Use callback to return string - objective-c

Question

I've working on/learning this all afternoon.
Following an example here: http://themainthread.com/blog/2012/09/communicating-with-blocks-in-objective-c.html, I have managed to setup a callback to get the result of an asynchronous call to a web service i have.
The web service takes a key code and the app transforms it and passes it back for authentication.
With my code below, how can I change the method from a void to an NSString that I can call to return my pass code?
-(void) showPassCode{
getAuthCodeAndMakePassCodeCompleteBlock callback = ^(BOOL wasSuccessful, NSString *passCode) {
if (wasSuccessful) {
NSLog(#"Code is: %#", passCode);
} else {
NSLog(#"Unable to fetch code. Try again.");
}
};
[self getAuthCodeAndMakePassCode:#"myAuthCode" withCallback:callback];
}
Ideally, I want it to work or look like this:
-(NSString *) strPassCode{
getAuthCodeAndMakePassCodeCompleteBlock callback = ^(BOOL wasSuccessful, NSString *passCode) {
if (wasSuccessful) {
return passCode;
} else {
return nil;
}
};
[self getAuthCodeAndMakePassCode:#"myAuthCode" withCallback:callback];
}

Answer 1

Without knowing the specifics of your code and how you query the server, I have to imagine it would look something like:
-(void)getAuthCodeWithCallback:(void (^)(NSString* authCode))callback
{
//make server call, synchronously in this example
NSString* codeReturnedFromServer = [self getServerCodeSynchronous];
callback(codeReturnedFromServer);
}
//some calling code
[self getAuthCodeWithCallback:^(NSString* authCode) {
NSLog(#"Code is: %#", authCode);
}];
If the method that gets your auth code from the server is asynchronous, it would look something like this:
-(void)getAuthCodeWithCallback:(void (^)(NSString* authCode))callback
{
//make server call, asynchronously in this example
[self someMethodCallToQueryCodeFromServerWithCallback:^(NSError* error, NSString* code) {
if (error) {
//handle error
}
else
callback(code);
}
}