RxSwift: Subjects

In this blog, I will talk about subjects & variables in RxSwift. I hope you must be knowing about RxSwift. If you don’t then read my earlier blog https://medium.com/@arnavgupta180/shift-from-swift-to-rxswift-8dece8af9f4 for better understanding of concepts of RxSwift.

Subjects

A Subject is a sort of bridge or proxy that acts both as an observer and as an Observable. A subject can emit it’s elements even if it has no observers.

Subject Types

There are three commonly used subject types. They all behave almost the same with one difference: each one does something different with values emitted before the subscription happened.

Publish Subject :

the publish subject will ignore all elements that were emitted before subscribe have happened.

Replay Subject :

Replay subject will repeat the last N number of values, even the ones before the subscription happen. The N is the buffer, so for our example it’s 3.

Behavior Subject :

Behavior subject will repeat only the one last value. Moreover, it’s initiated with a starting value, unlike the other subjects.

Example

let subject = defined below with output w.r.t all cases of subjects.
 
let observable : Observable<String> = subject 
subject.onNext("Before Subscribe Value First")
 subject.onNext("Before Subscribe Value Second")
 subject.onNext("Before Subscribe Value Third")
observable.subscribe(onNext: { text in      
  print(text)   
 
 }).addDisposableTo(disposeBag) 
 subject.onNext("After Subscribe")

OUTPUT

Output
1) PublishSubject
 let subject = PublishSubject<String>()         
 
Output:
              
After Subscribe              //no values repeat before subscibe

2) ReplaySubject with buffer size 3
let subject = ReplaySubject<String>().create(bufferSize: 3) 
Output:
Before Subscribe Value First   //on subscribe will repeat buffer 
Before Subscribe Value Second  // size value eg last 3
Before Subscribe Value Third
After Subscribe

3) BehaviorSubject : It has initial value
let subject = BehaviorSubject<String>(value: "Initial value") 
Output:
Before Subscribe Value Third   //on subscribe will repeat last value 
After Subscribe