RxJS Observables are classsss (if you know how to use them) but there are quite a few common issues people encounter. If you don't use them correctly you can end up with memory leaks. If you don't use them efficiently you can end up with a lot of unnecessarily verbose code.
How to get memory leaks
In RxJS, each time you subscribe to an Observable or Subject, you then need to unsubscribe. If you don't unsubscribe, the subscription will exist in memory and any time the subject emits a value, the logic in the subscription will run.
export class MemoryLeakComponent {
// Our subject that we will subscribe to
subjectA$: Subject<number> = new Subject<number>();
constructor(){
// Create your memory leaks
this.subjectA$.subscribe(value => {
// logic goes here ...
});
this.subjectA$.subscribe(value => {
// logic goes here ...
});
this.subjectA$.subscribe(value => {
// logic goes here ...
});
this.subjectA$.subscribe(value => {
// logic goes here ...
});
}
}
When you navigate away from this component and then navigate back, the subscriptions still exist and 4 more subscriptions will be added upon each return to this component. If you had, for example, subscribed to a Subject from an injected service, the logic in your subscription would run in the background any time a value was emitted from the Subject, even though the instance of the component it was relating to no longer exists. This can lead to memory leaks and performance issues.
To avoid this, you need to unsubscribe which comes with it's own caveats.
How to end up with unnecessarily verbose code
export class VerboseSubscriptionComponent implements OnDestroy {
// Our subject that we will subscribe to
subjectA$: Subject<number> = new Subject<number>();
// The references we will use for our subscriptions;
subscriptionA: Subscription;
subscriptionB: Subscription;
subscriptionC: Subscription;
subscriptionD: Subscription;
constructor(){
// Create our verbose subscriptions
this.subscriptionA = this.subjectA$.subscribe(value => {
// logic goes here ...
});
this.subscriptionB = this.subjectA$.subscribe(value => {
// logic goes here ...
});
this.subscriptionC = this.subjectA$.subscribe(value => {
// logic goes here ...
});
this.subscriptionD = this.subjectA$.subscribe(value => {
// logic goes here ...
});
}
ngOnDestroy(){
// Manually unsubscribe from each subscription
this.subscriptionA.unsubscribe();
this.subscriptionB.unsubscribe();
this.subscriptionC.unsubscribe();
this.subscriptionD.unsubscribe();
}
}
This solves the memory leak issue but leaves room for error and is very verbose if we have a large amount of subscriptions. If at any point one of our subscripton variables is unsubscribed from or if it is never actually assigned to, when unsubscribe() is called in ngOnDestroy() it will error and break our app.
Below are some solutions that completely avoid this issue.
A solution involving (wo)man's best friend; LOOPS!
We can create an Array<Subscription> to store all our subscriptions. Then we itterate over the array in the ngOnDestroy() method. With this solution, our code is much less verbose, we don't need to worry about keeping track of individual subscriptions and which one has or hasn't been unsubscribed from.
export class LoopSubscriptionComponent implements OnDestroy {
// Our subject that we will subscribe to
subjectA$: Subject<number> = new Subject<number>();
// An array to hold our subscriptions
subscriptions: Array<Subscription> = new Array<Subscription>()
constructor(){
// Add our subscription to
this.subscriptions.push(
this.subjectA$.subscribe(value => {
// logic goes here ...
})
);
}
ngOnDestroy(){
// We loop our subscriptions array and unsubscribe
this.subscriptions.forEach(subscription => subscription.unsubcribe());
}
}
This is the solution I used to use when I thought I was really smart and cool. I have since been humbled and use the below, superior, solution.
A better solution involving the takeUntil() pipeable operator
Using the takeUntil() pipeable operator, we no longer need to call unsubscribe() on each individual subscription. This operator takes an observable as a paramater, and when a value is emitted from that observable, it will gracefully close the subscription is is applied to.
export class TakeUntilComponent implements OnDestroy {
// Our magical observable that will be passed to takeUntil()
private readonly ngUnsubscribe$: Subject<void> = new Subject<void>();
// Our subject that we will subscribe to
subjectA$: Subject<number> = new Subject<number>();
constructor() {
this.subjectA$
.pipe(takeUntil(this.ngUnsubscribe$))
.subscribe(value => {
// logic goes here ...
});
}
ngOnDestroy(){
// Emit a value so that takeUntil will handle the closing of our subscriptions;
this.ngUnsubscribe$.next();
// Unsubscribe from our unsubscriber to avoid creating a memory leak
this.ngUnsubscribe$.unsubscribe();
}
}
Outro
Hopefully you learned something new here! I am by no means an expert but I write this hoping you can skip the first 3 itterations of learning and jump straight into the best pattern to work with (best being my opinion ofcourse).
If you know of a better pattern or solution, or if there are any issues with what I have written above, please let me know!
And most importantly, have a nice day ♥
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