Single starting to contribute to Trust Wallet in 2017 I’ve had the opportunity touch reactive programming with RxJava. We needed to use RxJava to keep asynchronous code like network calls and db updates manageable.

To be honest it has been a steep learning curve and I quickly realized that the reactive programming was a far cry from the imperative code I was used to writing. Here’s a trick I picked up recently.

RxJava has the Observable object at it’s core and can be thought of as a stream of future events which you can subscribe and observe. Single is a special type of Observable which yields one result. We use Single often because we often want to fetch just one object.

Say you have async functions fa(), fb() and fc(A a, B b) which yield three singles Single<A>, Single<B> and Single<C>. fc(A a, B b) depends on the inputs from fb() and fc(). What is the best way to construct Single<C>? This is the best way I’ve found:

Single<A> fa() { ... }
Single<B> fb() { ... }
Single<C> fc(A a, B b) { ... }

Single<C> = fa().flatMap( a -> fb()
    		.flatMap( b -> fc(a, b) )
        )