Swift 约束以编程方式产生奇怪的行为

Question

我正在尝试以编程方式布置我的所有约束，因为我必须根据用户输入添加和删除框。当我运行一个在 ViewDidLoad 中应用我所有约束的函数时，它工作得很好。但是如果我再次运行它，在将它们全部删除之后，我的 2 个标签会消失在我的导航栏后面。我不知道为什么！请记住，如果我从不运行下面粘贴的函数，我的屏幕将是空白的，因为我对从故事板启动没有任何限制。我的目标是弄清楚为什么我的标签和按钮在同一段代码再次运行时消失。

下面是一张图片，我将在下面 post 的代码最初是运行在 ViewDidLoad 中：

下图是同一段代码在按下下一个按钮后再次运行时的图片。这只是一个测试，以后我需要在更改约束后重新设置所有约束：

下面是调试视图层次结构的图片：

创建约束的代码如下。请记住，我只有 5 个元素，我的主视图、滚动视图、作为标签的 aglAltitudeLabel、作为按钮的 nextButton 和作为文本输入的 aglAltitude：

    //Remove all pre-existing contraints
    var constraints:NSArray = aglAltitudeLabel.constraints()
    aglAltitudeLabel.removeConstraints(constraints)
    constraints = aglAltitude.constraints()
    aglAltitude.removeConstraints(constraints)
    constraints = nextButton.constraints()
    nextButton.removeConstraints(constraints)
    constraints = scrollView.constraints()
    scrollView.removeConstraints(constraints)
    constraints = view.constraints()
    self.view.removeConstraints(constraints)
    constraints = calculateButton.constraints()
    calculateButton.removeConstraints(constraints)

    //Make scrollview fit normal View, accounting for Navigation bar
    self.view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("|[first(\(self.view.frame.width))]|", options: nil, metrics: nil, views: ["first": scrollView]))
    self.view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|-64-[first(\(self.view.frame.height-64))]|", options: nil, metrics: nil, views: ["first": scrollView]))

    //Set the heights for the label and button
    let labelHeight = NSLayoutConstraint(item: aglAltitudeLabel, attribute: NSLayoutAttribute.Height, relatedBy: NSLayoutRelation.Equal, toItem: nil, attribute: NSLayoutAttribute.NotAnAttribute, multiplier: 1.0, constant: 20)
    scrollView.addConstraint(labelHeight)
    let buttonHeight = NSLayoutConstraint(item: nextButton, attribute: NSLayoutAttribute.Height, relatedBy: NSLayoutRelation.Equal, toItem: nil, attribute: NSLayoutAttribute.NotAnAttribute, multiplier: 1.0, constant: 30)
    scrollView.addConstraint(buttonHeight)

    //Setup the vertical layout
    scrollView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|-8-[first(30)]-8-|", options: nil, metrics: nil, views: ["first": aglAltitude]))


    //Setup the horizontal layout
    scrollView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("|-8-[first(97)]-8-[second(>=100)]-8-[third(46)]-8-|", options: nil, metrics: nil, views: ["first": aglAltitudeLabel, "second": aglAltitude, "third": nextButton]))

    //Align baselines - this also occurs with the option above of AlignAllBaselines and leaving this code out
    let aglLabelBaseline = NSLayoutConstraint(item: aglAltitudeLabel, attribute: NSLayoutAttribute.Baseline, relatedBy: NSLayoutRelation.Equal, toItem: aglAltitude, attribute: NSLayoutAttribute.Baseline, multiplier: 1.0, constant: 0)
    scrollView.addConstraint(aglLabelBaseline)
    let nextButtonBaseline = NSLayoutConstraint(item: nextButton, attribute: NSLayoutAttribute.Baseline, relatedBy: NSLayoutRelation.Equal, toItem: aglAltitude, attribute: NSLayoutAttribute.Baseline, multiplier: 1.0, constant: 0)
    scrollView.addConstraint(nextButtonBaseline)

    self.view.layoutSubviews()

感谢您的帮助。我确定我做错了 55 件事。

编辑：如果我更改最后一段代码：

    //Align baselines - this also occurs with the option above of AlignAllBaselines and leaving this code out
    let aglLabelBaseline = NSLayoutConstraint(item: aglAltitudeLabel, attribute: NSLayoutAttribute.Baseline, relatedBy: NSLayoutRelation.Equal, toItem: aglAltitude, attribute: NSLayoutAttribute.Baseline, multiplier: 1.0, constant: 0)
    scrollView.addConstraint(aglLabelBaseline)
    let nextButtonBaseline = NSLayoutConstraint(item: nextButton, attribute: NSLayoutAttribute.Baseline, relatedBy: NSLayoutRelation.Equal, toItem: aglAltitude, attribute: NSLayoutAttribute.Baseline, multiplier: 1.0, constant: 0)
    scrollView.addConstraint(nextButtonBaseline)

为此：

    //Align baselines - this also occurs with the option above of AlignAllBaselines and leaving this code out
    let aglLabelBaseline = NSLayoutConstraint(item: aglAltitudeLabel, attribute: NSLayoutAttribute.Top, relatedBy: NSLayoutRelation.Equal, toItem: scrollView, attribute: NSLayoutAttribute.Top, multiplier: 1.0, constant: 14)
    self.view.addConstraint(aglLabelBaseline)
    let nextButtonBaseline = NSLayoutConstraint(item: nextButton, attribute: NSLayoutAttribute.Top, relatedBy: NSLayoutRelation.Equal, toItem: scrollView, attribute: NSLayoutAttribute.Top, multiplier: 1.0, constant: 8)
    self.view.addConstraint(nextButtonBaseline)

它完全符合我的预期。为什么不同？

Answer 1

在不知道您的要求是什么的情况下，很难给您好的建议。这是我如何做的一个例子。我在代码中添加了所有视图，并为视图添加了背景颜色，以便我可以更好地查看它们的布局。我假设滚动视图只添加一次，并且永远不会删除，所以它对 self.view 的约束应该只在 viewDidLoad 中添加一次。如果您不需要更改其他视图的高度，它们也是如此（请注意，我将高度限制添加到视图本身，而不是它的父视图）。唯一需要删除并重新添加的是滚动视图及其子视图之间的约束。

class ViewController: UIViewController {

    var aglAltitudeLabel = UILabel()
    var aglAltitude = UITextField()
    var nextButton = UIButton.buttonWithType(.System) as UIButton
    var scrollView = UIScrollView()
    var viewsDict: NSDictionary!

    override func viewDidLoad() {
        super.viewDidLoad()
        aglAltitudeLabel.text = "AGL Altitude"
        aglAltitudeLabel.backgroundColor = UIColor.lightGrayColor()
        aglAltitudeLabel.setTranslatesAutoresizingMaskIntoConstraints(false)
        aglAltitude.setTranslatesAutoresizingMaskIntoConstraints(false)
        aglAltitude.backgroundColor = UIColor.yellowColor()
        aglAltitudeLabel.addConstraint(NSLayoutConstraint(item: aglAltitudeLabel, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: 30))
        scrollView.setTranslatesAutoresizingMaskIntoConstraints(false)
        nextButton.setTranslatesAutoresizingMaskIntoConstraints(false)
        nextButton.setTitle("Next", forState: .Normal)
        nextButton.backgroundColor = UIColor.lightGrayColor()
        nextButton.addConstraint(NSLayoutConstraint(item: nextButton, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: 30))
        self.view.addSubview(scrollView)
        scrollView.addSubview(aglAltitude)
        scrollView.addSubview(aglAltitudeLabel)
        scrollView.addSubview(nextButton)
        scrollView.backgroundColor = UIColor(red: 1, green: 1, blue: 0.8, alpha: 1)

        viewsDict = ["scrollView": scrollView, "aglAltitudeLabel": aglAltitudeLabel, "aglAltitude":aglAltitude, "nextButton":nextButton] as NSDictionary

        self.view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("|[scrollView]|", options: nil, metrics: nil, views: viewsDict))
        self.view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|-64-[scrollView]|", options: nil, metrics: nil, views: viewsDict))

        self.redoConstraints()
    }


    @IBAction func redoConstraints() {

        scrollView.removeConstraints(scrollView.constraints())

        scrollView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|-8-[aglAltitude(30)]-8-|", options: nil, metrics: nil, views: viewsDict))

        scrollView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("|-8-[aglAltitudeLabel(97)]-8-[aglAltitude(>=100)]-8-[nextButton(46)]-8-|", options: .AlignAllBottom , metrics: nil, views: viewsDict))
    }
}

请注意，我将固定宽度和高度排除在滚动视图的约束之外。没有必要这样做，这会导致它在轮换时无法正常工作。

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