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angular-rxjs-grail

Forked from https://github.com/DeborahK/Angular-RxJS course repo.

Demo project demonstrating RxJS and Observable patterns within an Angular application. This approach is different to using the @ngrx package commonly used in Angular projects. Which gives us a store, actions, effects, reducer etc. Instead the following pattern of Observable actions and services is used to create a reactive application without a Store:

Actions: Creating an action stream using a BehaviorSubject and calling .next is similar to dispatching an action in NgRx:

  // Action stream
  private categorySelectedSubject = new BehaviorSubject<number>(0);
  categorySelectedAction$ = this.categorySelectedSubject.asObservable();

  onSelected(userName: string): void {
    this.categorySelectedSubject.next(category);
  }

Selecting data: Instead of using NgRx selectors or effects, state is queried directly from the service and subscribed to in the component with combineLatest.

  // Merge Data stream with Action stream
  // To filter to the selected category
  products$ = combineLatest([
    this.productService.productsWithCRUD$,
    this.categorySelectedAction$
  ])
    .pipe(
      map(([products, selectedCategoryId]) =>
        products.filter(product =>
          selectedCategoryId ? product.categoryId === selectedCategoryId : true
        )),
      catchError(err => {
        this.errorMessageSubject.next(err);
        return EMPTY;
      })
    );

Querying data: Alternatively to NgRx effects$, Observable services and data from other Observables are combined to provide the component with an Observable to subscribe to.

  // Combine products with categories
  // Map to the revised shape.
  // Be sure to specify the type to ensure after the map that it knows the correct type
  productsWithCategory$ = combineLatest([
    this.products$,
    this.productCategoryService.productCategories$
  ]).pipe(
    map(([products, categories]) =>
      products.map(product => ({
        ...product,
        price: product.price * 1.5,
        category: categories.find(c => product.categoryId === c.id).name,
        searchKey: [product.productName]
      }) as Product)
    ),
    shareReplay(1)
  );

Hot vs Cold Observables

Hot observables can produce events in the absence of observers, whereas cold observables don’t become active until they have a subscription. A result of this is that hot streams will often have much longer lifespans than their cold counterparts. Whereas a cold stream will shut down when the subscriber shuts down, a hot one can continue running after the end of a subscription.

Subjects

An RxJS Subject is a special type of Observable that allows values to be multicasted to many Observers.

Subject:

A Subject is like an Observable, but can multicast to many Observers. Subjects are like EventEmitters: they maintain a registry of many listeners.

A Subject sends data to subscribed observers. Any previously emitted data is not sent to new observers.

BehaviorSubject:

BehaviorSubject stores the latest value emitted to its consumers, and whenever a new Observer subscribes, it will immediately receive the “current value”.

const subject = new BehaviorSubject(0); // 0 is the initial value

ReplaySubject:

A ReplaySubject is similar to a BehaviorSubject in that it can send old values to new subscribers, but it can also record a part of the Observable execution.

const subject = new ReplaySubject(2); // buffer 2 values for new subscribers

AsyncSubject:

A AsyncSubject will emit the last value to observers when the sequence is completed.

const subject = new AsyncSubject();

Stackblitz

Stackblitz Demo: https://stackblitz.com/edit/angular-todos-deborahk?file=src%2Fapp%2Fapp.component.ts

RxJS Tutorials

Some youtube tutorials on RxJS principles and operators.

Development server

Run ng serve for a dev server. Navigate to http://localhost:4200/. The app will automatically reload if you change any of the source files.

Code scaffolding

Run ng generate component component-name to generate a new component. You can also use ng generate directive|pipe|service|class|guard|interface|enum|module.

Build

Run ng build to build the project. The build artifacts will be stored in the dist/ directory. Use the --prod flag for a production build.

Running unit tests

Run ng test to execute the unit tests via Karma.