iOS自定义对话框

1. 在Storyboard中新建一个ViewController命名为MyDialogViewController，将背景色设置为黑色半透明(使对话框的背景变暗)，新建一个View居中显示用来放置对话框内容。
2. 弹出MyDialogViewController前将其modalPresentationStyle设置为UIModalPresentationStyle.overFullScreen。

    private func showDialog(){
        let storyboard=UIStoryboard(name: "Main", bundle: nil)
        let viewController=storyboard.instantiateViewController(withIdentifier: "MyDialogViewController")
        viewController.modalPresentationStyle=UIModalPresentationStyle.overFullScreen
        present(viewController,animated: false, completion: nil)
    }