在Swift中以编程方式创建的UIActivityIndi​​catorView不显示

Question

我实际上是为UIView创建了一个扩展。所以我只使用ViewController的第一个视图来执行此操作：

view.displayBusy()      // you don't need 'self.view' in Swift
...
view.displayNotBusy()

我也试过这个：

view.displayBusy()
DispatchQueue.global().async {
   doSomething()
   DispatchQueue.main.async {
      doSomethingComplete()
   }
}
func doSomethingComplete() {
   view.displayNotBusy()
}

也不工作。

这是我的扩展。我添加了子视图，我把它带到了前面。但我看不到它。我还设置断点并在更新约束后检出框架。它位于市中心。我已经测试了没有'displayNotBusy'，并且'hidesWhenStopped'设置为false只是为了确保它没有通过忙碌区域直接跳转。我做了128号。

我看不到它。有谁知道为什么我没有看到它？

extension UIView {
    func displayBusy() {
        var busyView : UIActivityIndicatorView? = viewWithTag(0x4255) as? UIActivityIndicatorView
        if busyView == nil {
            busyView = UIActivityIndicatorView(frame: CGRect(x: 50, y: 50, width: 32, height: 32))
            busyView!.tag = 0x4255
            addSubview(busyView!)

            busyView!.translatesAutoresizingMaskIntoConstraints = false
            let widthConstraint = NSLayoutConstraint(item: busyView!, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 32)
            let heightConstraint = NSLayoutConstraint(item: busyView!, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 32)
            addConstraints([widthConstraint, heightConstraint])

            let horConstraint = NSLayoutConstraint(item: busyView!, attribute: .centerX, relatedBy: .equal, toItem: self, attribute: .centerX, multiplier: 1, constant: 0)
            let verConstraint = NSLayoutConstraint(item: busyView!, attribute: .centerY, relatedBy: .equal, toItem: self, attribute: .centerY, multiplier: 1, constant: 0)
            addConstraints([horConstraint, verConstraint])

            // Now make it all happen
            setNeedsUpdateConstraints()
            setNeedsLayout()
            layoutIfNeeded()
        }
        self.bringSubview(toFront: busyView!)
        busyView!.hidesWhenStopped = true
        busyView!.isHidden = false
        busyView!.startAnimating()
    }

    func displayNotBusy() {
        let busyView : UIActivityIndicatorView? = viewWithTag(0x4255) as? UIActivityIndicatorView
        if busyView != nil {
            busyView!.stopAnimating()
        }
    }
}