Completion handlers and return values

Question

I want to call a method which will return a value from its completion handler. The method performs asynchronously and I don't want to return a value before all the body of the method has been executed. Here is some faulty code to illustrate what I am trying to achieve:

// This is the way I want to call the method
NSDictionary *account = [_accountModel getCurrentClient];

// This is the faulty method that I want to fix
- (NSDictionary *)getCurrentClient
{
    __block NSDictionary *currentClient = nil;
    NXOAuth2Account *currentAccount = [[[NXOAuth2AccountStore sharedStore] accounts] lastObject];

    [NXOAuth2Request performMethod:@"GET"
                        onResource:[NSURL URLWithString:[NSString stringWithFormat:@"%@/clients/%@", kCatapultHost, currentAccount.userData[@"account_name"]]]
                    usingParameters:nil
                        withAccount:currentAccount
                sendProgressHandler:nil
                    responseHandler:^ (NSURLResponse *response, NSData *responseData, NSError *error) {
                        NSError *jsonError;

                        currentClient = [NSJSONSerialization JSONObjectWithData:responseData
                                                                        options:kNilOptions
                                                                            error:&jsonError];
                    }];

    return currentClient;
}

I don't want the getCurrentClient method to return a value until the NXOAuth2Request has finished. I can't return the current client inside the response handler of the request either. So what are my options?

Answer 1

You need to change getCurrentClient to take in a completion block instead of returning a value.

For example:

-(void)getCurrentClientWithCompletionHandler:(void (^)(NSDictionary* currentClient))handler
{
    NXOAuth2Account *currentAccount = [[[NXOAuth2AccountStore sharedStore] accounts] lastObject];

    [NXOAuth2Request performMethod:@"GET"
                        onResource:[NSURL URLWithString:[NSString stringWithFormat:@"%@/clients/%@", kCatapultHost, currentAccount.userData[@"account_name"]]]
                   usingParameters:nil
                       withAccount:currentAccount
               sendProgressHandler:nil
                   responseHandler:^ (NSURLResponse *response, NSData *responseData, NSError *error) {
                        NSError *jsonError;

                        NSDictionary* deserializedDict = [NSJSONSerialization JSONObjectWithData:responseData
                                                                                         options:kNilOptions
                                                                                           error:&jsonError];
                        handler(deserializedDict);
                }];
}

It's important to remember that getCurrentClient will return immediately, while the network request is dispatched on another thread. Don't forget that if you want to update the UI using your response handler, you need to have your handler run on the main thread.