How to asynchronously download files in a specific order in iOS? - ios

Question

My iPhone app makes a asynchronous request with NSURLRequest to my site.
The response is JSON with 10 URLs to images. When I receive the asynchronous response I want to download the 10 files using a loop, creating an NSMutableArray of UIImage objects.
I tried to do this with the method NSData dataWithContentsOfURL, and it works but isn't asynchronous, so the user interface is blocked.
If I try to use the NSURL asynchronous method inside the response of this asynchronous method, when I receive the 10 responses with the images, I can't know if the images have been downloaded in order, and in my application the order is important.
What is a solution for downloading files in order, without blocking the UI?
My code:
// Create the request.
NSString *advertURL = [NSString stringWithFormat: #"http://www.myURL"];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:advertURL]
cachePolicy:NSURLRequestUseProtocolCachePolicy
timeoutInterval:60.0];
// Create url connection and fire request
imagesConnection = [[NSURLConnection alloc] initWithRequest:request delegate:self];
When I receive response:
//decode json with the urls
NSArray* json = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:&error];
//star the loop to downloading the ten images
for (int i=0; i<10; i++) {
//find image[i] url from json
NSString *fullImage_URL = json[i][#"url"];
//download image synchronously (this blocks the UI!!)
NSData * data = [NSData dataWithContentsOfURL:[NSURL URLWithString:fullImage_URL]];
//insert image in the array
arrayImages[i] = [UIImage imageWithData:data];
}
Then, when I'm sure the array has 10 images and they're in order, I can start to show images on the screen.

Answer 1

Instead of processing the JSON array and then looping and fetching all those images on the main thread, why not do it on a background thread. In the background, run the loop, and at the end, call another method on the main to signal the completed task.
//star the loop to downloading the ten images ... in the background
dispatch_queue_t currentQueue = dispatch_get_current_queue();
dispatch_retain(currentQueue);
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0, ^(void) {
for (int i=0; i<10; i++) {
//find image[i] url from json
NSString *fullImage_URL = json[i][#"url"];
//download image synchronously (this blocks the UI!!)
NSData * data = [NSData dataWithContentsOfURL:[NSURL URLWithString:fullImage_URL]];
//insert image in the array
arrayImages[i] = [UIImage imageWithData:data];
}
// Return to main queue
dispatch_async(currentQueue, ^{
// process arrayImages now
});
dispatch_release(currentQueue);
});
To read more about the dispatch queues, check this out:
https://developer.apple.com/library/ios/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html#//apple_ref/doc/uid/TP40008091-CH102-SW1
There are many other ways to do this. Personally, I'm a fan of using NSNotificationCenter myself, and the OBSERVER pattern.
https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSNotificationCenter_Class/Reference/Reference.html