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一个简单的单窗口App在运行时出现错误:

2016-04-07 14:28:48.411 BlurViewAndPopView[4364:168520] Warning: Attempt to present <UIAlertController: 0x7a0a4e00> on <BlurViewAndPopView.ViewController: 0x797757d0> whose view is not in the window hierarchy!2016-04-07 14:28:48.935 BlurViewAndPopView[4364:168520] Attempting to load the view of a view controller while it is deallocating is not allowed and may result in undefined behavior (<UIAlertController: 0x7a0a4e00>)

该app的功能是在root VC中弹出一个popover视图,其中包含一个表视图,在点击表视图中的某一行时回调root VC中的闭包,完成改行内容的对话框弹出效果.

分析如下:根据错误信息,可以清楚看到在试图弹出对话框时,root VC不在窗口的继承体系中,这意味着此时root VC不在window中.

查看popover的cellSelect回调方法:

override func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {let selectedItem = items[indexPath]selectionHandler?(selectedItem: selectedItem)dismissViewControllerAnimated(true, completion: nil)}

可以看到在调用root VC注册的selectionHandler闭包之后才做的dismissVC的操作,这显然顺序不对!

我们可以这样修改:

override func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {let selectedItem = items[indexPath]defer {selectionHandler?(selectedItem: selectedItem)}dismissViewControllerAnimated(true, completion: nil)}

好了!现在将selectionHandler的回调放到final中,即可保证在root VC中的操作是在dismissVC之后才开始的,这是root VC应该在window的继承体系中了.