RxJava (Android) - Intermediate step on error, retry after error resolution

Question

I am currently trying to get myself into RxJava.

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Now I have the following scenario:
I have to authenticate a user.

The steps that I need to do for this are the following:

1. Try to authenticate user using locally stored token
2. If authentication fails (due to token expiration) try to get a new token using a locally stored refresh token
3. Alternatives:
   • On success:
     1. Save the newly fetched token locally
     2. Try to authenticate again, if this fails, show error
   • On failure (because also the refresh token expired): Show login dialog
   • On failure (because of other reasons): Show error

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What I have already implemented are 2 Observables, one for the authentication, one for the refresh of the token. Those return 2 different types of results.

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I now struggle to chain those into the steps mentioned above. Is there someone that could give me a hint?

Or is this more a case for a separate class to handle this procedure instead of a RxJava chain (because too complex)?

Answer 1

I think this can be done in a simpler way using an error handling operator onErrorResumeNext() which pass control to another Observable rather than invoking onError(), if the observable encounters an error.

Let's assume your 2 observables are:

// Assuming authenticate returns an exception of typeA if it fails
Observable<A> authenticate;
// returns an exception of typeB if refresh fails
Observable<B> refresh;

Now let's create another observable using concatMap.

// Probably you want to emit token from refresh observable. If not,
// then I hope you will get the idea how it's done. 
Observable<Object> refreshThenAuthenticate = refresh.concatMap(new Func1<A, Observable<B>>() {
        @Override
        public Observable<A> call(B b) {
            // create an authentication observable using the token
            return createAuthenticateObservable(b.getToken());
        }
});
// Above can be written in much simpler form if you use Java 8 lambdas.


/* first authenticate will be executed, if it fails then control will
 * go to refreshThenAuthenticate.
 */
authenticate
.onErrorResumeNext(refreshThenAuthenticate)
.subscribe(new Observer<A>() {
            @Override
            public void onCompleted() {
               // login successful
            }

            @Override
            public void onError(Throwable e) {
                // if e of typeA show login dialog
                // else do something else
            }

            @Override
            public void onNext(A a) {

            }
        });