调用openWithCompletionHandler时，UIDocument的documentState永远不会出现在UIDocumentStateProgressAvailable上

Question

我正在尝试通过openWithCompletionHandler方法打开时获取UIDocument（iCloud文档）的下载进度（NSProgress）值。根据iOS 11 SDK，当进度值可用时，UIDocument对象将进入UIDocumentStateProgressAvailable状态。但它根本没有给予价值。任何的想法 ？感谢

CustomDocument *doc = [[CustomDocument alloc] initWithFileURL:documentURL];
            [[NSNotificationCenter defaultCenter] addObserverForName:UIDocumentStateChangedNotification object:doc queue:nil usingBlock:^(NSNotification *note) {
// get called only once when the document state is Normal
                if ([doc progress]) {
                    transitionController.loadingProgress = [doc progress]; // never get called as doc.progress is always nil
                }
            }];
            [doc openWithCompletionHandler:^(BOOL success) {
                if (success) {
                    // successfully opened without being in  UIDocumentStateProgressAvailable state
                }
                [[NSNotificationCenter defaultCenter] removeObserver:doc];
            }];