How to compose Observables to avoid the given nested and dependent callbacks?

Question

In this blog, he gives this (copy/pasted the following code) example for the callback hell. However, there is no mention of how the issue can be eliminated by using Reactive

Answer 1

According to your code. Suppose that the remote call are done using Observable.

 Observable  callRemoveServiceA()  { /* async call */  }

/* .... */

Observable  callRemoveServiceE(Integer f2) { /* async call */  }

What you want :

• call serviceA then call serviceB with the result of serviceA
• call serviceC then call serviceD and serviceE with the result of serviceC
• with the result of serviceE and serviceD, build a new value
• display the new value with the result of serviceB

With RxJava, you'll achieve this with this code :

Observable f3 = callRemoveServiceA() // call serviceA
            // call serviceB with the result of serviceA
            .flatMap((f1) -> callRemoveServiceB(f1)); 


Observable f4Andf5 = callRemoveServiceC() // call serviceC
                    // call serviceD and serviceE then build a new value
                    .flatMap((f2) -> callRemoveServiceD(f2).zipWith(callRemoveServiceE(f2), (f4, f5) -> f4 * f5));

// compute the string to display from f3, and the f4, f5 pair
f3.zipWith(f4Andf5, (childF3, childF4Andf5) -> childF3 + " => " + childF4Andf5)
            // display the value
            .subscribe(System.out::println);

the important part here is the use of flapMap and zip (or zipWith)

• flapMap will transform a value into another Observable. This Observable here will be your new asynchronos call. ( http://reactivex.io/documentation/operators/flatmap.html )
• zip will compose a new value from two different Observable. So you can build a new value with the result of two (or more) Obsevable ( http://reactivex.io/documentation/operators/zip.html )

You can get more info on flapMap here : When do you use map vs flatMap in RxJava?