RxJS: Understanding Subjects

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I see a lot of questions about subjects on Stack Overflow. Recently, I saw one that asked how an AsyncSubject should be used. The question prompted me to write this article to show why the various types of subjects are necessary and how they are used in RxJS itself.

In his article On the Subject of Subjects, Ben Lesh states that:

… [multicasting] is the primary use case for Subjects in RxJS.

We’ll look at multicasting in more detail later in the article, but for now it’s enough to know that it involves taking the notifications from a single, source observable and forwarding them to one or more destination observers.

This connecting of observers to an observable is what subjects are all about. They’re able to do it because subjects themselves are both observers and observables.

Let’s use an Angular component as an example: an awesome-component. Our component does some awesome stuff and has an internal observable that emits values as the user interacts with the component.

To enable parent components to connect to the observable, the awesome-component accepts an observer input property — which it subscribes to the observable. That means the parent could connect to the observable by specifying an observer, like this:

With the observer wired up, the parent is connected and receives values from the awesome-component. However, this is essentially the same as if the awesome-component had emitted its values using an output event. So why not use an event?

Observables have the advantage of being easy to manipulate. For example, it’s easy to add filtering and debouncing just by applying a few operators. But the parent component has an observer — not an observable — so how can we apply operators?

Subjects are both observers and observables, so if we create a Subject, it can be passed to the awesome-component (as an observer) and can have debouncing applied to it (as an observable), like this:

The subject connects the do-something-with-the-value observer with the awesome-component observable, but with the parent component’s choice of operators applied.

By using a Subject to compose an observable, the awesome-component can be used in different ways by different components. For example, another component might be interested in only the last-emitted value. That component could use the last operator:

Interestingly, there is another way that component could choose to receive only the last-emitted value from the awesome-component: it could use a different type of subject. An AsyncSubject emits only the last-received value, so an alternative implementation would be:

If using an AsyncSubject is equivalent to composing the observable using a Subject and the last operator, why complicate RxJS with the AsyncSubject class?

Well, it’s because subjects are primarily for multicasting.

The two are equivalent here, because there is a single subscriber — the do-something-with-the-value observer. In a multicasting situation, there can be multiple subscribers and applying the last operator to a Subject won’t effect the same behaviour as an AsyncSubject for late subscribers.

Let’s have a closer look at multicasting.

The core of RxJS’s multicasting infrastructure is implemented using a single operator: multicast. The multicast operator is applied to a source observable, takes a subject (or a factory that creates a subject) and returns an observable composed from the subject.

The multicast operator is somewhat like the awesome-component in our examples: we can obtain an observable that exhibits different behaviour simply by passing a different type of subject.

When a basic Subject is passed to multicast:

  • subscribers to the multicast observable receive the source’s next, error and complete notifications; and
  • late subscribers — i.e. those that subscribe after an error or complete notification has occurred — receive the error or complete notification.

It’s important to note that unless multicast is passed a factory, late subscribers don’t effect another subscription to the source.

To compose a multicast observable that forwards the source observable’s last-emitted next notification to all subscribers, it’s not enough to apply the last operator to a multicast observable that was created using a Subject. Late subscribers to such an observable won’t receive the last-emitted next notification; they will receive only the complete notification.

For late subscribers to receive the last-emitted next notification, the notification needs to be stored in the subject’s state. That’s what the AsyncSubject does and that’s why the AsyncSubject class is necessary.

There are two other subject variants: BehaviorSubject and ReplaySubject.

To understand the BehaviorSubject, let's have a look at another component-based example:

Here, the parent component connects to the awesome-component using a Subject and applies the startWith operator. Using startWith ensures that the parent receives the value "awesome" upon subscription, followed by the values emitted by the awesome-component — whenever they happen to be emitted.

In the same way that an AsyncSubject replaced the use of a Subject and the last operator, a BehaviorSubject could replace the use of a Subject and the startWith operator — with the BehaviorSubject's constructor taking the value that would otherwise have been passed to startWith.

However, using a Subject and the startWith operator won’t effect the desired behaviour in a multi-subscriber situation. The first subscriber will see the expected behaviour, but subsequent subscribers will always receive the startWith value — even if the source has already emitted a value.

If a BehaviorSubject is used, subsequent subscribers will receive the initial value if the source has not yet emitted or the most-recently-emitted value if it has. This is possible because the BehaviorSubject stores the value in its state.

There is no single-subscriber analogy for the ReplaySubject, as the concept of replaying already received notifications is inherently multi-subscriber. To facilitate the replaying of notifications to subsequent subscribers, the ReplaySubject stores the notifications in its state.

Now that we’ve seen what the various subjects do and why they are necessary, how should they be used? Well, it’s quite likely that the only subject class you will ever need to use will be a Subject.

A Subject works just fine for connecting an observer to an observable. And for the multicasting situations, there is an alternative.

RxJS contains multicasting operators that use the various subject classes and in the same way that I favour using RxJS observable creators (like fromEvent) over calls to Observable.create, for multicasting situations I favour using RxJS operators over explicit subjects:

  • publish or share can be used instead of a Subject;
  • publishBehaviour can be used instead of a BehaviorSubject;
  • publishLast can be used instead of an AsyncSubject; and
  • publishReplay or shareReplay can be used instead of a ReplaySubject.

The publish and share operators are covered in more detail in my articles:

This post is also published on my personal blog: ncjamieson.com.

RxJS core team member; front-end developer; mentor; speaker; open-source contributor

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