---

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

Table of Contents

• Compatibility with Angular Versions
• Install
• Usage
• API
  • SelectSnapshot
  • ViewSelectSnapshot
• Summary

Compatibility with Angular Versions

@ngxs-labs/select-snapshot	Angular
3.x	>= 10.0.5 < 13
4.x	>= 13 < 15
5.x	>= 15

📦 Install

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

$ npm install @ngxs-labs/select-snapshot
# Or if you're using yarn
$ yarn add @ngxs-labs/select-snapshot
# Or if you're using pnpm
$ pnpm install @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

@ngxs-labs/select-snapshot exposes @SelectSnapshot and @ViewSelectSnapshot decorators, they might be used to decorate class properties.

SelectSnapshot

@SelectSnapshot decorator should be used similarly to the @Select decorator. It will decorate the property to always return the latest selected value, whereas @Select decorates properties to return observable. Given 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);
  }
}

We don't have to inject the Store and call the selectSnapshot.

Behind the scenes, @SelectSnapshot sets up a getter that calls store.selectSnapshot with the provided selector on each access. In the above example, it roughly equates to setting up this property getter:

get token(): string | null {
  // ... inject `Store` in variable `store`
  return store.selectSnapshot(AuthState.token);
}

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;
}

The @ViewSelectSnapshot decorator will force the template to be updated whenever the progress property is changed on the state:

@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;
}

The decorator internally subscribes to store.select with the provided selector and calls markForCheck() whenever the state is updated (and the selector emits).

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).