swift - 添加 NSLayoutConstraint 后， View 消失

Question

当我向 View 添加 NSLayoutConstraint 时，会导致 View 消失。

我正在使用以下代码:

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()

当我调试 View 层次结构时，我看到 View 存在，即使它不可见。有关详细信息，请参阅下面的屏幕截图 - 只有约束可见，而不是 View :

Answer 1

这里需要讨论的事情太多了。

1. 当您使用以下内容时

   test.translatesAutoresizingMaskIntoConstraints = false
   

   那么它将完全依赖于约束来定位和调整 View 的大小。因此您需要设置 View 的高度和宽度约束。

   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)
   

最后这将是您正在寻找的代码

            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: 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