Writing operators, source-like (fromAsync) or intermediate-like (flatMap) has always been a hard task to do in RxJava. There are many rules to obey, many cases to consider but at the same time, many (legal) shortcuts to take to build a well performing code. Now writing an operator specifically for 2.x is 10 times harder than for 1.x. If you want to exploit all the advanced, 4th generation features, that's even 2-3 times harder on top (so 30 times harder in total).

(If you have been following my blog about RxJava internals, writing operators is maybe only 2 times harder than 1.x; some things have moved around, some tools popped up while others have been dropped but there is a relatively straight mapping from 1.x concepts and approaches to 2.x concepts and approaches.)

In this article, I'll describe the how-to's from the perspective of a developer who skipped the 1.x knowledge base and basically wants to write operators that conforms the Reactive-Streams specification as well as RxJava 2.x's own extensions and additional expectations/requirements.

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