使用 UILabel 以编程方式自动布局不起作用

Question

所以我想以编程方式在我的视图中将 UILabel 水平和垂直居中。我关注 this topic 但它不起作用。我该怎么做？

p/s：这是我的代码：

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        let noDataLabel = UILabel()
        noDataLabel.text = "No data :("
        noDataLabel.textColor = UIColor.redColor()
        noDataLabel.font = UIFont.systemFontOfSize(20)
        noDataLabel.sizeToFit()
        self.view.addSubview(noDataLabel)
        NSLayoutConstraint(item: noDataLabel, attribute: .CenterX, relatedBy: .Equal, toItem: self.view, attribute: .CenterX, multiplier: 1.0, constant: 0.0).active = true
        NSLayoutConstraint(item: noDataLabel, attribute: .CenterY, relatedBy: .Equal, toItem: self.view, attribute: .CenterY, multiplier: 1.0, constant: 0.0).active = true
    }
}

Answer 1

您还需要为 uilabel 创建大小限制，如下所示，并将其添加到您的 superView

 NSLayoutConstraint(item: noDataLabel, attribute: NSLayoutAttribute.Width, relatedBy: NSLayoutRelation.Equal, toItem: nil, attribute: NSLayoutAttribute.NotAnAttribute, multiplier: 1, constant: 100)
 NSLayoutConstraint(item: noDataLabel, attribute: NSLayoutAttribute.Height, relatedBy: NSLayoutRelation.Equal, toItem: nil, attribute: NSLayoutAttribute.NotAnAttribute, multiplier: 1, constant: 40)

您的实施将类似于

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        let noDataLabel = UILabel()
        noDataLabel.text = "No data :("
        noDataLabel.textColor = UIColor.redColor()
        noDataLabel.font = UIFont.systemFontOfSize(20)
        noDataLabel.sizeToFit()

        noDataLabel.translatesAutoresizingMaskIntoConstraints = false

        self.view.addSubview(noDataLabel)

        let horizontal = NSLayoutConstraint(item: noDataLabel, attribute: .CenterX, relatedBy: .Equal, toItem: self.view, attribute: .CenterX, multiplier: 1.0, constant: 0.0).active = true
        let vertical = NSLayoutConstraint(item: noDataLabel, attribute: .CenterY, relatedBy: .Equal, toItem: self.view, attribute: .CenterY, multiplier: 1.0, constant: 0.0).active = true
        let width = NSLayoutConstraint(item: noDataLabel, attribute: NSLayoutAttribute.Width, relatedBy: NSLayoutRelation.Equal, toItem: nil, attribute: NSLayoutAttribute.NotAnAttribute, multiplier: 1, constant: 100)
        let height = NSLayoutConstraint(item: noDataLabel, attribute: NSLayoutAttribute.Height, relatedBy: NSLayoutRelation.Equal, toItem: nil, attribute: NSLayoutAttribute.NotAnAttribute, multiplier: 1, constant: 40)

        self.view.addConstraint(horizontal)
        self.view.addConstraint(vertical)
        self.view.addConstraint(width)
        self.view.addConstraint(height)
    }
}

Answer 2

将此添加到您的代码中：

 noDataLabel.translatesAutoresizingMaskIntoConstraints = false

来自苹果文档：

By default, the autoresizing mask on a view gives rise to constraints that fully determine the view's position. This allows the auto layout system to track the frames of views whose layout is controlled manually (through -setFrame:, for example). When you elect to position the view using auto layout by adding your own constraints, you must set this property to NO. IB will do this for you.

使用 UILabel 以编程方式自动布局不起作用

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