Bypassing http response header Cache-Control: how to set cache expiration?

Question

All http responses from a server come with the headers that inform our app not to cache the responses:

Cache-Control: no-cache
Pragma: no-cache
Expires: 0
         


        

Answer 1

You can implement a custom NSURLCache that only returns cached responses that has not expired.

Example:

#import "CustomURLCache.h"

NSString * const EXPIRES_KEY = @"cache date";
int const CACHE_EXPIRES = -10;

@implementation CustomURLCache

// static method for activating this custom cache
+(void)activate {
    CustomURLCache *urlCache = [[CustomURLCache alloc] initWithMemoryCapacity:(2*1024*1024) diskCapacity:(2*1024*1024) diskPath:nil] ;
    [NSURLCache setSharedURLCache:urlCache];
}

-(NSCachedURLResponse *)cachedResponseForRequest:(NSURLRequest *)request {
    NSCachedURLResponse * cachedResponse = [super cachedResponseForRequest:request];
    if (cachedResponse) {
        NSDate* cacheDate = [[cachedResponse userInfo] objectForKey:EXPIRES_KEY];
        if ([cacheDate timeIntervalSinceNow] < CACHE_EXPIRES) {
            [self removeCachedResponseForRequest:request];
            cachedResponse = nil;
        }
    }

    return cachedResponse;
}

- (void)storeCachedResponse:(NSCachedURLResponse *)cachedResponse forRequest:(NSURLRequest *)request {
    NSMutableDictionary *userInfo = cachedResponse.userInfo ? [cachedResponse.userInfo mutableCopy] : [NSMutableDictionary dictionary];
    [userInfo setObject:[NSDate date] forKey:EXPIRES_KEY];
    NSCachedURLResponse *newCachedResponse = [[NSCachedURLResponse alloc] initWithResponse:cachedResponse.response data:cachedResponse.data userInfo:userInfo storagePolicy:cachedResponse.storagePolicy];

    [super storeCachedResponse:newCachedResponse forRequest:request];
}

@end

If this does not give you enough control then I would implement a custom NSURLProtocol with a startLoading method as below and use it in conjunction with the custom cache.

- (void)startLoading
{
    NSMutableURLRequest *newRequest = [self.request mutableCopy];
    [NSURLProtocol setProperty:@YES forKey:@"CacheSet" inRequest:newRequest];

    NSCachedURLResponse *cachedResponse = [[NSURLCache sharedURLCache] cachedResponseForRequest:self.request];
    if (cachedResponse) {  
        [self connection:nil didReceiveResponse:[cachedResponse response]];
        [self connection:nil didReceiveData:[cachedResponse data]];
        [self connectionDidFinishLoading:nil];
    } else {
        _connection = [NSURLConnection connectionWithRequest:newRequest delegate:self];
    }
}

Some links:

• Useful info on NSURLCache
• Creating a custom NSURLProtocol