在 Swift 中以编程方式添加约束不起作用

Question

我添加了以下代码以使以编程方式添加的视图居中：

let horizontalConstraint = NSLayoutConstraint(item: newView!, attribute: NSLayoutAttribute.CenterX, relatedBy: NSLayoutRelation.Equal, toItem: view, attribute: NSLayoutAttribute.CenterX, multiplier: 1, constant: 0)
view.addConstraint(horizontalConstraint)

它不起作用。视图未居中。它还在左边。

编辑：

    override func viewDidLoad() {
    super.viewDidLoad()

    newView = LineChartView(frame: CGRectMake(0, 0, 50, 50))
    newView?.delegate = self
    newView?.drawBordersEnabled = true
    newView?.noDataText = "No Data"
    newView?.noDataTextDescription = "No Data"
    newView?.borderColor = UIColor.blackColor()

    self.view.addSubview(newView!)

Answer 1

朋友，你需要选择一边，如果你使用自动布局，不要用frame初始化你的对象。试试这样的...

var newView:LineChartView!

override func viewDidLoad() {
    super.viewDidLoad()

    newView = LineChartView()
    newView.translatesAutoresizingMaskIntoConstraints = false
    newView.delegate = self
    newView.drawBordersEnabled = true
    newView.noDataText = "No Data"
    newView.noDataTextDescription = "No Data"
    newView.borderColor = UIColor.blackColor()
    self.view.addSubview(newView)

    let width = NSLayoutConstraint(item: newView, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: 50)
    let height = NSLayoutConstraint(item: newView, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: 50)
    newView.addConstraint(width)
    newView.addConstraint(height)
    let x = NSLayoutConstraint(item: newView, attribute: .CenterX, relatedBy: .Equal, toItem: self.view, attribute: .CenterX, multiplier: 1, constant: 0)
    let y = NSLayoutConstraint(item: newView, attribute: .CenterY, relatedBy: .Equal, toItem: self.view, attribute: .CenterY, multiplier: 1, constant: 0)
    self.view.addConstraint(x)
    self.view.addConstraint(y)

}

如果您的 LineChartView 对象是 UIView 的子类，那么这应该可以工作，并且您的超级视图中间应该有一个 50x50 的对象。

如果你要在代码中做这样的约束，你应该考虑使用 Apples Visual Formatting Language。