Getting data back from an NSURLSession via POST with JSON

Question

Since NSURLConnection is depreated I need to move to an NSURLSession. I have a URL and some data I need to input as JSON. Then the result should be JSON coming back. I see something like so:

NSError *error;

NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
NSURL *url = [NSURL URLWithString:@"[JSON SERVER"];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url
                                                       cachePolicy:NSURLRequestUseProtocolCachePolicy
                                                   timeoutInterval:60.0];

[request addValue:@"application/json" forHTTPHeaderField:@"Content-Type"];
[request addValue:@"application/json" forHTTPHeaderField:@"Accept"];

[request setHTTPMethod:@"POST"];
NSDictionary *mapData = [[NSDictionary alloc] initWithObjectsAndKeys: @"TEST IOS", @"name",
                     @"IOS TYPE", @"typemap",
                     nil];
NSData *postData = [NSJSONSerialization dataWithJSONObject:mapData options:0 error:&error];
[request setHTTPBody:postData];


NSURLSessionDataTask *postDataTask = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {

}];

[postDataTask resume];

I this the correct approach?

My requirements are: 1. Turn my key value pairs into JSON. 2. Pass in the URL and JSON to a reusable function. 3. Get the JSON data returned. 4. Parse the JSON data returned.

Answer 1

Have the callers to your method to provide a completion handler which processes the data returned and update the UI to indicate completion.

You can copy the pattern found in the SDK, as follows:

- (void)makeRequest:(NSString *)param completion:(void (^)(NSDictionary *, NSError *))completion;

Implement it like this:

// in the same scope
NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];

- (void)makeRequest:(NSString *)param
         completion:(void (^)(NSDictionary *, NSError *))completion {

    // your OP code goes here, e.g.
    NSError *error;
    NSMutableURLRequest *request = // maybe the param is the url for this request
   // use the already initialized session
    NSURLSessionDataTask *postDataTask = [session dataTaskWithRequest:request 
        completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {

        // call the completion handler in EVERY code path, so the caller is never left waiting
        if (!error) {
            // convert the NSData response to a dictionary
            NSDictionary *dictionary = [NSJSONSerialization JSONObjectWithData:data options:0 error:&error];
            if (error) {
                // there was a parse error...maybe log it here, too
                completion(nil, error);
            } else {
                // success!
                completion(dictionary, nil);
            }
        } else {
            // error from the session...maybe log it here, too
            completion(nil, error);
        }
    }];
    [postDataTask resume];
}

Code that calls this method will look like this:

// update the UI here to say "I'm busy making a request"
// call your function, which you've given a completion handler
[self makeRequest:@"https://..." completion:^(NSDictionary *someResult, NSError *error) {
    // here, update the UI to say "Not busy anymore"
    if (!error) {
        // update the model, which should cause views that depend on the model to update
        // e.g. [self.someUITableView reloadData];
    } else {
        // handle the error
    }
}];

Notice a couple things: (1) the return type is void, the caller expects nothing to be returned from this method, and makes no assignment when calling it. The data "returned" is provided as parameters to the completion handler, which is called later, after the asnych request is complete, (2) the signature of the completion handler matches exactly what the caller declared in the completion block ^(NSDictionary *, NSError *), this is just a suggestion, typical for network requests.