如何向UIDocumentInteractionController预览添加按钮

Question

我UIDocumentInteractionController用于文件预览:

interactionController = UIDocumentInteractionController.FromUrl(url);    
interactionController.Delegate = new UIDocumentInteractionControllerDelegateClass(this);    
InvokeOnMainThread(delegate{interactionController.PresentPreview(true);}
});

如何在视图顶部添加自定义按钮以执行某些操作？