Downloading multiple files in batches in iOS

Question

I have an app that right now needs to download hundreds of small PDF's based on the users selection. The problem I am running into is that it is taking a significant amount of time because every time it has to open a new connection. I know that I could use GCD to do an async download, but how would I go about doing this in batches of like 10 files or so. Is there a framework that already does this, or is this something I will have to build my self?

Answer 1

By the way, iOS 7 (and Mac OS 10.9) offer URLSession and URLSessionDownloadTask, which handles this quite gracefully. If you just want to download a bunch of files, you can do something like:

NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
NSURLSession              *session       = [NSURLSession sessionWithConfiguration:configuration];

NSString      *documentsPath = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)[0];
NSFileManager *fileManager   = [NSFileManager defaultManager];

for (NSString *filename in self.filenames) {
    NSURL *url = [baseURL URLByAppendingPathComponent:filename];
    NSURLSessionTask *downloadTask = [session downloadTaskWithURL:url completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
        NSString *finalPath = [documentsPath stringByAppendingPathComponent:filename];

        BOOL success;
        NSError *fileManagerError;
        if ([fileManager fileExistsAtPath:finalPath]) {
            success = [fileManager removeItemAtPath:finalPath error:&fileManagerError];
            NSAssert(success, @"removeItemAtPath error: %@", fileManagerError);
        }

        success = [fileManager moveItemAtURL:location toURL:[NSURL fileURLWithPath:finalPath] error:&fileManagerError];
        NSAssert(success, @"moveItemAtURL error: %@", fileManagerError);

        NSLog(@"finished %@", filename);
    }];
    [downloadTask resume];
}

Perhaps, given that your downloads take a "significant amount of time", you might want them to continue downloading even after the app has gone into the background. If so, you can use backgroundSessionConfiguration rather than defaultSessionConfiguration (though you have to implement the NSURLSessionDownloadDelegate methods, rather than using the completionHandler block). These background sessions are slower, but then again, they happen even if the user has left your app. Thus:

- (void)startBackgroundDownloadsForBaseURL:(NSURL *)baseURL {
    NSURLSession *session = [self backgroundSession];

    for (NSString *filename in self.filenames) {
        NSURL *url = [baseURL URLByAppendingPathComponent:filename];
        NSURLSessionTask *downloadTask = [session downloadTaskWithURL:url];
        [downloadTask resume];
    }
}

- (NSURLSession *)backgroundSession {
    static NSURLSession *session = nil;

    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfiguration:kBackgroundId];
        session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:[NSOperationQueue mainQueue]];
    });

    return session;
}

#pragma mark - NSURLSessionDownloadDelegate

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {
    NSString *documentsPath    = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)[0];
    NSString *finalPath        = [documentsPath stringByAppendingPathComponent:[[[downloadTask originalRequest] URL] lastPathComponent]];
    NSFileManager *fileManager = [NSFileManager defaultManager];

    BOOL success;
    NSError *error;
    if ([fileManager fileExistsAtPath:finalPath]) {
        success = [fileManager removeItemAtPath:finalPath error:&error];
        NSAssert(success, @"removeItemAtPath error: %@", error);
    }

    success = [fileManager moveItemAtURL:location toURL:[NSURL fileURLWithPath:finalPath] error:&error];
    NSAssert(success, @"moveItemAtURL error: %@", error);
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didResumeAtOffset:(int64_t)fileOffset expectedTotalBytes:(int64_t)expectedTotalBytes {
    // Update your UI if you want to
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite {
    // Update your UI if you want to
}

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
    if (error)
        NSLog(@"%s: %@", __FUNCTION__, error);
}

#pragma mark - NSURLSessionDelegate

- (void)URLSession:(NSURLSession *)session didBecomeInvalidWithError:(NSError *)error {
    NSLog(@"%s: %@", __FUNCTION__, error);
}

- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session {
    AppDelegate *appDelegate = (id)[[UIApplication sharedApplication] delegate];
    if (appDelegate.backgroundSessionCompletionHandler) {
        dispatch_async(dispatch_get_main_queue(), ^{
            appDelegate.backgroundSessionCompletionHandler();
            appDelegate.backgroundSessionCompletionHandler = nil;
        });
    }
}

By the way, this assumes your app delegate has a backgroundSessionCompletionHandler property:

@property (copy) void (^backgroundSessionCompletionHandler)();

And that the app delegate will set that property if the app was awaken to handle URLSession events:

- (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)())completionHandler {
    self.backgroundSessionCompletionHandler = completionHandler;
}

For an Apple demonstration of the background NSURLSession see the Simple Background Transfer sample.