问题描述

我正在使用 NSFileManager 的 startDownloadingUbiquitousItemAtURL 将文件从 iCloud 下载到本地副本(在这种情况下本地还没有文件的副本).我似乎找不到这个过程的回调.我需要回调来告诉我的应用程序请求的文件已完成下载(到本地副本)，以便其他任务可以开始读取文件的内容并执行操作.

I am using NSFileManager's startDownloadingUbiquitousItemAtURL to download file from iCloud to local copy (the local does not yet have the copy of the file in this case). I can't seem to find the callback for this process. I need the callback to tell my app that the requested file is finished downloaded (to local copy) so other task can start reading the content of the file and does stuff.

我可以检查文件是否已下载.但它会涉及不断的轮询.有没有办法在不设置计时器来轮询的情况下做到这一点?

I can check to see if the file is downloaded. But it would involve in polling constantly. Is there a way to do this without setting a timer to poll for this?

推荐答案

我相信这个方法打算和基于Apple 的文档.所以你需要像这样使用文件协调器:

I believe that this method is intended to be used in conjunction with file coordinators based on Apple's documentation. So you would need to use a file coordinator like so:

NSURL *itemURL = nil; // this is the URL you want to read from

__block NSData *data = nil;
NSError *error = nil;
NSFileCoordinator *coordinator = [[NSFileCoordinator alloc] initWithFilePresenter:nil];
[coordinator coordinateReadingItemAtURL:itemURL options:0 error:&error byAccessor:^(NSURL *newURL) {
    data = [NSData dataWithContentsOfURL:newURL];
}];

不过，这将是同步的，因此如果您想异步执行某项操作，可以这样使用块:

This will be synchronous, however, so if you wanted to do something asynchronously, you could use blocks as so:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // read info from the URL using the code above
    dispatch_async(dispatch_get_main_queue(), ^{
        // handle data read from the URL
    });
});

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