Angular 2 / Rxjs：我真的需要取消订阅吗？

Question

I understand that I must unsubscribe certains Observable (ie: Observables that have infinite value) when Components are destroyed in order to prevent memory leaks. I do not need to do that for finite Observables as they will complete and automatically unsubscribe .

But if I create an infinite Observable in my component (for example FormGroup.valueChanges , or QueryList.changes ), this one will be destroyed with the component that contains it, so I think that they will be no memory leak even if I do not unsubscribe from it.

Here is a simple example:

@Component({})
export class DummyComponent {
    form: FormGroup;

    constructor(private fb: FormBuilder) {
        this.form = this.fb.group({
            firstName: [''],
            lastName: ['']
        });
        this.form.valueChanges.subscribe(
            x => console.log(x)
        );
    }
}

Here, I do not unsubscribe from this.form.valueChanges ; when my component is destroyed, the this.form.valueChanges will be destroyed too.

Will there be a memory leak in this case?

Answer 1

As Babar had mention, you need to do the unsubscribe in order to stop those subscriptions to continue watching for changes.

For your particular case I think that you have a point.

One thing I do when I have a lot of subscriptions in the same component is the following.

First I create "subscriptions", an empty Array of Subscription type.

private subscriptions: Subscription[] = [];

Then every time I need to do subscribe I push it into the array

this.subscriptions.push(this.form.valueChanges.subscribe(x => console.log(x)));

And in the ngOnDestroy I unsubscribe to every subscription inside the array.

ngOnDestroy(): void {
  this.subscriptions.forEach((elem) => { elem.unsubscribe(); })
}

Answer 2

When your component is destroyed your subscribers are not they still waiting for any event came to them it will cost memory as well sometimes it may disturb your logics as well. for example router events subscription it will trigger everywhere in your app and execute the code you did inside subscription.

I second case if you switch between components and never unsubscribed your app will be hang up after a bunch of time because whenever you load the component new subscribers bind up.

@Component({})
export class DummyComponent implements OnDestroy {
 form: FormGroup;
 subscription: Subscription; // from rxjs
 constructor(private fb: FormBuilder) {
    this.form = this.fb.group({
        firstName: [''],
        lastName: ['']
    });
    this.subscription = this.form.valueChanges.subscribe(
        x => console.log(x)
    );
 }

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