This document discusses custom operators for RxJava 2. It provides an example of using an extension function to filter an Observable by a predicate and return the first matching element. It then describes issues with compose and how Kotlin can help address them. Finally, it presents a filterObservable operator that filters an Observable of Observables based on the results of a predicate observable. Pseudocode and a link to source code are provided.

1. Rxjava2 custom operator
Android Taipei
2018/11@Piccollage

3. Example: get first required element
val inputData = Observable.just(
User(1, "White Jame"),
User(2, "Blue Jame"),
User(3, "Green Jame"),
User(4, "White John"),
User(5, "Green John")
)
inputData.filter { it.name.contains("John") }
.firstElement()
.test()
.assertValues(User(4, "White John"))

4. Downside about compose
1. Need to always add a key word “compose”
2. Can not convert between Observable, Single, Maybe, Completable...

5. But we have kotlin now!!

6. Solution : extension function
fun <T> Observable<T>.first(predicate: (T) -> Boolean): Maybe<T> {
return this.filter(predicate)
.firstElement()
}

7. Pairwise

9. filterObservable

10. fun <T> Observable<Observable<T>>.filterObservable(predicateObservable:
(Observable<T>) -> Single<Boolean>): Observable<Observable<T>> {
val predicates = this.flatMap { predicateObservable(it).toObservable() }
return this.zipWith(predicates,
BiFunction { original: Observable<T>, check: Boolean ->
Pair(original, check)
})
.filter { (_, predicate) -> predicate }
.map { (observable, _) -> observable }
}

11. What The …...

13. Obs<Obs<T>>
Obs<Obs<T>> Obs<Boolean>
Obs<Boolean>>
Obs<Obs<T>>
Obs<Boolean>>
Obs<Obs<T>>
Obs<Obs<T>>
zip
filter

14. Source code
https://gist.github.com/hungyanbin/4cd67008d5f9b60bcb7c38dd93cdda3e