ios - How to cache using NSURLSession and NSURLCache. Not working

Question

I have a test app setup and it successfully downloads content from the network even if the user switches apps while a download is in progress. Great, now I have background downloads in place. Now I want to add caching. There is no point to me downloading images more than once, b/c of system design, given an image URL I can tell you the content behind that URL will never change. So, now I want to cache the results of my download using apple's built in in-memory/on-disk cache that I've read so much about (as opposed to me saving the file manually in NSCachesDirectory and then checking there before making new request, ick). In an attempt to get caching working on top of this working code, I added the following code:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Override point for customization after application launch.

    // Set app-wide shared cache (first number is megabyte value)
    [NSURLCache setSharedURLCache:[[NSURLCache alloc] initWithMemoryCapacity:60 * 1024 * 1024
                                                                diskCapacity:200 * 1024 * 1024
                                                                    diskPath:nil]];

    return YES;
}

When I create my session, I've added two NEW lines (URLCache and requestCachePolicy).

// Helper method to get a single session object
- (NSURLSession *)backgroundSession
{
    static NSURLSession *session = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfiguration:@"com.example.apple-samplecode.SimpleBackgroundTransfer.BackgroundSession"];
        configuration.URLCache = [NSURLCache sharedURLCache]; // NEW LINE ON TOP OF OTHERWISE WORKING CODE
        configuration.requestCachePolicy = NSURLRequestReturnCacheDataElseLoad;  // NEW LINE ON TOP OF OTHERWISE WORKING CODE
        session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
    });
    return session;
}

Then, just to be ultra redundant in an attempt to see caching success I switched my NSURLRequest line from

// NSURLRequest *request = [NSURLRequest requestWithURL:downloadURL]; // Old line, I've replaced this with...
NSURLRequest *request = [NSURLRequest requestWithURL:downloadURL cachePolicy:NSURLRequestReturnCacheDataElseLoad timeoutInterval:2*60]; // New line

Now, when I go to download the item a 2nd time, the experience is exaclty like the first!! Takes a long time to download and progress bar is animated slow and steady like an original download. I want the data in the cache immediately!! What am I missing???

----------------------------UPDATE----------------------------

Okay, thanks to Thorsten's answer, I've added the following two lines of code to my didFinishDownloadingToURL delegate method:

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)downloadURL {

    // Added these lines...
    NSLog(@"DiskCache: %@ of %@", @([[NSURLCache sharedURLCache] currentDiskUsage]), @([[NSURLCache sharedURLCache] diskCapacity]));
    NSLog(@"MemoryCache: %@ of %@", @([[NSURLCache sharedURLCache] currentMemoryUsage]), @([[NSURLCache sharedURLCache] memoryCapacity]));
    /*
    OUTPUTS:
    DiskCache: 4096 of 209715200
    MemoryCache: 0 of 62914560
    */
}

This is great. It confirms the cache is growing. I presume since I'm using a downloadTask (downloads to file as opposed to memory), that that's why DiskCache is growing and not memory cache first? I figured everything would go to memory cache until that overflowed and then disk cache would be used and that maybe memory cache was written to disk before the OS kills the app in the background to free up memory. Am I misunderstanding how Apple's cache works?

This is a step forward for sure, but the 2nd time I download the file it takes just as long as the first time (maybe 10 seconds or so) and the following method DOES get executed again:

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
    // This shouldn't execute the second time around should it? Even if this is supposed to get executed a second time around then shouldn't it be lightning fast? It's not.
    // On all subsequent requests, it slowly iterates through the downloading of the content just as slow as the first time. No caching is apparent. What am I missing?
}

What do you make of my edits above? Why am I not seeing the file returned very quickly on subsequent requests?

How can I confirm if the file is being served from the cache on the 2nd request?

Answer 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.