以编程方式添加 CenterX/CenterY 约束

Swift 3/Swift 4 的更新:

从 iOS 8 开始，您可以并且应该通过将约束的 isActive 属性设置为 true 来激活约束.这使约束能够将自己添加到正确的视图中.您可以通过将包含约束的数组传递给 NSLayoutConstraint.activate()

As of iOS 8, you can and should activate your constraints by setting their isActive property to true. This enables the constraints to add themselves to the proper views. You can activate multiple constraints at once by passing an array containing the constraints to NSLayoutConstraint.activate()

let label = UILabel(frame: CGRect.zero)
label.text = "Nothing to show"
label.textAlignment = .center
label.backgroundColor = .red  // Set background color to see if label is centered
label.translatesAutoresizingMaskIntoConstraints = false
self.tableView.addSubview(label)

let widthConstraint = NSLayoutConstraint(item: label, attribute: .width, relatedBy: .equal,
                                         toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 250)

let heightConstraint = NSLayoutConstraint(item: label, attribute: .height, relatedBy: .equal,
                                          toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 100)

let xConstraint = NSLayoutConstraint(item: label, attribute: .centerX, relatedBy: .equal, toItem: self.tableView, attribute: .centerX, multiplier: 1, constant: 0)

let yConstraint = NSLayoutConstraint(item: label, attribute: .centerY, relatedBy: .equal, toItem: self.tableView, attribute: .centerY, multiplier: 1, constant: 0)

NSLayoutConstraint.activate([widthConstraint, heightConstraint, xConstraint, yConstraint])

更好的解决方案:

由于最初回答了这个问题，因此引入了布局锚点，使创建约束变得更加容易.在这个例子中，我创建了约束并立即激活它们:

Since this question was originally answered, layout anchors were introduced making it much easier to create the constraints. In this example I create the constraints and immediately activate them:

label.widthAnchor.constraint(equalToConstant: 250).isActive = true
label.heightAnchor.constraint(equalToConstant: 100).isActive = true
label.centerXAnchor.constraint(equalTo: self.tableView.centerXAnchor).isActive = true
label.centerYAnchor.constraint(equalTo: self.tableView.centerYAnchor).isActive = true

或同样使用 NSLayoutConstraint.activate():

NSLayoutConstraint.activate([
    label.widthAnchor.constraint(equalToConstant: 250),
    label.heightAnchor.constraint(equalToConstant: 100),
    label.centerXAnchor.constraint(equalTo: self.tableView.centerXAnchor),
    label.centerYAnchor.constraint(equalTo: self.tableView.centerYAnchor)
])

注意:始终在创建和激活约束之前将您的子视图添加到视图层次结构中.

Note: Always add your subviews to the view hierarchy before creating and activating the constraints.

原答案:

约束引用self.tableView.由于您将标签添加为 self.tableView 的子视图，因此需要将约束添加到共同祖先":

The constraints make reference to self.tableView. Since you are adding the label as a subview of self.tableView, the constraints need to be added to the "common ancestor":

   self.tableView.addConstraint(xConstraint)
   self.tableView.addConstraint(yConstraint)

正如@mustafa 和@kcstricks 在评论中指出的，您需要将label.translatesAutoresizingMaskIntoConstraints 设置为false.执行此操作时，还需要指定带有约束的标签的 width 和 height，因为不再使用框架.最后，您还应该将 textAlignment 设置为 .Center，以便您的文本在标签中居中.

As @mustafa and @kcstricks pointed out in the comments, you need to set label.translatesAutoresizingMaskIntoConstraints to false. When you do this, you also need to specify the width and height of the label with constraints because the frame no longer is used. Finally, you also should set the textAlignment to .Center so that your text is centered in your label.

    var  label = UILabel(frame: CGRectZero)
    label.text = "Nothing to show"
    label.textAlignment = .Center
    label.backgroundColor = UIColor.redColor()  // Set background color to see if label is centered
    label.translatesAutoresizingMaskIntoConstraints = false
    self.tableView.addSubview(label)

    let widthConstraint = NSLayoutConstraint(item: label, attribute: .Width, relatedBy: .Equal,
        toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 250)
    label.addConstraint(widthConstraint)

    let heightConstraint = NSLayoutConstraint(item: label, attribute: .Height, relatedBy: .Equal,
        toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 100)
    label.addConstraint(heightConstraint)

    let xConstraint = NSLayoutConstraint(item: label, attribute: .CenterX, relatedBy: .Equal, toItem: self.tableView, attribute: .CenterX, multiplier: 1, constant: 0)

    let yConstraint = NSLayoutConstraint(item: label, attribute: .CenterY, relatedBy: .Equal, toItem: self.tableView, attribute: .CenterY, multiplier: 1, constant: 0)

    self.tableView.addConstraint(xConstraint)
    self.tableView.addConstraint(yConstraint)