Subjects

We've been using Subjects in this book since Chapter 2, Reactive Programming with RxPHP, but there're multiple different variants of the Subject class for more specific use cases where all of them are relevant to multicasting.

BehaviorSubject

The BehaviorSubject class extends the default Subject class and lets us set a default value that is passed to its observer right on subscription. Consider this very simple example of BehaviorSubject:

// behaviorSubject_01.php  
use Rx\Subject\BehaviorSubject; 
 
$subject = new BehaviorSubject(42); 
$subject->subscribe(new DebugSubject()); 

When DebugSubject subscribes to the BehaviorSubject class, the default value 42 is emitted immediately. This is a similar functionality to using the startWith() operator.

The output is then just a single line:

$ php behaviorSubject_01.php
15:11:54 [] onNext: 42 (integer)

ReplaySubject

The ReplaySubject class internally contains an array of the last N values it received and automatically...