Use a Injectable() class as a factory for your reducers

medium-ngrx-reducer-class.ts

@Injectable()
export class AccountReducer {
  // with real DI inside
  constructor (private usefulService: UsefulService) {}

  createReducer() {
    // handle specific action with the help of your injected service and return new states, this is the pure reducer function
    return (state: AccountState, action: Action) => {
      if(action.type === 'SomethingUseful' /* or better still action instanceof SomethingUsefulAction, but this makes for another post ;-)*/) {
        return {...state, {useful: this.usefulService.something(action.payload)}}
      }
    }
  }
}