NSURLCache cachedresponseforrequest no data

why the responseCache is nil? i'd run this post and really get the responseObject from cache. How can i get the responseCache?

manager.requestSerializer = [AFJSONRequestSerializer serializer];
manager.responseSerializer = [AFJSONResponseSerializer serializer];

manager.requestSerializer.cachePolicy=NSURLRequestReturnCacheDataElseLoad;

[manager POST:URL parameters:paramdic progress:^(NSProgress * _Nonnull uploadProgress) {
} success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {

    NSData * data=[NSJSONSerialization dataWithJSONObject:responseObject options:NSJSONWritingPrettyPrinted error:nil];

    NSURLCache * cache=[NSURLCache sharedURLCache];

    NSCachedURLResponse * responseCache=[cache cachedResponseForRequest:task.originalRequest];

    NSCachedURLResponse * response=[[NSCachedURLResponse alloc]initWithResponse:task.response data:data userInfo:nil storagePolicy:NSURLCacheStorageAllowed];


    [cache storeCachedResponse:response forRequest:task.originalRequest];




} failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
    NSLog(@"%@",error);
}];

Solution

There are three reasons it is nil at that point:

POST requests are not cached by any iOS/OS X networking code because they are not guaranteed to be idempotent (i.e. they can have side effects, such as storing data on the server). The only way a POST request will ever get stored in an NSURLCache is if you explicitly add it.
POST requests are not cached because NSURLCache uses the URL as the lookup key. Because the URL does not (cannot) include the POST body, any POST operations to the same URL would be returned for a different POST request, which is almost certainly not what you would want. So if you do add it, you'll have to add some custom rewriting of the URL on its way into the cache and custom lookup code to make the URLs unique enough based on specific POST body fields or whatever.
The cache is highly asynchronous, so cached data would not necessarily be available when the request's completion handler runs even if this were a GET request.

This is not necessarily a complete set of reasons. :-)

The cache is intended to reduce network traffic. You shouldn't generally consult it yourself. The normal lookup path used by NSURLSession et al performs checks for certain protocol caching policies (e.g. response expiration) that would not be performed by merely asking the cache if it has a response for a particular key.

If you need a general mechanism for storing a single response for later use by your app (rather than keeping it in memory), you should do so in your own internal dictionary (or, if the response is large, by using a download task and moving the file into a temporary folder in your app's sandbox that you purge on every launch).