IOS download image asynchronously and cache it

gistfile1.m

__block ASIHTTPRequest* req=[[ASIHTTPRequest alloc] initWithURL:[NSURL URLWithString:url]];
req.cachePolicy=ASIOnlyLoadIfNotCachedCachePolicy;
req.downloadCache=[ASIDownloadCache sharedCache];
        
[req setCompletionBlock:^{
    UIImage* img=[UIImage imageWithData:[req responseData]];
}];
        
[req setFailedBlock:^{
    NSLog(@"Download Failed");
}];
        
[req startAsynchronous];