Firebase - Android - fetchProvidersForEmail - Why are all the calls asynchronous?

Question

I'm new to Firebase and I'm having a lot of problems with the fact that all the tasks are called asynchronously.

For example, I am trying to use fetchProvidersForEmail to know if I should direct the user to sign up or log in. However, by the time the task finishes, it's too late.

I am not sure if it's clear but here is my current code (which works) and below is the method I would want to create. How can I get that done?

public static void printProviders(String email) {
    FirebaseAuth auth = FirebaseAuth.getInstance();
    auth.fetchProvidersForEmail(email).addOnCompleteListener(new OnCompleteListener<ProviderQueryResult>() {
            @Override
            public void onComplete(@NonNull Task<ProviderQueryResult> task) {
                Log.d(TAG, "We have " + task.getResult().getProviders().size() + " results.");
                for (int i = 0; i < task.getResult().getProviders().size(); i++) {
                    Log.d(TAG, "Provider " + (i+1) + ": " + task.getResult().getProviders().get(i));
                }
            }
        }
    );
}

Here is the pseudo-code of the method I would want to create (of course, this doesn't work)...

    public static boolean emailIsRegistered(String email) {
    FirebaseAuth auth = FirebaseAuth.getInstance();
    auth.fetchProvidersForEmail(email).addOnCompleteListener(new OnCompleteListener<ProviderQueryResult>() {
         @Override
         public void onComplete(@NonNull Task<ProviderQueryResult> task) {
             if (task.getResult().getProviders().size() > 0) {
                 return true;
             }
             return false;
         }
    });
}

However, this does not work because the return statement is void for onComplete() and because the task is executed asynchronously...

I am new to this. I tried to search through StackOverflow but couldn't find anything that helped me. Hopefully someone can help.

Thank you!

score 4 · Accepted Answer · edited May 23 '17 at 12:09

When you call fetchProvidersForEmail that information is not available in the APK of your app. The Firebase client has to call out to the servers to get this information.

Given the nature of the internet, this means that it will take an undetermined amount of time before the result comes back from those servers.

The client has a few options on what to do in the meantime:

wait until the data is available
continue executing and calling you back when the data is available

Waiting for the data would mean that your code stays simple. But it also means that your app is blocked while the data is being looked up. So: no spinner animation would run, the user can't do anything else (which may be fine for your app, but not for others), etc. This is considered a bad user experience. So bad in fact, that Android will show an Application Not Responding dialog if your app is in this state for 5 seconds.

So instead, the Firebase SDKs choose the other option: they let your code continue, while they're retrieveing the data from the servers. Then when the data is retrieved, they call back into a code block you provided. Most modern web APIs are built this way, so the sooner you come to grips with it, the sooner you can efficiently use those APIs.

The easiest way I found to grasps asynchronous programming is by reframing your problems. Right now you're trying to "first determine if the email is already used, then sign the user up or in".

if (emailIsRegistered(email)) {
    signInUser(email);
}
else {
    signUpUser(email);
}

This approach leads to a emailIsRegistered method that returns a boolean, something that is impossible with asynchronous methods.

Now let's reframe the problem to "determine if the email is already used. When we know this, sign the user up or in".

This leads to a different piece of code:

public static boolean emailIsRegistered(String email) {
FirebaseAuth auth = FirebaseAuth.getInstance();
auth.fetchProvidersForEmail(email).addOnCompleteListener(new OnCompleteListener<ProviderQueryResult>() {
     @Override
     public void onComplete(@NonNull Task<ProviderQueryResult> task) {
         if (task.getResult().getProviders().size() > 0) {
             signUserIn(email);
         }
         signUserUp(email);
     }
});

We've moved the calls to sign the user up or in into the emailIsRegistered method and invoke then when the result becomes available.

Now this of course hard-coded the follow up action into the emailIsRegistered method, which makes it harder to re-use. That's why you quite often see a callback being passed into these functions. A great example of that is the OnCompleteListener that you're already using. Once the Firebase client gets the result from the servers, it calls the onComplete method that you passed in.

Learning to deal with asynchronous calls is both hard and important. I'm not sure if this is my best explanation of the concepts ever. So I'll include some previous explanations (from both me and others):

Firebase - Android - fetchProvidersForEmail - Why are all the calls asynchronous?

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