How to cache using NSURLSession and NSURLCache. Not working

Question 1

Note that the following SO post helped me solve my problem: Is NSURLCache persistent across launches?

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Set app-wide shared cache (first number is megabyte value)
    NSUInteger cacheSizeMemory = 500*1024*1024; // 500 MB
    NSUInteger cacheSizeDisk = 500*1024*1024; // 500 MB
    NSURLCache *sharedCache = [[NSURLCache alloc] initWithMemoryCapacity:cacheSizeMemory diskCapacity:cacheSizeDisk diskPath:@"nsurlcache"];
    [NSURLCache setSharedURLCache:sharedCache];
    sleep(1); // Critically important line, sadly, but it's worth it!
}

In addition to the sleep(1) line, also note the size of my cache; 500MB.

According to docs you need a cache size that is way bigger than what you're trying to cache.

The response size is small enough to reasonably fit within the cache. (For example, if you provide a disk cache, the response must be no larger than about 5% of the disk cache size.)

So for example if you want to be able to cache a 10MB image, then a cache size of 10MB or even 20MB will not be enough. You need 200MB. Honey's comment below is evidence that Apple is following this 5% rule. For an 8Mb he had to set his cache size to minimum 154MB.

Question 2

Solution - first get all info u need it something like this

- (void)loadData
{
    if (!self.commonDataSource) {
        self.commonDataSource = [[NSArray alloc] init];
    }

    [self setSharedCacheForImages];

    NSURLSession *session = [self prepareSessionForRequest];
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:[BaseURLString stringByAppendingPathComponent:@"app.json"]]];
    [request setHTTPMethod:@"GET"];
    __weak typeof(self) weakSelf = self;
    NSURLSessionDataTask *dataTask = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
        if (!error) {
            NSArray *jsonResponse = [NSJSONSerialization JSONObjectWithData:data options:kNilOptions error:&error];
            weakSelf.commonDataSource = jsonResponse;
            dispatch_async(dispatch_get_main_queue(), ^{
                [weakSelf updateDataSource];
            });
        }
    }];
    [dataTask resume];
}

- (void)setSharedCacheForImages
{
    NSUInteger cashSize = 250 * 1024 * 1024;
    NSUInteger cashDiskSize = 250 * 1024 * 1024;
    NSURLCache *imageCache = [[NSURLCache alloc] initWithMemoryCapacity:cashSize diskCapacity:cashDiskSize diskPath:@"someCachePath"];
    [NSURLCache setSharedURLCache:imageCache];
}

- (NSURLSession *)prepareSessionForRequest
{
    NSURLSessionConfiguration *sessionConfiguration = [NSURLSessionConfiguration defaultSessionConfiguration];
    [sessionConfiguration setHTTPAdditionalHeaders:@{@"Content-Type": @"application/json", @"Accept": @"application/json"}];
    NSURLSession *session = [NSURLSession sessionWithConfiguration:sessionConfiguration];
    return session;
}

After you need to download each file - in my case - make parsing of response and download images. Also before making request you need to check if cache already have response for your request - something like this

NSString *imageURL = [NSString stringWithFormat:@"%@%@", BaseURLString ,sourceDictionary[@"thumb_path"]];
NSURLSession *session = [self prepareSessionForRequest];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:imageURL]];
[request setHTTPMethod:@"GET"];

NSCachedURLResponse *cachedResponse = [[NSURLCache sharedURLCache] cachedResponseForRequest:request];
if (cachedResponse.data) {
    UIImage *downloadedImage = [UIImage imageWithData:cachedResponse.data];
    dispatch_async(dispatch_get_main_queue(), ^{
        cell.thumbnailImageView.image = downloadedImage;
    });
} else {
NSURLSessionDataTask *dataTask = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
    if (!error) {
        UIImage *downloadedImage = [UIImage imageWithData:data];
        dispatch_async(dispatch_get_main_queue(), ^{
            cell.thumbnailImageView.image = downloadedImage;
        });
    }
}];
    [dataTask resume];
}

After that you can also check result with xCode Network Analyzer.

Also note as mentionted by @jcaron and documented by Apple

NSURLSession won't attempt to cache a file larger than 5% of the cache size

Result something like

enter image description here

Question 3

Once you set the cache and the session, you should use the session-methods to download your data:

- (IBAction)btnClicked:(id)sender {
    NSString *imageUrl = @"http://placekitten.com/1000/1000";
    NSURLSessionDataTask* loadDataTask = [session dataTaskWithRequest:[NSURLRequest requestWithURL:[NSURL URLWithString:imageUrl]] completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
        UIImage *downloadedImage = [UIImage imageWithData:data];
        NSLog(@"ImageSize: %f, %f", downloadedImage.size.width, downloadedImage.size.height);
        NSLog(@"DiskCache: %i of %i", [[NSURLCache sharedURLCache] currentDiskUsage], [[NSURLCache sharedURLCache] diskCapacity]);
        NSLog(@"MemoryCache: %i of %i", [[NSURLCache sharedURLCache] currentMemoryUsage], [[NSURLCache sharedURLCache] memoryCapacity]);
    }];
    [loadDataTask resume]; //start request
}

After the first call, the image is cached.