Can I avoid getting a message from an observable by exclusively using complete: () =>?

I'm currently diving into the world of observables in Angular and stumbled upon the Angular observable documentation. In the definitions section, it mentions:

"A handler for the execution-complete notification. Delayed values can continue to be delivered to the next handler after execution is complete"

This got me curious about how exactly this mechanism works.

Question - Does this imply that if I send a "message", there's a chance I might not receive it if I solely rely on the option complete: () =>

this.someService.getMessage().subscribe({message => {
  error: err => console.error('Observer got an error: ' + err),
  complete: () => console.log(message.whatever),
}
});

Answer №1

Is it possible that sending a "message" may not be received if only the option complete is used?

The complete handler should not be used to receive messages sent through an Observable. Its purpose is to respond when the Observable is marked as complete. Here's an example of how it should be utilized:

this.someService.getMessage().subscribe({
  next: message => console.log(message);
  error: err => console.error('Observer encountered an error: ' + err),
  complete: () => console.log('Observer was closed'),
});

In the scenario where the Observable emits a message ("hello") before being completed, you would observe:

hello
Observer was closed

Additionally, there is an important point to note:

Delayed values can still be processed by the next handler after completion.

This indicates that occasionally, the Observable might be marked as complete prior to receiving the final message. This situation could appear like this:

Observer was closed
hello

This outcome may be unexpected, which is why it's emphasized in the documentation so that developers are aware and can handle such cases accordingly.

If the next handler is omitted like below:

this.someService.getMessage().subscribe({
  error: err => console.error('Observer encountered an error: ' + err),
  complete: () => console.log('Observer was closed'),
});

... then regardless of the number of messages received from the subscription, you will simply see:

Observer was closed

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