@ngxs-labs/select-snapshot

---

Flexibile decorator, an alternative for the @Select but selects a snapshot of the state

Table of Contents

• Install
• Usage
• API
  • SelectSnapshot
  • ViewSelectSnapshot
• Summary

📦 Install

To install @ngxs-labs/select-snapshot run the following command:

npm install @ngxs-labs/select-snapshot
# of if you use yarn
yarn add @ngxs-labs/select-snapshot

🔨 Usage

Import the NgxsSelectSnapshotModule into your root application module:

import { NgModule } from '@angular/core';
import { NgxsModule } from '@ngxs/store';
import { NgxsSelectSnapshotModule } from '@ngxs-labs/select-snapshot';

@NgModule({
  imports: [NgxsModule.forRoot(states), NgxsSelectSnapshotModule.forRoot()],
})
export class AppModule {}

API

There are 2 decorators exposed publicly. These are @SelectSnapshot and @ViewSelectSnapshot. They can be used to decorate class properties.

SelectSnapshot

@SelectSnapshot decorator behaves the same as the @Select decorator. The only difference is @SelectSnapshot decorated property will always return the current state value whereas @Select decorated property returns an Observable. Let's look at the following example:

import { SelectSnapshot } from '@ngxs-labs/select-snapshot';

@Injectable()
export class TokenInterceptor {
  @SelectSnapshot(AuthState.token) token: string | null;

  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    if (this.token) {
      req = req.clone({
        setHeaders: {
          Authorization: `Bearer ${this.token}`,
        },
      });
    }

    return next.handle(req);
  }
}

As you may notice we don't have to inject the Store class and invoke the selectSnapshot on it.

ViewSelectSnapshot

@ViewSelectSnapshot is a decorator that should decorate class properties that are used in templates (e.g. renderable or passed as bindings). Given the following example:

@Component({
  selector: 'app-progress',
  template: `
    <div>
      <div [style.width.%]="progress"></div>
    </div>
  `,
  changeDetection: ChangeDetectionStrategy.OnPush,
})
export class ProgressComponent {
  // 🚫 Do not use `SelectSnapshot` since `progress` is used in the template.
  @SelectSnapshot(ProgressState.getProgress) progress: number;
}

Why? Because if the progress state gets updated then Angular has to check that view and update it. This view will not get updated because it's marked as OnPush, which means it's constantly in CheckOnce state. How to make the above example work?

@Component({
  selector: 'app-progress',
  template: `
    <div>
      <div [style.width.%]="progress"></div>
    </div>
  `,
  changeDetection: ChangeDetectionStrategy.OnPush,
})
export class ProgressComponent {
  // ✔️ Our view will be checked and updated.
  @ViewSelectSnapshot(ProgressState.getProgress) progress: number;
}

How does it work? The @ViewSelectSnapshot decorator calls markForCheck() under the hood when the progress state gets updated.

Summary

We have looked at several examples of using both decorators. Consider to use the @SelectSnapshot if decorated properties are not used in templates! Consider to use the @ViewSelectSnapshot if decorated properties are used in templates (e.g. renderable or passed as bindings).