How can I keep track of multiple downloads with NSURLSession? - objective-c

Question

How can I keep track of multiple downloads with NSURLSession?
For example:
NSURLSessionConfiguration *sessionConfiguration = [NSURLSessionConfiguration defaultSessionConfiguration];
session = [NSURLSession sessionWithConfiguration:sessionConfiguration delegate:self delegateQueue:nil];
NSURLSessionDownloadTask *jsonTask = [session downloadTaskWithURL:[NSURL URLWithString:[urlString stringByAppendingString:#"iOSMenu.json"]]];
NSURLSessionDownloadTask *imageTask = [session downloadTaskWithURL:[NSURL URLWithString:[urlString stringByAppendingString:#"menu1#2x.png"]]];
NSURLSessionDownloadTask *titleTask = [session downloadTaskWithURL:[NSURL URLWithString:[urlString stringByAppendingString:#"titleBar#2x.png"]]];
[jsonTask resume];
[imageTask resume];
[titleTask resume];
I downloaded these three files and I can confirm it with NSLog, like this.
-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {
NSLog(#"%#", location);
}
But I don't know how can I access and differentiate these files. How can I do that?

Answer 1

The didFinishDownloadingToURL method passes you a reference to the NSURLSessionDownloadTask in that delegate method. From that, you can either refer to the task's taskIdentifier or refer to the task's originalRequest.URL property.
You want to make sure you move the file at location to somewhere you'll have access to in the future (as when you return from didFinishDownloadingToURL, it will remove that temporary file if you don't do something with it).
For example, you might save the file to your documents folder. In this example, I'll grab the last path component of the original URL, create a path to a file in your documents folder, and move the file to new location:
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {
NSString *documentsPath = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)[0];
NSString *path = [documentsPath stringByAppendingPathComponent:[downloadTask.originalRequest.URL lastPathComponent]];
NSURL *newLocation = [NSURL fileURLWithPath:path];
NSError *error;
if (![[NSFileManager defaultManager] moveItemAtURL:location toURL:newLocation error:&error]) {
NSLog(#"failed to move %# to %#: %#", location, newLocation, error);
}
}
Alternatively, if you want to not rely upon the lastPathComponent of the original URL, you can maintain a dictionary mapping the task identifiers (or original URLs) to your new location you want to save the file. You can build that dictionary as you create the tasks, and then the didFinishDownloadingToURL could look up the desired destination using the task's identifier in that dictionary, and use that in the moveItemAtURL method. Clearly, if this is a background session, you'll want to make sure you save this cross reference to persistent storage so you can re-retrieve it when the app is restarted when the downloads finish.