Allow async function to only be called once (make others wait) - ios

Question

I have an authorisation function that will run in my app if it makes an unauthorised request. I originally built the auth function in a POC and now I have transferred it into the app I am working on. I've noticed an interesting issue where if I set the token to invalid on launch (and this only happens on launch) then about 4 auth requests are sent out from the app. From what I can tell, the response for the auth returns after the requests are sent out so a few stack up before the app realises it has a valid token. The function works perfectly if an auth token expires whilst using the app, this issue is purely on load.
I've decided the solution to this problem is to make the auth function exclusive. I did a little research into allowing a function to be called once and came across #synchronized. It made sense but I'm not too sure how I can implement it into my function. My auth function looks like this:
+ (void)requestAuthTokenWithBlock:(void (^)(void))completion {
NSLog(#"requestNewToken - Called: Requesting a new authorization bearer token.");
[[NSURLCache sharedURLCache] removeAllCachedResponses];
//Build request URL String
NSString *requestString = [NSString stringWithFormat:#"%#%#",baseURL,authRequestURL];
//Encode password so that it can be safely sent in request
NSString *encodedPassword = [kU1Password stringByAddingPercentEncodingForRFC3986];
//Populate post request with user credentials
NSString *post = [NSString stringWithFormat:#"client_id=%#&password=%#&grant_type=%#", kU1ClientId, encodedPassword, kU1GrantType];
//Encode post string & convert to type NSData
NSData *postData = [post dataUsingEncoding:NSASCIIStringEncoding allowLossyConversion:YES];
//Calculate the length of the post string
NSString *postLength = [NSString stringWithFormat:#"%lu",(unsigned long)[postData length]];
//Initialize url request
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
//Set the url for which you will pass your request data
[request setURL:[NSURL URLWithString:requestString]];
//Set HTTP method for request
[request setHTTPMethod:#"POST"];
//Set HTTP header field with length of post data
[request setValue:postLength forHTTPHeaderField:#"Content-Length"];
//Set the encoded value for HTTP Header field
[request setValue:#"application/x-www-form-urlencoded" forHTTPHeaderField:#"Content-Type"];
//Set the HTTP body of the urlrequest with our post data
[request setHTTPBody:postData];
//Create full request
NSURLSession *session = [NSURLSession sharedSession];
NSURLSessionDataTask *dataTask = [session dataTaskWithRequest:request
completionHandler:^(NSData *data, NSURLResponse *response, NSError *error){
NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *) response;
NSLog(#"Status Code: %ld\n - requestAuthTokenWithBlock - ",(long)httpResponse.statusCode);
NSString *message = [NSHTTPURLResponse localizedStringForStatusCode:httpResponse.statusCode];
NSLog(#"Message: %#", message);
//Check for an error, if there is no error we proceed.
if (!error) {
NSLog(#"requestAuthToken - Successful responce from server");
//Populate the auth object with the parse json data (handled entirely in the builder)
AuthToken *auth = [TokenBuilder authFromJSON:data error:&error];
//Save the auth & refresh tokens in the keychain
[SAMKeychain setPassword:auth.AuthToken forService:kServer account:kKeyAccessToken];
[SAMKeychain setPassword:auth.RefreshToken forService:kServer account:kKeyRefreshToken];
//Trigger completion block
if (completion)
{
completion();
}
}
else {
//Failed request
//Trigger completion block
if (completion)
{
completion();
}
}
}];
[dataTask resume];
}
And it only ever gets called from this switch statement if the app has a token:
switch (httpResponse.statusCode) {
case 200 ... 299:{
NSLog(#"SUCCESS");
NSLog(#"Performing any completion related functions!");
retryAttempts = 0;
completion(true, message, data, nil);
return;
}
case 401:{
if (retryAttempts == 0){
NSLog(#"401 Challenge - Retrying Authentication, First Attempt:");
}
else {
NSLog(#"401 Challenge - Retrying Authentication, Attempt %ld", (long)retryAttempts);
}
[self requestAuthTokenWithBlock:^(void){
NSLog(#"requestAuthTokenWithBlock - Completion block called");
[self dataTaskwithRequest:request method:method completion:completion];
}];
retryAttempts += 1;
break;
}
}
I copied out the #Synchronized snippet but didn't really know where to put it:
#synchronized (<#token#>) {
<#statements#>
}
I tried putting it into the token method where I call the datatask:
#synchronized (dataTask) {
[dataTask resume];
}
and the function still got called more than once. I'm not sure how to tackle this issue, I can add a call to this function from anywhere in the app and don't think I can control the requests themselves. I think the only place to protect the function from multiple calls is in the function itself. And as a sidenote I added a boolean value that gets set to false when the function starts and true when the function ends.
if (canAttempt == NO) {
return;
}
canAttempt = NO;
And now the functions that once got called 4 times now return as failed because they weren't authorised. Any ideas on how I can get the app to wait on this function before continuing?