Add `langChanged$` observable

Is your feature request related to a problem? Please describe.

TranslocoService provides an observable that emits when the language is about to change. But there is no observable that emits when the language has changed.

Current behavior

When using langChanges$ to trigger rerendering a pipe the pipe rerenders but the language has not been loaded yet and therefor the pipe rerenders in the same language as before. This would be useful for https://github.com/ngneat/transloco/issues/443

[gabriel-gn]

Added PR https://github.com/ngneat/transloco/pull/534

[mfodor]

I face with the very same problem in our project as well. I’ve already tried with events$ which can help when the first lang change happens, but it is not fired after that.

I need to translate computed keys programmatically in the controller. So this is working fine now:

    this.transloco.langChanges$.subscribe(() => {
      this.buildHRParams();

      // this is needed to fix translation when the other language is loaded for the first time and so is not available
      // but is not enough since this is never called again later
      this.transloco.events$.pipe(
        filter(e => e.type === 'translationLoadSuccess'),
        pluck('payload'),
        first()
      ).subscribe(() => {
        this.buildHRParams();
      });
    });

But is this really a good API to use? Why should I call buildHRParams() twice? Why should I subscribe in a subscription for the (kind of the) same event?

langChanges$ is fired before loading the other language file
events$ fired only when real loading happened and not the cache has been used. We could say that it’s “bug” should be fixed and fire a translationLoadSuccess event even when loaded from the cache but I’d still say that why should I do 2 subscription for the 1 event I am interested of.

It’s easy to see that there is a valid need to have a langChanged$ event which is fired always after having the new translations available.

I don’t know if there is any reason to fire the langChanges$ event before loading the translations but if there is none then even that could be changed to be fired after the loading.