noteBodyLabel,
        attribute: .Width,
        relatedBy: .Equal,
        toItem: view.superview,
        attribute: .NotAnAttribute,
        multiplier: 1, constant: UIScreen.mainScreen().bounds.width - 80)


    let topCons = NSLayoutConstraint(
        item: noteBodyLabel,
        attribute: .Top,
        relatedBy: .Equal,
        toItem: view.superview,
        attribute: .Bottom,
        multiplier: 1, constant: 10);


    /*
    let bottomCons = NSLayoutConstraint(
        item: noteBodyLabel,
        attribute: .Bottom,
        relatedBy: .Equal,
        toItem: ,
        attribute: .Bottom,
        multiplier: 1, constant: 0);
    */
    let leftCons = NSLayoutConstraint(
        item: noteBodyLabel,
        attribute: .Left,
        relatedBy: .Equal,
        toItem: view.superview,
        attribute: .Left,
        multiplier: 1, constant: 40);
    let rightCons = NSLayoutConstraint(
        item: noteBodyLabel,
        attribute: .Right,
        relatedBy: .Equal,
        toItem: view.superview,
        attribute: .Right,
        multiplier: 1, constant: 40);


  header.addSubview(noteBodyLabel)


  noteBodyLabel.addConstraints([topCons,leftCons,rightCons,widthCons])


        self.tableView.tableHeaderView = header

Answer 1

您的第一个约束（noteBodyLabel 上的宽度约束）是错误的.....toItem 应该为零

  var widthCons = NSLayoutConstraint(
        item: noteBodyLabel,
        attribute: .Width,
        relatedBy: .Equal,
        toItem: nil,
        attribute: .NotAnAttribute, // this means to toItem should be nil
        multiplier: 1, constant: UIScreen.mainScreen().bounds.width - 80)

Answer 2

正如错误警告您： A multiplier of 0 or a nil second item together with a location for the first attribute creates an illegal constraint of a location equal to a constant. Location attributes must be specified in pairs'

带有.NotAnAttribute 的宽度约束不应该有toItem 对象。试试toItem:nil