iOS close background thread - ios

Question

In my ViewController, I'm downloading a number of png files off the internet and adding them to UIImageViews, which I add to a UIScrollView.
However, if the user presses the back button in the navigation bar, I noticed that the background thread is still continuing to download those files. Also, I noticed huge memory spikes as a result, and I don't think my objects are properly getting released.
How can I close all threads when the user presses back in the navigation bar?
xcode 4.2.1, ARC
Thanks!
EDIT -- the following is in a for loop
NSURL* url = [[NSURL alloc] initWithString:[#"someurl"]];
NSData* imageData = [[NSData alloc]initWithContentsOfURL:url];
UIImage* image = [[UIImage alloc] initWithData:imageData];
UIImageView *imageView = [[UIImageView alloc] initWithImage:image];
imageView.frame = aFrame;
[self.screenshots addSubview:imageView]

Answer 1

You shouldn't be doing any downloading in your view controller. That's the source of the problem. Move your data into model classes. Do all of your downloading and data management there. The view controllers are just responsible for pulling data from the model and giving it to the views. View controllers are not responsible for fetching data from the network. As you've discovered, they can vanish at any time.
You don't want to abort downloads and then restart them every time someone moves between screens. You want to let the model classes know that some data is needed (because the user came to a screen), and then you want the model classes to post a notification (or provide a KVO change notification) that lets the view controllers know that new data is read.

Answer 2

You may want to consider NSOperationQueue and add new download operations via. NSURLConnection and add a supply your NSOperationQueue. You can have more then one source downloading at once and cancel all operations at once once you need to. Also you can set things like how many to download at a time.

Answer 3

Try this:
in your for loop, make a condition that would stop the downloads when you don't want them anymore:
declare class variable or properly
BOOL stopDownloading;
set it to false on when you start the downloads:
stopDownloading = NO;
modify your for loop to stop when you set it to NO:
if (!stopDownloading) {
NSURL* url = [[NSURL alloc] initWithString:[#"someurl"]];
NSData* imageData = [[NSData alloc]initWithContentsOfURL:url];
UIImage* image = [[UIImage alloc] initWithData:imageData];
UIImageView *imageView = [[UIImageView alloc] initWithImage:image];
imageView.frame = aFrame;
[self.screenshots addSubview:imageView];
}
so now, when back button is pressed, set
stopDownloading = YES;
it will stop all further downloads, however due to your setup, i don't immediately see a way to stop the one that is currently downloading (only subsequent ones).
To solve this problem, i would create a NSOperationQueue and add NSOperations for each download.
Or even simpler, use some existing open source solutions like AFNetworking. specifically these two:
AFImageRequestOperation A subclass of AFHTTPRequestOperation for downloading and processing images.
UIImageView+AFNetworking Adds methods to UIImageView for loading remote images asynchronously from a URL.