Waiting for an asyncronous call to finish within a completion block - ios

Question

I am currently working on a POC app, I have previously posted about it here. I am trying to handle automatic refreshing of an authentication token should my server give me a 401 error (unauthorised).
Here is my demo function that requests some information from the server (I can deliberately send it valid/invalid auth tokens)
NSInteger retryAttempts = 0;
NSInteger retryMax = 1;
- (void) requestDataForUser {
NSLog(#"requestDataForUser - Called");
//Indicate Network Activity
dispatch_async(dispatch_get_main_queue(), ^{
[UIApplication sharedApplication].networkActivityIndicatorVisible = TRUE;
});
//Build request URL String
NSString *requestString = [NSString stringWithFormat:#"%#%#%#",baseURL,requestURL,#"3"];//Change to allow change in username here.
//Get auth token
NSString *accessToken = [SAMKeychain passwordForService:kServer account:kKeyAccessToken];
NSString *requestAuthorization = [NSString stringWithFormat:#"%# %#", #"Bearer", accessToken];
//Initialize url request
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
//Set the url for the request
[request setURL:[NSURL URLWithString:requestString]];
//Set HTTP method for request
[request setHTTPMethod:#"GET"];
//Set HTTP header field with the authorization token
[request setValue:requestAuthorization forHTTPHeaderField:#"Authorization"];
//Create full request
NSURLSession *session = [NSURLSession sharedSession];
__weak typeof (self) weakSelf = self;
NSURLSessionDataTask *dataTask = [session dataTaskWithRequest:request
completionHandler:^(NSData *data, NSURLResponse *response, NSError *error){
NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *) response;
NSLog(#"Status Code: %ld\n",(long)httpResponse.statusCode);
NSString *message = [NSHTTPURLResponse localizedStringForStatusCode:httpResponse.statusCode];
NSLog(#"Message: %#", message);
NSLog(#"requestDataForUser - Responce from server");
//Check for an error, if there is no error we proceed.
if (!error) {
if (retryAttempts <= retryMax) {
switch (httpResponse.statusCode) {
case 200 ... 299:
NSLog(#"SUCCESS");
NSLog(#"Performing any completion related functions!");
break;
case 401:
NSLog(#"401 Challenge - Retrying Authentication, Attempt %ld", (long)retryAttempts);
[weakSelf refreshAuth];
[weakSelf requestDataForUser];//retries this function
retryAttempts += 1;
break;
}}
else {
NSLog(#"401 Error Recieved - Retried credentials %ld time(s), please check your details are correct", (long)retryMax);
retryAttempts = 0; //Reset retry counter
//Alert controller?
}
//Get que and perform any UI changes
dispatch_async(dispatch_get_main_queue(), ^{
[UIApplication sharedApplication].networkActivityIndicatorVisible = FALSE;
});
}
else {
//Failed request
NSLog(#"requestDataForUser - error : %#", error.description);
dispatch_async(dispatch_get_main_queue(), ^{
[UIApplication sharedApplication].networkActivityIndicatorVisible = FALSE;
});
}
}];
[dataTask resume];
}
The problems I am having with this come in the 401 challenge section of the request. What I want to do is request/refresh a new token (refresh in the final iteration but currently my server is a bit hit/miss on token refreshes so I am requesting a new token in this example). So lets look at my server challenge section:
case 401:
NSLog(#"401 Challenge - Retrying Authentication, Attempt %ld", (long)retryAttempts);
[weakSelf refreshAuth];
[weakSelf requestDataForUser];//retries this function
retryAttempts += 1;
break;
So i am printing out the attempt number here, I can manually set the amount of times that this 'block' is retried until it gives up and throws an error at the user. Next it will call for an auth token, retry the request and increase retryAttempts by 1.
My problem is that when I request a new token I'm doing it asynchronously so the request is sent off and then my function retries itself (obviously without a new token) and then it throws the error. And then my token returns and prints to the console that a new token returned successfully.
I have had a look at semaphores but I can't seem to get them to work (as my requestAuthToken method has no completion block). Is there anyway I can force the auth request to be syncronous?
I have also tried to get my requestAuth method to return a BOOL and loop the bool within the 401 block until it becomes true, however it never gets set to true and the while loop goes on forever.
Any and all help is appreciated!

Answer 1

Assuming you can change implementation of requestAuth, add a completion handler parameter to requestAuth function.
Inside requestAuth implementation, call that handler after token is received. Then in requestDataForUser:
case 401:
NSLog(#"401 Challenge - Retrying Authentication, Attempt %ld", (long)retryAttempts);
[weakSelf refreshAuth withCompletionHandler:weakself.requestDataForUser];
retryAttempts += 1;
break;
Otherwise, use NSOperationQueue and set maximum concurrent operation to 1:
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
queue.maxConcurrentOperationCount = 1;
[queue addOperationWithBlock:^{
[weakself requestAuth];
}];
[queue addOperationWithBlock:^{
[weakself requestDataForUser];
}];