我的应用程序中有基本的 iCloud 支持(同步更改、使无处不在等)，但到目前为止，一个关键的遗漏是缺少对云中存在(或有更改)的文件的“下载”支持，但与当前磁盘上的内容不同步。

我在我的应用程序中添加了以下方法，基于一些 Apple 提供的代码，并进行了一些调整:

下载方式:

- (BOOL)downloadFileIfNotAvailable:(NSURL*)file {
    NSNumber*  isIniCloud = nil;

    if ([file getResourceValue:&isIniCloud forKey:NSURLIsUbiquitousItemKey error:nil]) {
        // If the item is in iCloud, see if it is downloaded.
        if ([isIniCloud boolValue]) {
            NSNumber*  isDownloaded = nil;
            if ([file getResourceValue:&isDownloaded forKey:NSURLUbiquitousItemIsDownloadedKey error:nil]) {
                if ([isDownloaded boolValue])
                    return YES;

                // Download the file.
                NSFileManager*  fm = [NSFileManager defaultManager];
                NSError *downloadError = nil;
                [fm startDownloadingUbiquitousItemAtURL:file error:&downloadError];
                if (downloadError) {
                    NSLog(@"Error occurred starting download: %@", downloadError);
                }
                return NO;
            }
        }
    }

    // Return YES as long as an explicit download was not started.
    return YES;
}

- (void)waitForDownloadThenLoad:(NSURL *)file {
    NSLog(@"Waiting for file to download...");
    id<ApplicationDelegate> appDelegate = [DataLoader applicationDelegate];
    while (true) {
        NSDictionary *fileAttribs = [[NSFileManager defaultManager] attributesOfItemAtPath:[file path] error:nil];
        NSNumber *size = [fileAttribs objectForKey:NSFileSize];

        [NSThread sleepForTimeInterval:0.1];
        NSNumber*  isDownloading = nil;
        if ([file getResourceValue:&isDownloading forKey:NSURLUbiquitousItemIsDownloadingKey error:nil]) {
            NSLog(@"iCloud download is moving: %d, size is %@", [isDownloading boolValue], size);
        }

        NSNumber*  isDownloaded = nil;
        if ([file getResourceValue:&isDownloaded forKey:NSURLUbiquitousItemIsDownloadedKey error:nil]) {
            NSLog(@"iCloud download has finished: %d", [isDownloaded boolValue]);
            if ([isDownloaded boolValue]) {
                [self dispatchLoadToAppDelegate:file];
                return;
            }
        }

        NSNumber *downloadPercentage = nil;
        if ([file getResourceValue:&downloadPercentage forKey:NSURLUbiquitousItemPercentDownloadedKey error:nil]) {
            double percentage = [downloadPercentage doubleValue];
            NSLog(@"Download percentage is %f", percentage);
            [appDelegate updateLoadingStatusString:[NSString stringWithFormat:@"Downloading from iCloud (%2.2f%%)", percentage]];
        }
    }
}

以及启动/检查下载的代码:

 if ([self downloadFileIfNotAvailable:urlToUse]) {
            // The file is already available. Load.
            [self dispatchLoadToAppDelegate:[urlToUse autorelease]];
        } else {
            // The file is downloading. Wait for it.
            [self performSelector:@selector(waitForDownloadThenLoad:) withObject:[urlToUse autorelease] afterDelay:0];
        }

据我所知上面的代码看起来不错，但是当我在设备 A 上进行大量更改时，保存这些更改，然后打开设备 B(以提示在设备上下载B) 这是我在控制台中看到的:

2012-03-18 12:45:55.858 MyApp[12363:707] Waiting for file to download...
2012-03-18 12:45:58.041 MyApp[12363:707] iCloud download is moving: 0, size is 101575
2012-03-18 12:45:58.041 MyApp[12363:707] iCloud download has finished: 0
2012-03-18 12:45:58.041 MyApp[12363:707] Download percentage is 0.000000
2012-03-18 12:45:58.143 MyApp[12363:707] iCloud download is moving: 0, size is 101575
2012-03-18 12:45:58.143 MyApp[12363:707] iCloud download has finished: 0
2012-03-18 12:45:58.144 MyApp[12363:707] Download percentage is 0.000000
2012-03-18 12:45:58.246 MyApp[12363:707] iCloud download is moving: 0, size is 101575
2012-03-18 12:45:58.246 MyApp[12363:707] iCloud download has finished: 0
2012-03-18 12:45:58.246 MyApp[12363:707] Download percentage is 0.000000
2012-03-18 12:45:58.347 MyApp[12363:707] iCloud download is moving: 0, size is 177127
2012-03-18 12:45:58.347 MyApp[12363:707] iCloud download has finished: 0
2012-03-18 12:45:58.347 MyApp[12363:707] Download percentage is 0.000000
2012-03-18 12:45:58.449 MyApp[12363:707] iCloud download is moving: 0, size is 177127
2012-03-18 12:45:58.449 MyApp[12363:707] iCloud download has finished: 0
2012-03-18 12:45:58.450 MyApp[12363:707] Download percentage is 0.000000

所以无论出于何种原因:

1. 文件下载开始没有错误
2. 文件下载状态的文件属性总是返回未下载、未下载完成、进度为0%。
3. 我永远陷入了循环，即使在两次检查之间文件大小发生变化也是如此。

我做错了什么？

最佳答案

多年后，我仍然遇到下载文件的周期性问题(尽管在其他答案中的一些变通方法之后更为罕见)。因此，我联系了 Apple 的开发人员，要求进行技术审查/讨论，这就是我的发现。

定期检查相同 NSURL 的下载状态，即使您正在重新创建它，也不是检查状态的首选方法。我不知道为什么不行 - 看起来它应该 起作用，但它不起作用。相反，一旦您开始下载文件，您应该为下载的进度在 NSNotificationCenter 注册一个观察者，维护对该文件的查询的引用。这是提供给我的确切代码示例。我已经在我的应用程序中实现了它(通过一些特定于应用程序的调整)，它的表现似乎更加合适。

- (void)download:(NSURL *)url
{
    dispatch_queue_t q_default;
    q_default = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(q_default, ^{

        NSError *error = nil;
        BOOL success = [[NSFileManager defaultManager] startDownloadingUbiquitousItemAtURL:url error:&error];
        if (!success)
        {
            // failed to download
        }
        else
        {
            NSDictionary *attrs = [url resourceValuesForKeys:@[NSURLUbiquitousItemIsDownloadedKey] error:&error];
            if (attrs != nil)
            {
                if ([[attrs objectForKey:NSURLUbiquitousItemIsDownloadedKey] boolValue])
                {
                    // already downloaded
                }
                else
                {
                    NSMetadataQuery *query = [[NSMetadataQuery alloc] init];
                    [query setPredicate:[NSPredicate predicateWithFormat:@"%K > 0", NSMetadataUbiquitousItemPercentDownloadedKey]];
                    [query setSearchScopes:@[url]]; // scope the search only on this item

                    [query setValueListAttributes:@[NSMetadataUbiquitousItemPercentDownloadedKey, NSMetadataUbiquitousItemIsDownloadedKey]];

                    _fileDownloadMonitorQuery = query;

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

                    [self.fileDownloadMonitorQuery startQuery];
                }
            }
        }
    });
}

- (void)liveUpdate:(NSNotification *)notification
{
    NSMetadataQuery *query = [notification object];

    if (query != self.fileDownloadMonitorQuery)
        return; // it's not our query

    if ([self.fileDownloadMonitorQuery resultCount] == 0)
        return; // no items found

    NSMetadataItem *item = [self.fileDownloadMonitorQuery resultAtIndex:0];
    double progress = [[item valueForAttribute:NSMetadataUbiquitousItemPercentDownloadedKey] doubleValue];
    NSLog(@"download progress = %f", progress);

    // report download progress somehow..

    if ([[item valueForAttribute:NSMetadataUbiquitousItemIsDownloadedKey] boolValue])
    {
        // finished downloading, stop the query
        [query stopQuery];
        _fileDownloadMonitorQuery = nil;
    }
}

关于objective-c - 下载iCloud文件的方法？非常困惑？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/9760236/