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我创建了一个UIAlertController

let alertC = UIAlertController(title: "Title", message: "Message", preferredStyle: UIAlertControllerStyle.Alert)
alertC.addTextFieldWithConfigurationHandler(addTextField)
alertC.addAction(UIAlertAction(title: "Cancel", style: UIAlertActionStyle.Default, handler: nil))
alertC.addAction(UIAlertAction(title: "OK", style: UIAlertActionStyle.Default, handler: okButton))
presentViewController(alertC, animated: true, completion: nil)

但之后我想改变UIAlertController的高度？我怎样才能做到这一点？