如何向UIDocumentInteractionController预览添加按钮

Question

我正在使用UIDocumentInteractionController进行文件预览：

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？