icloud doesn't update the download precess

问题:

Usually, my code is ok to download the files from iCloud. But sometimes the downloading process will not be updated.

Following are some code snips:

start download:

            NSFileManager*  fm = [NSFileManager defaultManager];
        if (![fm startDownloadingUbiquitousItemAtURL:file error:nil]) {
            return NO;
        }

        /////////////////////////
        id query = [[icloudClass alloc] init];
        _query = query;

        [query setSearchScopes:[NSArray arrayWithObject:NSMetadataQueryUbiquitousDocumentsScope]];

        NSArray *listItems = [[file path] componentsSeparatedByString:@"/"];

        NSPredicate *pred = [NSPredicate predicateWithFormat: @"%K == %@", NSMetadataItemFSNameKey, [listItems objectAtIndex:[listItems count] -1] ];
        NSLog(@"filename = %@", [listItems objectAtIndex:[listItems count] -1]);
        [query setPredicate:pred];

       [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(updateDownloading:) name:NSMetadataQueryDidUpdateNotification object:query];

        [query startQuery];

update download:

    -(void)updateDownloading:(NSNotification *)notification {
    id query = [notification object];


    if ([query resultCount] /*>=*/ == 1) {
        for (int i=0; i<[query resultCount]; ++i) {
            NSMetadataItem *item = [query resultAtIndex:i];

            //debug
            NSNumber* isDownloading = [item valueForAttribute:NSMetadataUbiquitousItemIsDownloadingKey];
            NSNumber* isDownloaded  = [item valueForAttribute:NSMetadataUbiquitousItemIsDownloadedKey];
            NSNumber* perDownloading = [item valueForAttribute:NSMetadataUbiquitousItemPercentDownloadedKey];

            NSLog(@"isDownloaded=%@", [isDownloaded boolValue]?@"Yes":@"No");
            NSLog(@"isDownloading=%@", [isDownloading boolValue]?@"Yes":@"No");
            NSLog(@"percent downloaded=%f", [perDownloading doubleValue]);
            //

            if ([isDownloaded boolValue]) {

                [query disableUpdates];
                [query stopQuery];
                [[NSNotificationCenter defaultCenter] removeObserver:self name:NSMetadataQueryDidUpdateNotification object:query];
                _query = nil;


                NSLog(@"----------end download!------------------");



            }

        }//end for
    } 
}

please note that the update function will only be invoked once with the following console print, and never will be invoked even restart the app.

Jan 10 15:20:27 unknown *[1383] <Warning>: isHasConflict=No
Jan 10 15:20:27 unknown *[1383] <Warning>: isDownloaded=No
Jan 10 15:20:27 unknown *[1383] <Warning>: isDownloading=No
Jan 10 15:20:27 unknown *[1383] <Warning>: percent downloaded=0.000000

Does anyone met this problem before?