NSURLCache - Disk caching for GET request with parameters not working

Question

I'm trying to cache some HTTP requests to disk using NSURLCache. After doing some tests with AFNetworking and not working, I created a simple example using NSURLRequest.

I initialize the cache on the AppDelegate like this, setting the memory size to 0, forcing it to go always to Disk:

NSUInteger sz = 1024*1024*20;
NSURLCache *cache = [[NSURLCache alloc] initWithMemoryCapacity:0 diskCapacity:sz diskPath:nil];
[NSURLCache setSharedURLCache:cache];

I use the following code to store the response:

NSURLCache *urlCache = [NSURLCache sharedURLCache];

NSString *urlString = @"http://localhost:8000/cities/api/cities/?lang=en";
NSHTTPURLResponse *response = nil;
NSError *error = nil;

NSURL *finalUrl = [NSURL URLWithString:urlString];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:finalUrl];
request.cachePolicy = NSURLRequestReturnCacheDataElseLoad;

NSData *data = nil;

data = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];

NSCachedURLResponse *cachedResponse = [[NSCachedURLResponse alloc] initWithResponse:response data:data userInfo:nil storagePolicy:NSURLCacheStorageAllowed];
[urlCache storeCachedResponse:cachedResponse forRequest:request];

My request has the HTTP Header Cache-Control like:

response['cache-control'] = 'max-age=36000, public'

I see the response is cached in the file Cache.db.

And later, in the next execution or even in the same one, I use the following code to try to get the response from the Cache:

NSCachedURLResponse *cachedResponse = [urlCache cachedResponseForRequest:request];
response = cachedResponse.response;
data = cachedResponse.data;

The problem is that cachedResponse is always null when the request has GET parameters on it. If the request is "http://localhost:8000/cities/api/cities/", the result is stored and recovered later. But when it does have GET parameters, something is preventing the method [NSURLCache cachedResponseForRequest:request] from finding the request.

Giving this, I subclassed NSURLCache and implemented that method like this:

- (id)initWithMemoryCapacity:(NSUInteger)memoryCapacity diskCapacity:(NSUInteger)diskCapacity diskPath:(NSString *)path {
self = [super initWithMemoryCapacity:memoryCapacity diskCapacity:diskCapacity diskPath:path];
if (self) {
    NSArray *docPaths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
    NSString *dir = (NSString*)[docPaths objectAtIndex:0];
    dir = [dir stringByAppendingString:@"/uk.co.airsource.TestURLCache/nsurlcache"];
    NSString *path = [dir stringByAppendingFormat:@"/Cache.db"];
    self.db = [[FMDatabase alloc] initWithPath:path];
    [self.db open];
    int o = 4;
}
return self;

}

- (NSCachedURLResponse *) cachedResponseForRequest:(NSURLRequest *)request {
NSCachedURLResponse *cachedURLResponse = [super cachedResponseForRequest:request];
if (cachedURLResponse == nil && [request.HTTPMethod isEqualToString:@"GET"]) {
    //[db executeQuery:@"select * from test where a = ?", @"hi'",  nil];
    NSLog(@"%@", request.URL.absoluteString);
    FMResultSet *cfurl_cache_response = [self.db executeQuery:@"select * from cfurl_cache_response where request_key = ? limit 1", request.URL.absoluteString, nil];
    if ([cfurl_cache_response next]) {
        id entry_ID = [cfurl_cache_response objectForColumnName:@"entry_ID"];
        [cfurl_cache_response close];
        if (entry_ID != [NSNull null]) {
            FMResultSet *cfurl_cache_blob_data = [self.db executeQuery:@"select * from cfurl_cache_blob_data where entry_ID = ? limit 1", entry_ID, nil];
            if ([cfurl_cache_blob_data next]) {
                id response_object = [cfurl_cache_blob_data objectForColumnName:@"response_object"];
                [cfurl_cache_blob_data close];
                FMResultSet *cfurl_receiver_data = [self.db executeQuery:@"select * from cfurl_cache_receiver_data where entry_ID = ? limit 1", entry_ID, nil];
                if ([cfurl_receiver_data next]) {
                    id receiver_data = [cfurl_receiver_data objectForColumnName:@"receiver_data"];
                    [cfurl_receiver_data close];
                    if (response_object != [NSNull null] && receiver_data != [NSNull null] && response_object && receiver_data) {
                        NSURLResponse *urlResponse = [[NSURLResponse alloc] initWithURL:request.URL MIMEType:[[request allHTTPHeaderFields] objectForKey:@"Accept"] expectedContentLength:[(NSData *)response_object length] textEncodingName:nil];
                        cachedURLResponse = [[NSCachedURLResponse alloc] initWithResponse:urlResponse data:receiver_data userInfo:nil storagePolicy:NSURLCacheStorageAllowed];
                    }
                }
            }
        }
    }
}
return cachedURLResponse;

}

In this case under the GET request with parameters, it does follow every single line until getting the NSURLResponse that was cached before.

Any ideas on why is it not working normally when the request has GET parameters?

Answer 1

The problem appeared to be with not specifying memory capacity, if you specify the memory capacity as follows,

Here I am specifying memory capacity to be 1 MB of size

[[NSURLCache alloc] initWithMemoryCapacity:1*1024*1024 diskCapacity:sz diskPath:nil];

it will work, it appears to me as if the memory capacity is needed no matter, how small the capacity is but cannot be empty. Then you will be able to extract stored cached as follows,

NSCachedURLResponse *cachedResponse = [[NSURLCache sharedURLCache] cachedResponseForRequest:request];
response = cachedResponse.response;
data = cachedResponse.data;

Answer 2

Finally I overrided NSURLCache to be able to achieve what I was looking for:

1. GET requests (with parameters) being cached to Disk
2. Requests going to cache while "Cache-Control max-age" value says it is not expired, and going to the server again after expiration, storing the new response and starting the expiration time over
3. If offline, always return the response (if it exists) from cache

NTURLCache - overrides NSURLCache

I seriously think this is an issue that is not working properly in iOS 8.1

Answer 3

The issue persists for iOS 11.x as well, though I have an iPad Mini running version iOS 9.3.5 which has perfectly working cache. Something is definetely broken with NSURLCache/URLCache