Load image with headers and cache it - Objective-C (macOS apps) - ios

Question

I need to load big image from thumbnail url with forHTTPHeaderField and cache it so that not to load again and again from url. Below is the code I am using.
NSString *urlStr = [NSString stringWithFormat:#"%#",medialurl];
NSURL *url = [NSURL URLWithString:urlStr];
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
[request setHTTPMethod:#"GET"];
[request setURL:url];
[request addValue:#"application/json" forHTTPHeaderField:#"Content-Type"];
[request addValue:#"abcd" forHTTPHeaderField:#"ABC"];
[request addValue:#"xyzzy" forHTTPHeaderField:#"XYZ"];
[RestService fetchDataWithCompletionBlock:request completionHandler:^(NSData * _Nonnull responseData, NSURLResponse * _Nonnull response, NSError * _Nonnull error) {
NSString *responseStr = [[NSString alloc] initWithData:responseData encoding:NSUTF8StringEncoding];
NSLog(#"responseStr: %#", responseStr);
NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *) response;
long code = (long)[httpResponse statusCode];
NSLog(#"response status code: %ld", code);
completionBlock(responseData);
}];

Answer 1

Would setting a different cachePolicy on your request help?
request.cachePolicy = NSURLRequestReturnCacheDataElseLoad;
Use existing cache data, regardless or age or expiration date, loading from originating source only if there is no cached data.
There is a note on the cachePolicy property which states:
This property is ignored for requests used to construct NSURLSessionUploadTask and NSURLSessionDownloadTask objects, as caching is not supported by the URL Loading System for upload or download requests.
So, if that is how you're performing your request, this might not work and you would probably have to manage your caching for this request yourself.
You can check to see if there is a cached response for your request with the following (assuming you are using the shared cache and not creating your own):
NSCachedURLResponse* cachedResponse = [[NSURLCache sharedURLCache] cachedResponseForRequest:request];
if ( cachedResponse )
{
NSHTTPURLResponse* response = (NSHTTPURLResponse*)cachedResponse.response;
NSLog(#"Cached Response: %#", response.allHeaderFields);
}
else
{
NSLog(#"No Cached Response");
}
If your endpoint uses ETags, you can also store the value of the ETag header field from the cached response and then compare it to what you get back when you make your request, this would tell you if the response you get back was indeed from the cache or not.
If the above tells you there is no cached response for your request, then you'd have to look into why it might not be caching previous responses. Perhaps there is an issue with the "Cache-Control" header coming back from the server and you are not permitted to cache the responses? If that were the case and you have no control over the server side, you would have to use another method to cache your images.