将pdf / doc文件保存到“文件”应用程序将返回错误的完成值

Question

我正在使用以下代码将pdf文件保存到文件应用中：

let activityController = UIActivityViewController.init(activityItems: [url], applicationActivities: nil)

activityController.excludedActivityTypes = [.postToWeibo, .assignToContact, .saveToCameraRoll, .addToReadingList, .postToFlickr, .postToVimeo, .postToTencentWeibo, .openInIBooks, .assignToContact, .postToTwitter
    ]

activityController.completionWithItemsHandler = { (activityType, completed, returnedItems, error) in
   //completed returns false even though file is saved
}

那么如何让用户知道我的应用中文件已保存？