斯威夫特：在上海华与NSLayoutConstraint

Question

我试图在父视图中心的视图，但预期其不居中不能居中的UIView。这是我的代码，结果的截图。

emptyView.translatesAutoresizingMaskIntoConstraints = false 
self.view.addSubview(emptyView) 
let centerXConstraint = NSLayoutConstraint(item: emptyView, attribute: .centerX, relatedBy: .equal, toItem: self.view, attribute: .centerX, multiplier: 1, constant: 0) 
let centerYConstraint = NSLayoutConstraint(item: emptyView, attribute: .centerY, relatedBy: .equal, toItem: self.view, attribute: .centerY, multiplier: 1, constant: 0) 
self.view.addConstraints([centerXConstraint]) 
self.view.addConstraints([centerYConstraint]) 

Answer 1

下面是代码，

import UIKit 

class ViewController: UIViewController 
{ 
    @IBOutlet weak var emptyView: UIView! 

    override func viewDidLoad() 
    { 
     super.viewDidLoad() 
     self.emptyView.translatesAutoresizingMaskIntoConstraints = false 
     let centerXConstraint = NSLayoutConstraint(item: emptyView, attribute: .centerX, relatedBy: .equal, toItem: self.view, attribute: .centerX, multiplier: 1, constant: 0) 
     let centerYConstraint = NSLayoutConstraint(item: emptyView, attribute: .centerY, relatedBy: .equal, toItem: self.view, attribute: .centerY, multiplier: 1, constant: 0) 
     let widthConstraint = NSLayoutConstraint(item: emptyView, attribute: NSLayoutAttribute.width, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 100) 
     let heightConstraint = NSLayoutConstraint(item: emptyView, attribute: NSLayoutAttribute.height, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 100) 

     self.view.addConstraints([centerXConstraint, centerYConstraint, widthConstraint, heightConstraint]) 
     self.view.layoutIfNeeded() 
    } 
} 

，没有必要对高度和宽度约束

视图控制器接口：

输出：

Answer 2

如果您使用的标签按钮或具有固有大小的任何其他视图试试这个代码

emptyView.translatesAutoresizingMaskIntoConstraints = false 
let widthConstraint = NSLayoutConstraint(item: emptyView, attribute: NSLayoutAttribute.width, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: emptyView.frame.size.width) 
let heightConstraint = NSLayoutConstraint(item: emptyView, attribute: NSLayoutAttribute.height, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: emptyView.frame.size.height) 
var constraints = NSLayoutConstraint.constraints(
     withVisualFormat: "V:[superview]-(<=1)-[label]", 
     options: NSLayoutFormatOptions.alignAllCenterX, 
     metrics: nil, 
     views: ["superview":view, "label": emptyView]) 

view.addConstraints(constraints) 

// Center vertically 
constraints = NSLayoutConstraint.constraints(
     withVisualFormat: "H:[superview]-(<=1)-[label]", 
     options: NSLayoutFormatOptions.alignAllCenterY, 
     metrics: nil, 
     views: ["superview":view, "label": emptyView]) 

view.addConstraints(constraints) 
view.addConstraints([ widthConstraint, heightConstraint])