How to emit an item while waiting another Flowable to emit

Question

I'm using RxJava3 and I have the following code setup where I want to emit an item in the middle, between the first and second flowable. Is there a way to do it?

firstFlowable.firstElement()
//I want to emit an item here
.flatMap { secondFlowable.firstElement() }

The reason I want to do this is because after firstFlowable initializes there is a long period of time until the secondFlowable initializes and I want to notify the UI with a message that there the operation has started and I'm waiting on the data computation from secondFlowable.

I tried to use startWithItem, but that initializes my whole chain at the beginning, but I want to emit only after firstFlowable produces its first value.

Answer 1

You could use merge to inject a value, then act based on the value type. In case secondFlowable finishes immediately, you may want to avoid displaying the string after all via takeUntil.

firstFlowable.firstElement()
.flatMap(v ->
    Maybe.<Object>merge(
        secondFlowable.firstElement(),
        Maybe.just("Second in progress")
    )
)
.takeUntil(v -> !(v instanceof String))
.observeOn(mainThread())
.subscribe(v -> {
   if (v instanceof String) {
       // display message here
   } else {
       // cast and display results of second
   }
});