Question

In my app I have a tableView filled with contents from a server. To download these contents I use NSURLConnection and I create a NSMutableArray (tableItems) to hold and to manage the addresses to the images I want to use.

In connectionDidFinishLoading, after populating tableItems, there is this for-loop:

for (int i = 0; i < [tableItems count]; i++) {

        // HERE I CHECK IF THE IMAGE I'M LOOKING FOR IS ALREADY ON DISK
        NSString *pathToCheckImage = [NSString stringWithFormat:@"%@/%@.png", pathToPreviews, [tableItems objectAtIndex:i]];
        NSData *dataOfCheckImage = [NSData dataWithContentsOfFile:pathToCheckImage];
        UIImage *checkImage = [UIImage imageWithData:dataOfCheckImage];

        NSString *pathToImage;
        UIImage *image;
        if (checkImage != nil) {
            // THE IMAGE IS ALREADY ON DISK SO I GET IT FROM TEMP FOLDER
            image = checkImage;
        } else {

            // THE IMAGE IS NEW SO I HAVE TO GET THE IMAGE FROM THE SERVER
            pathToImage = [NSString stringWithFormat:@"http://www.SERVER.com/SERVER_PATH/%@.png", [tableItems objectAtIndex:i]];

            NSURL *url = [NSURL URLWithString:pathToImage];
            NSData *data = [NSData dataWithContentsOfURL:url];
            image = [UIImage imageWithData:data];

            // AND SAVE IT ON DISK
            NSString *path = [NSString stringWithFormat:@"%@/%@.png", pathToPreviews, [tableItems objectAtIndex:i]];
            [self cacheImageOnDisk:image withPath:path];
        }

        if (image != nil) {
            [arrayOfImages addObject:image];
        }

}

This code is working, even if, depending on the number and size of the images I have to download from the server, it can take 1 or 2 minutes to perform its task.

The problem is that if the user quits (home button pushed) while this for-loop is running it keeps on performing its task until the end, even if it needs 1 minute to finish it.

During this period, launching the app again inevitably ends up crashing on startup.

I’ve tried to stop this for-loop on exit but applicationDidEnterBackground, applicationWillResignActive and applicationWillTerminate are not called until for-loop ends its task.

I’ve tried to set “application does not run in background”, but nothing changed.

Any suggestion will be really appreciated.

Answer 1

You should not be downloading images on the main thread. The app won’t be responsive. Grand Central Dispatch is an easy method to accomplish these tasks.

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    NSUInteger count = [tableItems count];
    for (int i = 0; i < count; i++) {
        // TODO: check if app didResignActive and break

        UIImage *image = ... // get image from disk or network

        // Add the image to the array on the main thread to avoid any threading issues
        dispatch_sync(dispatch_get_main_queue(), ^{
            [arrayOfImages addObject:image];
        });
    }
});