How to Compose Multiple Dependent Observables in RxJava and Aggregate Their Results?

How can I combine several Observables in RxJava that depend on each other and gather all the results at the end?

Observable<ResultType> observable1 = Observable.create(emitter -> { /* Logic */ });
Observable<ResultType> observable2 = observable1.flatMap(result1 -> Observable.create(emitter -> { /* Logic that uses result1 */ }));

In RxJava, composing multiple Observables that depend on each other allows for an efficient flow of asynchronous data. This involves chaining Observables using operators such as `flatMap`, `map`, and `zip`, facilitating data transformation and aggregation of results before sending them downstream.

Observable<ResultType> observable1 = Observable.just("Data 1");

Observable<ResultType> observable2 = observable1.flatMap(data1 -> {
    return Observable.just("Computed from " + data1);
});

Observable<List<String>> combined = Observable.zip(observable1, observable2, (result1, result2) -> {
    return Arrays.asList(result1, result2);
});

combined.subscribe(results -> {
    // Handle collected results
    System.out.println(results);
});

Causes

• Dependency between Observables requires sequential processing.
• Need to collect results once all Observables have emitted values.

Solutions

• Use `flatMap` for sequential dependencies where the output of one Observable is required as input for another.
• Employ `zip` when Observables can operate in parallel, gathering results once all Observables complete.
• Utilize `toList()` to collect all the items emitted by the Observables at the end.

Mistake: Not handling errors properly in Observables.

Solution: Ensure to use `onErrorReturn`, `onErrorResumeNext`, or other error-handling operators.

Mistake: Overly complicated chaining leading to hard-to-read code.

Solution: Break down complex chains into smaller, manageable methods.

Mistake: Ignoring thread management, resulting in blocking operations.

Solution: Utilize `subscribeOn()` and `observeOn()` to manage threading effectively.