Prevent memory leaks in Angular 2?

Question

In Angular 2 are there any specific pitfalls regarding memory management, I should be aware of?

What are the best practices to manage the state of components in order to avoid possible leaks?

Specifically, I've seen some people unsubscribing from HTTP observables in the ngOnDestroy method. Should I always do that?

In Angular 1.X I know that when a $scope is destroyed, all listeners on it are destroyed as well, automatically. What about observables in Angular 2 components?

@Component({
  selector: 'library',
  template: `
    <tr *ngFor="#book of books | async">
        <td>{{ book.title.text }}</td>
        <td>{{ book.author.text }}</td>
    </tr>
  `
})
export class Library {
    books: Observable<any>;

    constructor(private backend: Backend) {
        this.books = this.backend.get('/texts'); // <-- does it get destroyed
                                                 //     with the component?
    }
};

Answer 1

You do not have to unsubscribe from standard subscriptions like after http.get(). But you DO have to unsubscribe from subscription on your custom Subjects. If you have some component and inside it you subscribing to some Subject in your service, then every time you showing that component new subscription will be added to the Subject.

Please check this out: Good solution to make your components 'clean'

My personal approach - all my components are extended from this nice class:

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

/**
 * A component that cleans all subscriptions with oneself
 * https://stackoverflow.com/questions/38008334/angular-rxjs-when-should-i-unsubscribe-from-subscription
 * @class NeatComponent
 */
export abstract class NeatComponent implements OnDestroy, OnInit {
// Add '.takeUntil(this.ngUnsubscribe)' before every '.subscrybe(...)'
// and this subscriptions will be cleaned up on component destroy.

  protected ngUnsubscribe: Subject<any> = new Subject();

  public ngOnDestroy() {
    this.ngUnsubscribe.next();
    this.ngUnsubscribe.complete();
  }

  public ngOnInit(){}
}

And I just adding super() call to constructor and .takeUntil(this.ngUnsubscribe) before every subscribe:

import { NeatComponent } from '../../types/neat.component';

@Component({
  selector: 'category-selector',
  templateUrl: './category-selector.component.pug'
})
export class CategorySelectorComponent extends NeatComponent {

  public constructor(
    private _shopService: ShopsService
  ) { super(); }

  public ngOnInit() {
    this._shopService.categories.takeUntil(this.ngUnsubscribe)
      .subscribe((categories: any) => {
        // your code here
      })
  }
}

Answer 2

As requested by @katspaugh

In your specific case there's no need to unsubscribe manually since that's the Async pipe's job.

Check the source code for AsyncPipe. For brevity I'm posting the relevant code

class AsyncPipe implements PipeTransform, OnDestroy {
    // ...
    ngOnDestroy(): void {
        if (isPresent(this._subscription)) {
          this._dispose();
        }
    }

As you can see the Async pipe implements OnDestroy, and when it's destroyed it checks if is there some subscription and removes it.

You would be reinventing the wheel in this specific case (sorry for repeating myself). This doesn't mean you can't/shouldn't unsubscribe yourself in any other case like the one you referenced. In that case the user is passing the Observable between components to communicate them so it's good practice to unsubscribe manually.

I'm not aware of if the framework can detect any alive subscriptions and unsubscribe of them automatically when Components are destroyed, that would require more investigation of course.

I hope this clarifies a little about Async pipe.