Angular和rxJS。 流永远不会完成

Question

I have a menu in angular app and this menu controls the selected client in my web application.

As it is :

• My menu is a component which shares the selected client through a service.
• The selected client is shared as an Observable.
• The observable never complete. It just emit the new selected client when the menu is clicked.
• This is a replay subject. Each new subscriber retrieve the last emitted client.

it seemed like a good design but I encountered some problem when making new observables based on the client one (never completing observables). AFAIK this is due that the first observable never completesand this property will propagate.

//Command
export interface Command {
    productName: string;
    qty: number;
}

//The service
@Injectable()
export class ClientService {
private _client: ReplaySubject<Client> = new ReplaySubject(1);

    setClient(client: Client) { // The menu component calls setClient on a user click
        this._client.next(client);
    }
    getClient(): Observable<Client> { // getClient is heavilly called in child component to observe client selection events.
        return this._client.asObservable();
    }
}


getCommands(): Observable<Command> { //Used in a template with async pipe

    //In a child component
    const commandsObs = this.clientService.getClient()
    //.take(1) //I am forced to use take(1) to get getCommands() observer to complete
    .flatMap(c => {
        return Observable.merge(getCommandsPromise1(c), getCommandsPromise2(c));
    })
    .reduce((acc, next) => Object.assign({}, acc, next))
    .map(next => {
        return finishMakingCommand(next));
    })
    .catch(err => /* Some error management code*/)
}

getCommandsPromise1(client: Client): Promise<any> {
    //REST call returning a promise
    return Promise.resolve({ 'command1': { 'productName': 'toy', qty: 1 } });
}
getCommandsPromise2(client: Client): Promise<any> {
    //REST call returning a promise
    return Promise.resolve({ 'command2': { 'productName': 'another toy', qty: 1 } });
}

finishMakingCommand(commands: any): Command[] {
    // Flattens 'command1' and 'command2' to a list of commands
    return [{'productName': 'toy', qty: 1}, {'productName': 'another toy', qty: 2}];
}

I'd like to know if more experienced developers think a never ending observable is a good design and what are the alternatives to avoid never ending observables.

Answer 1

you can use unsubscribe function to stop this: for example: mySubscribtion: Subscription;

 this.mySubscription = somthing.subscribe((...) => {...})

and then you can unsubscibe on event or onDestroy that way:

this.mySubscribtion.unsubscribe();

Answer 2

As I mentioned above, think about observables as laying a pipeline. The only time there's a problem is when the water keeps coming (as with Observable.interval(1000) source - it's going to keep ticking). In that situation, if you manually subscribe to the observable, you also need to unsubscribe.

But, as you said, you are using the async pipe and Angular takes care of the unsubscribing there.

With you menu clicks, one click will send one value. It's quite common for an observable never to complete (ie never receive the completed event), it's not a requirement.

The completed event is generally useful for aggregating operators such as toArray() so that they know the whole set of values.

I would suggest just using

const commandsObs = this.clientService.getClient();

and in the template (example)

<div>{{ commandObs | async }}</div>

Forget about the flatMap, unless it's intending to do something fancy - let me know.

Edit - suggested changes to new sample code

You can try moving the reduce and map inside the flatmap, as they are intended to process the results of the inner observable (Observable.merge).

const commandsObs = this.clientService.getClient()
  .flatMap(c => {
    return Observable.merge(getCommandsPromise1(c), getCommandsPromise2(c))
      .reduce((acc, next) => Object.assign({}, acc, next))
      .map(next => finishMakingCommand(next) )
  });

An alternative version to try,

const commandsObs = this.clientService.getClient()
  .map(c => Observable.forkJoin(getCommandsPromise1(c), getCommandsPromise2(c) );