[NgRx] NgRx Data Fetching Solution - How to Load Data Only If Needed

We have a reoslver, which everytime we want visit '/courses' route, it will be triggered, then api will be called, data will be loaded.

import { Injectable } from "@angular/core";
import {
  Resolve,
  ActivatedRouteSnapshot,
  RouterStateSnapshot
} from "@angular/router";
import { Observable } from "rxjs";
import { Store } from "@ngrx/store";
import { AppState } from "../reducers";
import { CoursesAction } from "./actions-types";
import { tap, first, finalize } from "rxjs/operators";
import { adapter } from "./reducers/courses.reducers";

@Injectable()
export class CoursesResolver implements Resolve<any> {
  loading = false;
  constructor(private store: Store<AppState>) {}

  resolve(
    route: ActivatedRouteSnapshot,
    state: RouterStateSnapshot
  ): Observable<any> {
    return this.store.pipe(
      tap(() => {
        if (!this.loading) {
          this.loading = true;
          this.store.dispatch(CoursesAction.loadAllCourse());
        }
      }),
      // this resolve need to complete, so we can use first()
      first(),
      finalize(() => (this.loading = false))
    );
  }
}

So how to prevent reload the data if we have already loaded the data once?

Add one prop into state: 'allCoursesLoaded'

import { Course, compareCourses } from "../model/course";
import { EntityState, createEntityAdapter } from "@ngrx/entity";
import { createReducer, on } from "@ngrx/store";
import { CoursesAction } from "../actions-types";
/*
export interface CoursesState {
  entities: { [key: number]: Course };
  ids: number[];
}*/

export interface CoursesState extends EntityState<Course> {
  /**Extend the entity here */
  allCoursesLoaded: boolean;
}

export const adapter = createEntityAdapter<Course>({
  sortComparer: compareCourses
  // selectId: course => course.id // NgRx use 'id' by default
});

export const initCoursesState = adapter.getInitialState({
  allCoursesLoaded: false
});

export const coursesReducer = createReducer(
  initCoursesState,
  on(CoursesAction.allCoursesLoaded, (state, action) =>
    adapter.addAll(action.courses, { ...state, allCoursesLoaded: true })
  )
);

export const { selectAll } = adapter.getSelectors();

Now we can modify the resolver:

import { Injectable } from "@angular/core";
import {
  Resolve,
  ActivatedRouteSnapshot,
  RouterStateSnapshot
} from "@angular/router";
import { Observable } from "rxjs";
import { Store, select } from "@ngrx/store";
import { AppState } from "../reducers";
import { CoursesAction } from "./actions-types";
import { tap, first, finalize, filter } from "rxjs/operators";
import { adapter } from "./reducers/courses.reducers";
import { selectAllCoursesLoaded } from "./courses.selectors";

@Injectable()
export class CoursesResolver implements Resolve<any> {
  loading = false;
  constructor(private store: Store<AppState>) {}

  resolve(
    route: ActivatedRouteSnapshot,
    state: RouterStateSnapshot
  ): Observable<any> {
    return this.store.pipe(
      select(selectAllCoursesLoaded),
      tap(courseLoaded => {
        if (!this.loading && !courseLoaded) {
          this.loading = true;
          this.store.dispatch(CoursesAction.loadAllCourse());
        }
      }),
      // this resolve need to complete, so we can use first()
      filter(courseLoaded => courseLoaded),
      first(),
      finalize(() => (this.loading = false))
    );
  }
}