Serial dispatch queue not waiting

Question

I need to log in to Facebook with my app, fetch some data and AFTER this is finished, login to Firebase also. I am using dispatch queues for this but it is not working. My blocks don't wait for each other to finish at all.

Here is my code:

dispatch_queue_t loginQueue = dispatch_queue_create("com.balazsvincze.loginQueue", DISPATCH_QUEUE_SERIAL);


//Log in to Facebook
    dispatch_async(loginQueue, ^{
        FBSDKLoginManager *login = [[FBSDKLoginManager alloc] init];
        [login logInWithReadPermissions: @[@"public_profile",@"user_friends"] fromViewController:viewController handler:^(FBSDKLoginManagerLoginResult *result, NSError *error) {

            if(!error){
                NSLog(@"Logged into Facebook");
                self.facebookUserID = [[FBSDKAccessToken currentAccessToken] userID];

                //Load my profile
                [FBSDKProfile loadCurrentProfileWithCompletion:^(FBSDKProfile *profile, NSError *error){
                    if(!error){
                        self.name = profile.name;
                    }

                    else{
                        NSLog(@"Could not load profile: %@",error);
                        completionResult(NO);
                        return;
                    }

                }];
            }

            else{
                NSLog(@"Could not login: %@",error);
                completionResult(NO);
                return;
            }

        }];
    });

    //Log in to Firebase
    dispatch_async(loginQueue, ^{
        FIRAuthCredential *credential = [FIRFacebookAuthProvider credentialWithAccessToken:[FBSDKAccessToken currentAccessToken].tokenString];

        [[FIRAuth auth] signInWithCredential:credential  completion:^(FIRUser *user, NSError *error) {

            if(!error){
                NSLog(@"Logged into Firebase");
                self.firebaseUserID = user.uid;
            }

            else{
                NSLog(@"Could not login: %@",error);
                completionResult(NO);
                return;
            }
        }];
    });

Any ideas? Thanks!

The Facebook login and the firebase logins are themselves asynchronous, so your async dispatched blocks exit while the asynchronous logins are still running. See my explanation here http://stackoverflow.com/questions/42145427/how-does-a-serial-queue-private-dispatch-queue-know-when-a-task-is-complete/42146095?noredirect=1#comment71500175_42146095 — Paulw11, Feb 10 '17 at 23:56
Thanks! I read it but I am afraid that I still don't understand... Please see my edited answer! — Balázs Vincze, Feb 11 '17 at 00:08
You don't want the first wait; you only want a wait when you want to block pending completion of the previous task. You still need your dispatch_async or you will block the main thread on the wait. you could also use a dispatch_group_notify since the two operations are independent and you just want to know when both have completed. — Paulw11, Feb 11 '17 at 00:11
Okay, so I tried it with dispatch_group_notify, however they still don't wait for each other... What am I doing wrong? — Balázs Vincze, Feb 11 '17 at 00:14
You need to put the dispatch_group_notify *after* the other asynchronous blocks. I will revert your question, since your changed code is worse and add an answer — Paulw11, Feb 11 '17 at 00:19
OMG, those changes completely the question! Thanks, Paulw11 for reverting. — Rob, Feb 11 '17 at 00:21
So, I return to my original comment that you should either (a) just nest them like Russell suggests; (b) nest them but put the subsequent calls in separate methods, avoiding the ugly nested blocks situation; or (c) use solutions like asynchronous `NSOperation` custom subclasses or promises ([PromiseKit](http://promisekit.org) or [RXPromise](https://github.com/couchdeveloper/RXPromise)), though those are probably over-engineered if this is just a one-time issue. — Rob, Feb 11 '17 at 00:25
Thanks for the suggestions @Rob! All of them are asynchronous calls so would putting them into separate functions mean that those separate functions would need completion blocks? — Balázs Vincze, Feb 11 '17 at 00:27
Yep, if you want to pass some completion handler that the final function will call when it's done, you can do that. — Rob, Feb 11 '17 at 00:37

score 2 · Answer 1 · answered Feb 11 '17 at 00:07

2

make the call to login in Firebase within the completion handler for Facebook, and it will all work as expected

answered Feb 11 '17 at 00:07

Russell

5,436
2
19
27

Thanks! But I was trying to avoid nesting blocks – Balázs Vincze Feb 11 '17 at 00:07

Paulw11 · Answer 2 · 2017-02-11T00:36:25.163

Although you are dispatching asynchronous tasks on a serial dispatch queue, those tasks themselves dispatch asynchronous tasks and then end; so your tasks have completed but the logins haven't.

As @Rob suggested, splitting your code into separate tasks using something like NSOperation will result in cleaner, more scalable code.

However, here is how you can use a dispatch_group and dispatch_group_notify to schedule some code when the asynchronous operations have completed:

dispatch_group_t dispatchGroup = dispatch_group_create();

//Log in to Facebook
dispatch_group_enter(dispatchGroup);

FBSDKLoginManager *login = [[FBSDKLoginManager alloc] init];
[login logInWithReadPermissions: @[@"public_profile",@"user_friends"] fromViewController:viewController handler:^(FBSDKLoginManagerLoginResult *result, NSError *error) {

    if(!error){
        NSLog(@"Logged into Facebook");
        self.facebookUserID = [[FBSDKAccessToken currentAccessToken] userID];

        //Load my profile
        dispatch_group_enter(dispatchGroup);
        [FBSDKProfile loadCurrentProfileWithCompletion:^(FBSDKProfile *profile, NSError *error){
            if(!error){
                self.name = profile.name;
            }
            else{
                NSLog(@"Could not load profile: %@",error);
                completionResult(NO);
            }
            dispatch_group_leave(dispatchGroup);
        }];
    }
    else{
        NSLog(@"Could not login: %@",error);
        completionResult(NO);
    }
    dispatch_group_leave(dispatchGroup);
}];

//Log in to Firebase
dispatch_group_enter(dispatchGroup);

FIRAuthCredential *credential = [FIRFacebookAuthProvider credentialWithAccessToken:[FBSDKAccessToken currentAccessToken].tokenString];

[[FIRAuth auth] signInWithCredential:credential  completion:^(FIRUser *user, NSError *error) {

    if(!error){
        NSLog(@"Logged into Firebase");
        self.firebaseUserID = user.uid;
    }

    else{
        NSLog(@"Could not login: %@",error);
        completionResult(NO);
    }
    dispatch_group_leave(dispatchGroup);
}];

dispatch_group_notify(dispatchGroup,dispatch_get_main_queue(),^{
    NSLog(@"All login tasks completed");
    // Do whatever
});

Serial dispatch queue not waiting

2 Answers2