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Understanding TakeUntil Operator in RxJS

Introduction

What is TakeUntil Operator?

How does TakeUntil Operator work?

Usage of TakeUntil Operator

Benefits of Using TakeUntil Operator

FAQs

Introduction

RxJS is a popular library for writing reactive programs using Observables. One of the most important concepts in RxJS is hot and cold Observables. Cold observables are those where the subscription starts the execution of the observable from its initial value, whereas hot observables have no impact on subscription starting the observable execution.

This article will focus on the TakeUntil operator in RxJS, which helps you to complete subscriptions to an observable under certain specified conditions. We will look at what TakeUntil is, how it works, and the benefits of using it.

What is TakeUntil Operator?

The TakeUntil operator in RxJS is used to complete an observable stream when another observable, called the take until observable, emits a particular value.

In simple terms, you can use this operator to unsubscribe from an observable stream (such as an HTTP request) automatically. When we use this operator, we can subscribe to an observable stream, and this observable will continue to emit values until the take-until observable emits a value.

How does TakeUntil Operator work?

The TakeUntil operator works by receiving two Observables: the source Observable and an Observable, called the notifier Observable. The takeUntil operator continues to listen to the source observable until the notifier observable emits a value.

Once the notifier observable emits a value, the TakeUntil operator completes the subscription to the source observable.

Here’s an example to illustrate this concept:

“`
import { interval, Subject } from ‘rxjs’;
import { takeUntil } from ‘rxjs/operators’;

const source = interval(1000);
const stopper = new Subject();

source.pipe(takeUntil(stopper))
.subscribe(
value => console.log(value),
err => console.log(`Error: ${err}`),
() => console.log(‘Completed’)
);

setTimeout(() => {
stopper.next();
stopper.complete();
}, 5000);
“`

The code above sets up an interval Observable, which emits an integer (starting from 0) every second.

We then set up a Subject called ‘stopper,’ which we will use as the notifier Observable.

We pipe the interval Observable through the takeUntil operator and pass in the stopper Observable.

The takeUntil operator listens for a value emitted by the stopper Observable. Once the stopper Observable emits a value, the takeUntil operator completes the subscription to the interval Observable, and the subscription ends.

Usage of TakeUntil Operator

The TakeUntil operator can be used in various scenarios where we want to control the lifecycle of a subscription.
Here are some examples of how we can use it:

Example One:

Suppose you have a component in a single-page application that makes API requests and displays the results. If the user navigates away from the component, you may want to cancel any in-progress API requests to clean up resources.

You can use TakeUntil to unsubscribe from the observable when the user navigates away from the component.

“`
import { Component, OnDestroy } from ‘@angular/core’;
import { fromEvent, interval, Subject } from ‘rxjs’;
import { takeUntil } from ‘rxjs/operators’;

@Component({
selector: ‘app-stopwatch’,
template: ‘

{{time}}

‘,
})
export class StopwatchComponent implements OnDestroy {

private stopper = new Subject();
time = 0;

constructor() {
interval(1000).pipe(takeUntil(this.stopper))
.subscribe(value => {
this.time += 1;
console.log(this.time);
});

fromEvent(window, ‘beforeunload’)
.subscribe(() => {
this.stopper.next();
this.stopper.complete();
});
}

ngOnDestroy(): void {
this.stopper.next();
this.stopper.complete();
}
}
“`

The code above sets up an interval observable that emits an integer value (starting from 0) every second. This observable takes the stopper observable, which completes the subscription when it emits a value.

In the constructor, we set up a window beforeunload event observable. We subscribe to this observable and listen for a beforeunload event. When this event is fired, we emit a value and then complete the subscription to cancel any in-progress API requests.

We also implement OnDestroy, and when the component is destroyed, we ensure that we clean up the resources so that it does not cause any memory leaks.

Example Two:

Suppose you have multiple simultaneous HTTP requests to send to a server. We can use TakeUntil to cancel these requests when the user navigates away from the page.

“`
import { Component, OnDestroy } from ‘@angular/core’;
import { HttpClient } from ‘@angular/common/http’;
import { Subject } from ‘rxjs’;
import { takeUntil } from ‘rxjs/operators’;

@Component({
selector: ‘app-http-requests’,
template: ‘

{{ value }}

‘,
})
export class HttpRequestsComponent implements OnDestroy {

private stopper = new Subject();
values = [];

constructor(private http: HttpClient) {
this.http.get(‘https://jsonplaceholder.typicode.com/posts/1’)
.pipe(takeUntil(this.stopper))
.subscribe(value => {
this.values.push(value);
});

this.http.get(‘https://jsonplaceholder.typicode.com/posts/2’)
.pipe(takeUntil(this.stopper))
.subscribe(value => {
this.values.push(value);
});
}

ngOnDestroy(): void {
this.stopper.next();
this.stopper.complete();
}
}
“`

The code above sets up two HTTP requests and subscribes to each of them. The takeUntil operator takes the stopper observable and completes the subscription when the stopper observable emits a value.

In ngOnDestroy, we emit a value and complete the Stopper Subject. This operation cancels any in-progress API requests when the user navigates away from the page.

Benefits of Using TakeUntil Operator

Using the TakeUntil operator has several benefits:

1. Helps in avoiding memory leaks by tracking the lifetime of a subscription: When multiple subscriptions are active, TakeUntil allows you to end them when not required. By doing this, it reduces the possibility of unwanted subscriptions building up in memory.

2. Minimizes Network Latency: TakeUntil provides us an option to stop requests that are no longer required. It reduces the load on the server, thus minimizing network latency.

3. Easy to implement: Implementing the TakeUntil operator is a straightforward process, which is simple to understand without memorizing any complicated APIs.

FAQs

1) What is the difference between take and takeUntil operator?

The take operator extracts a specific number of values from an observable. In contrast, the takeUntil operator finishes the subscription of the observable when another observable emits a specific value.

2) Is unsubscribing from observables essential?

Yes, If you are not unsubscribing from observables that are no longer needed, it may lead to memory leaks. When you no longer require the subscription, it is essential to cancel them since it can cause a considerable amount of memory usage.

3) What is the significance of the stopper.complete() method?

The stopper.complete() method is used to signal to the observer that the observable has completed its job. It also frees any resources associated with the subscription.

4) Can multiple observable streams be used simultaneously with TakeUntil?

Yes, multiple observable streams can be used simultaneously with TakeUntil to cancel the subscriptions associated with them. In such scenarios, use multiple TakeUntil operators explicitly for each stream.

In conclusion, the TakeUntil operator is a powerful tool in RxJS that can significantly assist in the management of subscriptions and reducing the likelihood of memory leaks or unwanted subscriptions. Combining this with other RxJS operators can provide even more effective solutions in reactive programming.

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