Observable in component lifecycle in Angular - Mini Project: Build & Apply

Observable in component lifecycle

📖 Scenario: You are building a simple Angular component that listens to a timer observable. This timer emits a number every second. You want to show the current timer value in the component and clean up the subscription when the component is destroyed.

🎯 Goal: Create an Angular standalone component that subscribes to a timer observable on initialization, updates a displayed counter, and unsubscribes when the component is destroyed.

📋 What You'll Learn

Create a standalone Angular component named TimerComponent

Use Angular's timer observable from rxjs that emits every 1000 milliseconds

Subscribe to the timer observable inside the component lifecycle

Store the emitted value in a component property named count

Unsubscribe from the observable when the component is destroyed to avoid memory leaks

Display the current count value in the component template

💡 Why This Matters

🌍 Real World

Timers and observables are common in Angular apps for live updates, animations, or polling data.

💼 Career

Understanding observables and component lifecycle is essential for Angular developers to manage data streams and resource cleanup properly.

1

Set up the Angular component and import timer

Create a standalone Angular component named TimerComponent. Import Component from @angular/core and timer from rxjs. Initialize a public property count with 0.

Angular

import { Component } from '@angular/core';
import { timer } from 'rxjs';

@Component({
  selector: 'app-timer',
  standalone: true,
  template: `Count: {{ count }}`
})
export class TimerComponent {
  count = 0;
  // Your code here
}

Hint

Start by importing Component and timer. Then create a standalone component with a count property set to 0.

2

Create a subscription property and timer observable

Inside TimerComponent, create a private property named subscription to hold the subscription. Create a timer observable named timer$ that emits every 1000 milliseconds using timer(0, 1000).

Angular

import { Component } from '@angular/core';
import { timer } from 'rxjs';

@Component({
  selector: 'app-timer',
  standalone: true,
  template: `Count: {{ count }}`
})
export class TimerComponent {
  count = 0;
  private subscription: any;
  timer$ = timer(0, 1000);
  // Your code here
}

Hint

Add a private property subscription to store the subscription. Create timer$ using timer(0, 1000) to emit immediately and then every second.

3

Subscribe to the timer observable and update count

In the ngOnInit() lifecycle method, subscribe to timer$. Update the count property with the emitted value inside the subscription callback.

Angular

import { Component, OnInit } from '@angular/core';
import { timer } from 'rxjs';

@Component({
  selector: 'app-timer',
  standalone: true,
  template: `Count: {{ count }}`
})
export class TimerComponent implements OnInit {
  count = 0;
  private subscription: any;
  timer$ = timer(0, 1000);

  ngOnInit() {
    // Subscribe to timer$ and update count
    // Your code here
  }
}

Hint

Implement ngOnInit(). Inside it, subscribe to timer$ and update count with the emitted value.

4

Unsubscribe from timer observable on component destroy

Implement the ngOnDestroy() lifecycle method. Inside it, unsubscribe from subscription to clean up the observable subscription.

Angular

import { Component, OnInit, OnDestroy } from '@angular/core';
import { timer } from 'rxjs';

@Component({
  selector: 'app-timer',
  standalone: true,
  template: `Count: {{ count }}`
})
export class TimerComponent implements OnInit, OnDestroy {
  count = 0;
  private subscription: any;
  timer$ = timer(0, 1000);

  ngOnInit() {
    this.subscription = this.timer$.subscribe(value => {
      this.count = value;
    });
  }

  ngOnDestroy() {
    // Unsubscribe here
    // Your code here
  }
}

Hint

Implement ngOnDestroy() and call unsubscribe() on subscription to avoid memory leaks.

Practice

1. What is the main reason to subscribe to an Observable inside the ngOnInit lifecycle hook in an Angular component?

easy

A. To start listening to data streams when the component is initialized

B. To stop listening to data streams when the component is destroyed

C. To create a new Observable instance

D. To update the component's template directly

Solution

1. Step 1: Understand ngOnInit role

   ngOnInit runs once when the component is created, ideal to start tasks like subscribing to Observables.

2. Step 2: Purpose of subscribing in ngOnInit

   Subscribing here means the component begins listening to data streams as soon as it is ready.

3. Final Answer:

   To start listening to data streams when the component is initialized -> Option A

4. Quick Check:

   Subscribe in ngOnInit = start listening [OK]

Hint: Subscribe in ngOnInit to start data listening early [OK]

Common Mistakes:

• Subscribing in ngOnDestroy instead of ngOnInit
• Expecting subscription to stop automatically
• Confusing subscription with Observable creation

2. Which of the following is the correct way to unsubscribe from an Observable in the ngOnDestroy lifecycle hook?

easy

A. this.subscription.unsubscribe();

B. this.subscription.subscribe();

C. this.subscription.complete();

D. this.subscription.destroy();

Solution

1. Step 1: Identify the unsubscribe method

   Angular subscriptions have an unsubscribe() method to stop listening to Observables.

2. Step 2: Match method to lifecycle hook

   In ngOnDestroy, calling unsubscribe() properly cleans up the subscription.

3. Final Answer:

   this.subscription.unsubscribe(); -> Option A

4. Quick Check:

   Unsubscribe method = unsubscribe() [OK]

Hint: Use unsubscribe() to stop Observable listening [OK]

Common Mistakes:

• Using subscribe() instead of unsubscribe() to stop
• Calling complete() which does not unsubscribe
• Using a non-existent destroy() method

3. Consider this Angular component code snippet:

export class TimerComponent implements OnInit, OnDestroy {
  time = 0;
  private subscription!: Subscription;

  ngOnInit() {
    this.subscription = interval(1000).subscribe(() => {
      this.time++;
    });
  }

  ngOnDestroy() {
    this.subscription.unsubscribe();
  }
}

What will be the value of time after 3 seconds if the component is displayed and then removed?

medium

A. undefined

B. 3

C. 0

D. Subscription error

Solution

1. Step 1: Understand the interval Observable

   The interval(1000) emits a value every 1000 milliseconds (1 second), triggering the subscription callback.

2. Step 2: Count increments before unsubscribe

   After 3 seconds, the callback runs 3 times, incrementing time from 0 to 3. The unsubscribe in ngOnDestroy stops further increments.

3. Final Answer:

   3 -> Option B

4. Quick Check:

   3 seconds = 3 increments [OK]

Hint: Count interval ticks before unsubscribe stops updates [OK]

Common Mistakes:

• Assuming time stays 0 without increments
• Expecting error due to missing unsubscribe
• Confusing unsubscribe timing with immediate stop

4. Identify the error in this Angular component code:

export class DataComponent implements OnInit {
  data$ = this.service.getData();

  ngOnInit() {
    this.data$.subscribe(data => {
      console.log(data);
    });
  }
}

What is the main problem here?

medium

A. Observable should not be assigned to a property

B. Subscription syntax is incorrect

C. Missing unsubscribe in ngOnDestroy causes memory leak

D. ngOnInit should not contain subscriptions

Solution

1. Step 1: Check subscription management

   The code subscribes to an Observable but never unsubscribes, risking memory leaks.

2. Step 2: Lifecycle hook missing unsubscribe

   Without ngOnDestroy to unsubscribe, the subscription stays active even after component destruction.

3. Final Answer:

   Missing unsubscribe in ngOnDestroy causes memory leak -> Option C

4. Quick Check:

   Always unsubscribe to avoid leaks [OK]

Hint: Always unsubscribe in ngOnDestroy to prevent leaks [OK]

Common Mistakes:

• Thinking subscription syntax is wrong
• Believing Observables can't be class properties
• Assuming ngOnInit forbids subscriptions

5. You want to create an Angular component that listens to a user input Observable and updates a display value. The Observable may emit multiple times quickly. Which approach best manages the subscription lifecycle and avoids memory leaks?

hard

A. Use a global variable to store data and avoid subscribing

B. Subscribe in the constructor and never unsubscribe

C. Subscribe in ngOnInit without unsubscribing, relying on Angular to clean up

D. Subscribe in ngOnInit and unsubscribe in ngOnDestroy using a Subscription object

Solution

1. Step 1: Proper subscription management

   Subscribing in ngOnInit ensures the component starts listening after creation.

2. Step 2: Unsubscribe in ngOnDestroy to prevent leaks

   Unsubscribing cleans up resources when the component is removed, avoiding memory leaks.

3. Final Answer:

   Subscribe in ngOnInit and unsubscribe in ngOnDestroy using a Subscription object -> Option D

4. Quick Check:

   Subscribe ngOnInit + unsubscribe ngOnDestroy = safe lifecycle [OK]

Hint: Pair subscribe in ngOnInit with unsubscribe in ngOnDestroy [OK]

Common Mistakes:

• Subscribing in constructor which is too early
• Not unsubscribing and causing leaks
• Using global variables instead of Observables