小编Vik*_*kas的帖子

我想添加一个按钮作为根视图的子视图.按钮必须水平放置在中心以及垂直于superview的中心.我以编程方式使用NSLayoutConstraints.这是我的代码.

import UIKit

class ViewController: UIViewController {

    var button : UIButton!

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    override func motionEnded(motion: UIEventSubtype, withEvent event: UIEvent?) {

        button = UIButton()
        button.setTitle("Hello World", forState: .Normal)
        button.backgroundColor = UIColor.blueColor()
        button.sizeToFit()
        self.view.addSubview(button)

        NSLayoutConstraint.activateConstraints([

            NSLayoutConstraint(item: button, attribute: .CenterX, relatedBy: .Equal, toItem: self.view, attribute: .CenterX, multiplier: 1, constant: 0), …