添加NSLayoutConstraint后，视图消失

Question

当我将NSLayoutConstraint添加到视图时，它导致视图消失。

我使用下面的代码：

let test:UIView = UIView(frame: CGRectMake(0, 0, 100, 100)) 
test.backgroundColor = UIColor.blackColor() 
self.view.addSubview(test) 
test.translatesAutoresizingMaskIntoConstraints = false 
let topCKCtr = NSLayoutConstraint(item: test, attribute: .CenterX, relatedBy: .Equal, toItem: test.superview, attribute: .CenterX, multiplier: 0.5, constant: 0) 
topCKCtr.active = true 

let topCKCtr1 = NSLayoutConstraint(item: test, attribute: .CenterY, relatedBy: .Equal, toItem: test.superview, attribute: .CenterY, multiplier: 0.5, constant: 0) 
topCKCtr1.active = true 


self.view.layoutIfNeeded() 
self.view.setNeedsDisplay() 

当调试视图层次，我看到的观点存在，即使它是不可见的。有关详细信息，请参见下面的屏幕截图 - 只有约束是可见的，而不是视图：

Answer 1

有很多事情需要在这里讨论。

1. 当您使用以下

   test.translatesAutoresizingMaskIntoConstraints = false 
   

   然后，它会完全依靠约束定位和大小view.So你需要设置的高度和角度的宽度限制。

   let topCKCtr2 = NSLayoutConstraint(item: test, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .Width, multiplier: 1.0, constant: 100) 
   
   let topCKCtr3 = NSLayoutConstraint(item: test, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .Height, multiplier: 1.0, constant: 100) 
   

最后这将是你正在寻找

  let test:UIView = UIView(frame: CGRectMake(0, 0, 100, 100)) 
    test.backgroundColor = UIColor.blackColor() 
    self.view.addSubview(test) 
    test.translatesAutoresizingMaskIntoConstraints = false 
    let topCKCtr = NSLayoutConstraint(item: test, attribute: .CenterX, relatedBy: .Equal, toItem: self.view, attribute: .CenterX, multiplier: 0.5, constant: 0) 
    topCKCtr.active = true 
    let topCKCtr1 = NSLayoutConstraint(item: test, attribute: .CenterY, relatedBy: .Equal, toItem: self.view, attribute: .CenterY, multiplier: 0.5, constant: 0) 
    topCKCtr1.active = true 

    let topCKCtr2 = NSLayoutConstraint(item: test, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .Width, multiplier: 1.0, constant: 100) 
    topCKCtr2.active = true 

    let topCKCtr3 = NSLayoutConstraint(item: test, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .Height, multiplier: 1.0, constant: 100) 
    topCKCtr3.active = true