An NSURLCache subclass supporting block-based filters.

GOFilteringURLCache.h

#import <Foundation/Foundation.h>

@interface GOFilteringURLCache : NSURLCache

- (void)addFilterForURL:(NSURL *)URL response:(NSCachedURLResponse *(^)(NSURLRequest *request))responseBlock;
- (void)removeFiltersForURL:(NSURL *)URL;

@end

GOFilteringURLCache.m

#import "GOFilteringURLCache.h"

@interface GOURLFilter : NSObject

@property (copy, nonatomic) NSURL *targetURL;
@property (copy, nonatomic) NSCachedURLResponse *(^responseBlock)(NSURLRequest *request);

- (instancetype)initWithTargetURL:(NSURL *)targetURL response:(NSCachedURLResponse *(^)(NSURLRequest *request))responseBlock;

@end

@implementation GOURLFilter

- (instancetype)initWithTargetURL:(NSURL *)targetURL response:(NSCachedURLResponse *(^)(NSURLRequest *request))responseBlock
{
    self = [super init];
    if (!self) {
        return nil;
    }
    
    _targetURL = targetURL;
    _responseBlock = responseBlock;
    
    return self;
}

- (NSCachedURLResponse *)triggerWithRequest:(NSURLRequest *)request
{
    NSCachedURLResponse *response = nil;
    if (self.responseBlock) {
        response = self.responseBlock(request);
    }

    return response;
}

@end

@interface GOFilteringURLCache ()

@property (strong, nonatomic) NSMutableArray *filters;

@end

@implementation GOFilteringURLCache

- (instancetype)initWithMemoryCapacity:(NSUInteger)memoryCapacity diskCapacity:(NSUInteger)diskCapacity diskPath:(NSString *)path
{
    self = [super initWithMemoryCapacity:memoryCapacity diskCapacity:diskCapacity diskPath:path];
    if (!self) {
        return nil;
    }
    
    _filters = [NSMutableArray array];
    return self;
}

- (void)addFilterForURL:(NSURL *)URL response:(NSCachedURLResponse *(^)(NSURLRequest *request))responseBlock
{
    if (!URL || !responseBlock) {
        return;
    }
    GOURLFilter *filter = [[GOURLFilter alloc] initWithTargetURL:URL response:responseBlock];
    [self.filters addObject:filter];
}

- (void)removeFiltersForURL:(NSURL *)URL
{
    NSArray *matchingFilters = [self.filters arrayByFilteringWithBlock:^BOOL(GOURLFilter *filter, NSUInteger idx) {
        return [filter.targetURL isEqual:URL];
    }];
    
    [self.filters removeObjectsInArray:matchingFilters];
}

- (NSCachedURLResponse *)cachedResponseForRequest:(NSURLRequest *)request
{
    NSString *URLString = request.URL.absoluteString;

    // See if we have any matching filters
    GOURLFilter *matchingFilter = [self.filters arrayByFilteringWithBlock:^BOOL(GOURLFilter *filter, NSUInteger idx) {
        return [filter.targetURL.absoluteString isEqual:URLString];
    }].firstObject;
    
    NSCachedURLResponse *response = nil;
    if (matchingFilter) {
        response = [matchingFilter triggerWithRequest:request];
    }
    if (response == nil) {
        response = [super cachedResponseForRequest:request];
    }
    
    return response;
}

@end

Usage.m

NSURL *filteredURL = [NSURL URLWithString:@"http://example.com"];

[((GOFilteringURLCache *)[NSURLCache sharedURLCache]) addFilterForURL:filteredURL response:^NSCachedURLResponse *(NSURLRequest *request) {
    // Creating a mocked response for the request with an empty body.
    NSURLResponse *response = [[NSURLResponse alloc] initWithURL:filteredURL MIMEType:kPlainTextMIMEType expectedContentLength:1 textEncodingName:nil];
    NSCachedURLResponse *cachedResponse = [[NSCachedURLResponse alloc] initWithResponse:response data:[NSData dataWithBytes:" " length:1]];
    
    return cachedResponse;

}];