What is a Subject?

• The Subject class extends the Observable class. This means that you can use Subjects the same way you are used to using Observables. You can subscribe() to notifications and you can use operators within the pipe().
• A Subject is both an Observable and an Observer. In other words, Subjects enable us to both emit notifications and listen for notifications.
• Subjects multicast. We'll learn more about this later.

Why do we have a Subject?

When we need to create an Observable where we can emit notifications from outside of the subscribe function.

Variations

The Subject class extends the Observable class, and in turn, there are several subclasses of the Subject:

• BehaviorSubject
• AsyncSubject
• ReplaySubject
• WebSocketSubject

Each variation behaves differently in how notifications are emitted and the timing of those notifications.

Creation

New-up a Subject() via:

const users = new Subject();

If we are using TypeScript, then we can also specify the generic type. The class signature is:

class Subject<T> extends Observable<T> implements SubscriptionLike {}

• Here we see that the Subject class extends the Observable class.
• The class also implements the SubscriptionLike interface which requires an unsubscribe() method.

Using TypeScript we specify the generic as follows:

const users = new Subject<User[]>();

In this example, the type for the next notification is an array of User objects.