Angular/Rxjs Conditional nested Observables

Question

I recently converted a function to dynamically retrieve data from disk rather than memory and during the process went from synchronously returning a variable to returning a Observable.

As part of this conversion every dependent function had to be converted to async via Observables and one of these functions performs a nested conditional call that gets the last scenario for a category or if there was no previous scenario for a category generates gets a default scenario:

public generateScenario(category: string, value: number = 1) : Observable<Scenario>
{
    return this.getHistory(category).map(scenario => {
        if(scenario === null)
        {
            return this.defaultService.getDefault(category, value);
        }
        else
        {
            return scenario;
        }
    });
}

Here this.defaultService.getDefault(category, value) returns an Observable which means that when the scenario === null branch is executed generateScenario() returns an Observable>.

How do I flatten this out and avoid returning a nested Observable?

.do is for when you don't want to return something and change the value of the stream. — jonrsharpe
– jonrsharpe, Commented Dec 11, 2017 at 21:14
@jonsharpe sorry made a mistake in the sample code, I actually meant to use map — cubiclewar
– cubiclewar, Commented Dec 12, 2017 at 1:42
Checkout this entire article on conditional work in RxJS blog.strongbrew.io/rxjs-patterns-conditionally-executing-work — KwintenP
– KwintenP, Commented Feb 22, 2019 at 8:14

Explosion Pills · Accepted Answer · 2017-12-11 21:13:46Z

2

You can flatten this out with various flattening operators in RxJS such as switch or merge. In this case, switch seems most appropriate:

this.getHistory(category).switchMap(scenario =>
  scenario === null ? this.defaultService.getCategory(category, value)
  : of(scenario)
);

The of is required the function passed to .switchMap must return an Observable to be flattened/unwrapped/etc.

answered Dec 11, 2017 at 21:13

Explosion Pills

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