Platform: NGRX Effects polling

[E1ena]

Is there best practices for polling?

Hi, I'm trying to polling backend using effects.
The problem is I want to re-rendere my app only if server response data changed (so I'm trying to avoid unnecessary state update).
Is it possible? As I know every effect must return action, which triggers reducer, which returns new state. So it either updates state or failes.

effects:

@Effect() solutions: Observable<Action> = this.actions.pipe(
ofType(SolutionActionTypes.GetSolutions),
switchMap(() =>
    timer(0, 3000).pipe(
          switchMap((action: any) => {
            return this.http.get<any>(`/url`, {observe: 'response'}).pipe(
                map((res: any) => {
                    if (res.status === 200) {
                        if(//data's changed) {
                            return {type: SolutionActionTypes.SetSolutions, payload: res.body};
                       } else {
                         ???
                      } 
                    })
                );
            })
        )
    )
);

[bossqone]

In our project we created NoOpAction which is then fired and it's not handled by any reducer.

_Note: Don't forget to add some "stop" action to your code, otherwise you will keep pulling forever. But of course it depends on your project, maybe it's desired functionality._

[E1ena]

In our project we created NoOpAction which is then fired and it's not handled by any reducer.

_Note: Don't forget to add some "stop" action to your code, otherwise you will keep pulling forever. But of course it depends on your project, maybe it's desired functionality._

Thanks for answer! That is NoOpAction? Can't find it in docs

[bossqone]

You need to create custom one, it's not the part of NgRx.
Something like:

{ type: SomeActionTypes.NoOp }

[E1ena]

You need to create custom one, it's not the part of NgRx.
Something like:

{ type: SomeActionTypes.NoOp }

I've tried this way. But after { type: SomeActionTypes.NoOp } comes, my reducer updates state. It's ever

   case SomeActionTypes.NoOp: {
        return {...state};
 }

or
default:
return state;

And then all my client side 'jumpes'

[bossqone]

Well, you shouldn't handle this action at all, so it will fall into default case in all reducers.

[bbonczek]

Hi, I've recently wrote an article on polling using ngrx effect. If anyone needs help here is the link: https://bbonczek.github.io/jekyll/update/2018/03/01/polling-with-ngrx.html

[dinhlambcs]

@bbonczek I read your article and have some questions, so can you help answer me ?

• How and when dispatch continuePolling action?
• Does your article support ngrx version 6.5 to up?
  Thanks

[bbonczek]

@dinhlambcs - there is no continuePolling action - there are only StartPolling and StopPolling actions. ContinuePolling$ effect is called on GetSubnetDevicesSucceded/Failed actions. I've pasted a part of an article below:

import { Injectable } from '@angular/core';
import { of } from 'rxjs';
import { Effect, Actions, ofType } from '@ngrx/effects';
import { map, switchMap, catchError, takeWhile, delay } from 'rxjs/operators';
import * as SubnetActions from './subnet.actions.ts';
import SubnetEntry from './subnetEntry.model.ts';

@Injectable()
export class SubnetEffects {
  constructor(
    private actions$: Actions<SubnetActions.SubnetActionsUnion>,
    private http: HttpClient
  ) {}

  private isPollingActive = false; // a flag that indicates whether polling is active or not

  // This effect starts polling. There is no other way of initializing polling that calling StartPollingSubnetDevices action.
  @Effect()
  startPolling$ = this.actions$.pipe(
    ofType(SubnetActions.SubnetActionTypes.StartPollingSubnetDevices),
    map(() => this.isPollingActive = false), // switch flag to true
    switchMap(() => {
      return this.http.get<SubnetEntry>('http://localhost:5000/api/subnet').pipe(
        switchMap(entries => new SubnetActions.GetSubnetDevicesSucceded({ entries })),
        catchError(error => of(new SubnetActions.GetSubnetDevicesFailed({ error })))
      ),
    }),
  );

  // This effect stops polling. There is no other way to stop polling that calling Stop PollingSubnetDevices action.
  @Effect()
  stopPolling$ = this.actions$.pipe(
    ofType(SubnetActions.SubnetActionTypes.StopPollingSubnetDevices),
    map(() => this.isPollingActive = false) // switch flag to false
  );

  // this effect repeats only if time since last response is at least 5 seconds.
  @Effect()
  continuePolling$ = this.actions$.pipe(
    ofType(
      SubnetActions.SubnetActionTypes.GetSubnetDevicesSucceded,
      SubnetActions.SubnetActionTypes.GetSubnetDevicesFailed
    ),
    takeWhile(() => this.isPollingActive), // do this only as long as flag is set to true
    switchMap(() => {
      return this.http.get<SubnetEntry>('http://localhost:5000/api/subnet').pipe(
        delay(5000),
        switchMap(entries => new SubnetActions.GetSubnetDevicesSucceded({ entries })),
        catchError(error => of(new SubnetActions.GetSubnetDevicesFailed({ error })))
      );
    })
  );
}

And I am not sure which version of ngrx was used.

[dinhlambcs]

@bbonczek: Need I dispatch startPolling action ? I try to dispatch StartPolling action but api only call once time. I'm using rxjs 6.5 with angular 7, ngrx 7