Is it necessary to unsubscribe from observables created by Http methods?

Question

Do you need to unsubscribe from Angular 2 http calls to prevent memory leak?

 fetchFilm(index) {
        var sub = this._http.get(`http://example.com`)
              


        

Answer 1

So the answer is no, you don't. Ng2 will clean it up itself.

The Http service source, from Angular's Http XHR backend source:

Notice how it runs the complete() after getting the result. This means it actually unsubscribes on completion. So you don't need to do it yourself.

Here is a test to validate:

  fetchFilms() {
    return (dispatch) => {
        dispatch(this.requestFilms());

        let observer = this._http.get(`${BASE_URL}`)
            .map(result => result.json())
            .map(json => {
                dispatch(this.receiveFilms(json.results));
                dispatch(this.receiveNumberOfFilms(json.count));
                console.log("2 isUnsubscribed",observer.isUnsubscribed);
                window.setTimeout(() => {
                  console.log("3 isUnsubscribed",observer.isUnsubscribed);
                },10);
            })
            .subscribe();
        console.log("1 isUnsubscribed",observer.isUnsubscribed);
    };
}

As expected, you can see that it is always unsubscribed automatically after getting the result and finishing with the observable operators. This happens on a timeout (#3) so we can check the status of the observable when it's all done and completed.

And the result

So, no leak would exist as Ng2 auto unsubscribes!

Nice to mention: This Observable is categorized as finite, on contrary to the infinite Observablewhich is an infinite stream of data can be emitted like DOM click listener for example.

THANKS, @rubyboy for help on this.

Answer 2

Calling the unsubscribe method is rather to cancel an in-progress HTTP request since this method calls the abort one on the underlying XHR object and remove listeners on the load and error events:

// From the XHRConnection class
return () => {
  _xhr.removeEventListener('load', onLoad);
  _xhr.removeEventListener('error', onError);
  _xhr.abort();
};

That said, unsubscribe removes listeners... So it could be a good idea but I don't think that it's necessary for a single request ;-)

Hope it helps you, Thierry

Answer 3

You should definitely read this article. It shows you why you should always unsubscribe even from http.

If after creating the request but before receiving an answer from the back-end you deem the component unnecessary and destroy it, your subscription will maintain the reference to the component thus creating a chance for causing memory leaks.

Update

The above affirmation seems to be true, but anyway, when the answer comes back the http subscription is destroyed anyway