Combining Observables

In the previous example, we used two Observable to create a simple sequence of network operations. The second asynchronous operation operated with the result of the first operation and the two operations that executed serially produced a String result that updates the UI.

In our next example, we will run two tasks in parallel and combine the results of both operations using a combining RxJava operator. Each operation will retrieve asynchronously a JSON Object from the network and combine both results in the JSON Object to produce the JSON String passed to the UI main Thread.

Since we only want to emit one Event or an error from the operation, we are going to use, for the first time, a special kind of Observer, Single.

While an Observable is able to invoke onNext, onError, and onCompleted Observer functions, a Single entity will only invoke either onSuccess or onError to a SingleSubscriber:

 // Success callback invoked on success
 void onSuccess(T value);

 // Callback...