How to Make One Observable Sequence Wait for Another to Complete Before Emitting

Introduction

In reactive programming, the asynchronous nature of Observable sequences often requires precise control over execution order. When one Observable must wait for another to complete before starting to emit data, selecting the right operators is crucial. This article discusses effective approaches using the RxJS library.

Using the concat Operator

The concat operator sequentially joins multiple Observables, ensuring that the next Observable is subscribed only after the previous one completes. Here is an example code snippet:

import { take } from 'rxjs/operators';
import { concat } from 'rxjs';

const one = someObservable.pipe(take(1));
const two = someOtherObservable.pipe(take(1));
concat(one, two).subscribe(() => {
  // Handle logic
});

In this example, concat guarantees that two starts emitting only after one has fully completed. This method is straightforward and ideal for scenarios requiring strict sequential execution.

Using the publish Operator

If the Observables need to remain separate but require ordered execution, the publish operator can be used. Code implementation is as follows:

import { take, publish } from 'rxjs/operators';

const one = someObservable.pipe(take(1));
const two = someOtherObservable.pipe(take(1), publish());

two.subscribe(() => {
  // Handle logic for two
});

one.subscribe({
  next: () => { /* Handle logic for one */ },
  complete: () => two.connect()
});

Here, publish converts two into a connectable Observable, and it only starts emitting when one completes, triggered by the connect method.

Additional Methods

Beyond the above, operators like flatMap or switchMap can also be employed. For instance:

const one = someObservable.pipe(take(1));
const two = someOtherObservable.pipe(take(1));

one.pipe(
  switchMap(() => two)
).subscribe(() => {
  // Handle logic
});

This approach ensures order through chaining, but note that switchMap may cancel previous subscriptions when new values arrive.

Conclusion

By leveraging operators such as concat, publish, and flatMap, developers can flexibly manage the execution order of Observables. Choosing the appropriate method based on specific requirements ensures the correctness and efficiency of asynchronous data streams.