The Quatraits of RxSwift - Part 3

In Part Two of this series, I went over four different Traits used in RxSwift.

Now, Part Three will go over four kinds of Subjects : BehaviorSubject, AsyncSubject, PublishSubject, and ReplaySubject.

Subjects

A base Subject can receive and send events, acting as both observer and observable, but we still need some concrete classes of Subjects to resolve some ambiguities and add some additional constraints and behaviors.

“Born This Way”

Behavior’s an inborn nature —
then reflects current events —
you see One at their worst — or best —
watch their latest contents —

Explanation

BehaviorSubjects, upon being subscribed to, emit either a) an initial value provided on init or b) the most recent received event. They’re useful for acting as the backing private BehaviorSubject<T> for an exposed public Driver<T>¹.

“Wash Your Hands When Finished”

A sink retains the latest stream –
seen only when its stopped –
unplugged – A sink’s an endless pipe –
(Well, endless but when clogged!).

Explanation

An AsyncSubject can receive any number of onNext events and won’t emit any of them until it receives a Complete event. After this Complete, then and only then will the AsyncSubject<T> emit to all of its observers the last T received. This Subject is useful for one-and-done GET API calls.

“Self-Published”

pre-gutenberg: an empty shelf –
no written memory –
now: Publish books for each event –
tapped Subjectivity –

Explanation

Ignoring that books existed before the printing press (but for the average person, their bookshelf was bare), a PublishSubject<T>, as opposed to a BehaviorSubject, will not provide a default or last received T on subscription. Instead, a PublishSubject will emit each new one, not bothering to remember the previous. They’re useful for acting as the backing private PublishSubject<T> for an exposed public Signal<T>².

“Deja Entendu”

“Now – what’d you say? I couldn’t hear.”
“I’ll tell what I recall.
New words come out, I lose what was.”
Some speech replayed – not

Explanation

A ReplaySubject is a souped-up BehaviorSubject. Whereas a BehaviorSubject will emit only the initial/latest value, a ReplaySubject contains a buffer (the max capacity determined on init) of received Ts and emits, upon being subscribed to, this buffer. This buffer might also have a time window also, so events received outside a certain duration are removed from the buffer. Unlike BehaviorSubjects, a ReplaySubject doesn’t have an initial T and will return its buffered values even after an error.

Onward!

This third part introduced you to different kinds of Subjects in Reactive programming. Next, in Part Four of this series, I will go over some traits that are exclusive flavors of RxCocoa.