One time operation on First subscription of observable

Question

I am using Rxjs. I have one observable and multiple subscription from different sources. I wish to trigger one function only once after getting first subscription to the observable. Is there any way to achieve this?

Answer 1

Not sure if this fits your scenario, but I have dealt with this in the past as well and found it best to conditionally return a different observable after checking for an initialization variable of some kind. Below is a working example of what I mean.

Component wants a list of states from an API

this.statesService.getStates()
    .subscribe((states) => this.states = states);

Service wants to only get the states once from the API

    private _states: IState[];

    getStates(): Observable<IState[]> {
        if (!this._states) {
            // we don't have states yet, so return an observable using http
            // to get them from the API
            // then store them locally using tap
            return this.http.get<IState[]>('/options/states').pipe(
                tap((answer) => {
                    this._states = answer;
                }),
            );
        } else {
            // subsequent calls will just return an observable of the data needed
            return of(this._states);
        }
    }

In the case above, it's easy to return a conditional observable. Hopefully this provides you some ideas on how to handle your conditional (only on the first subscribe) scenario.