Angular Signals: How can we effectively prompt a data fetch when the input Signals undergo a change in value?

Question

Angular Signals: How can we effectively prompt a data fetch when the input Signals undergo a change in value?

As I delve into learning and utilizing Signals within Angular, I find it to be quite exhilarating. However, I have encountered some challenges in certain scenarios. I am struggling to come up with an effective approach when dealing with a component that has input signals, and I need to trigger a re-fetch of data whenever the input value changes.

Using computed is not feasible as it cannot handle asynchronous operations. The effect method, as per the documentation, is not meant to alter component state. Additionally, ngOnChanges is on its way to being deprecated in favor of Signals-based components and a zoneless approach.

Let's take a look at the following component:

@Component()
export class ChartComponent {
    dataSeriesId = input.required<string>();
    fromDate = input.required<Date>();
    toDate = input.required<Date>();

    private data = signal<ChartData | null>(null);
}

When any of the input signals receives a new value, I need to initiate a data re-fetch and update the private data signal. How should I tackle this issue? What would be considered best practice in this scenario? Should I use the effect method and disregard the guideline against modifying state?

angular typescript components angular-signals

Answer 1

Answer №1

Utilize Angular's rxjs-interop library to convert the incoming signals into an Observable, then use switchMap to retrieve the results and convert them back to a signal, as shown below:

import { toObservable, toSignal } from '@angular/core/rxjs-interop';

@Component({
  selector: 'app-chart',
  standalone: true,
  template: `
    {{data()}}
  `,
})
export class ChartComponent {
  dataSeriesId = input.required<string>();
  fromDate = input.required<Date>();
  toDate = input.required<Date>();

  params = computed(() => ({
    id: this.dataSeriesId(),
    from: this.fromDate(),
    to: this.toDate(),
  }));

  data = toSignal(
    toObservable(this.params).pipe(
      switchMap((params) => this.fetchData(params))
    )
  );

  private fetchData({ id, from, to }: { id: string; from: Date; to: Date }) {
    return of(`Example data for id [${id}] from [${from}] to [${to}]`).pipe(
      delay(1000)
    );
  }
}

Any modification to an input will result in triggering a new fetch operation.

See a working demonstration on StackBlitz

Answer 2

Utilize Angular's rxjs-interop library to convert the incoming signals into an Observable, then use switchMap to retrieve the results and convert them back to a signal, as shown below:

import { toObservable, toSignal } from '@angular/core/rxjs-interop';

@Component({
  selector: 'app-chart',
  standalone: true,
  template: `
    {{data()}}
  `,
})
export class ChartComponent {
  dataSeriesId = input.required<string>();
  fromDate = input.required<Date>();
  toDate = input.required<Date>();

  params = computed(() => ({
    id: this.dataSeriesId(),
    from: this.fromDate(),
    to: this.toDate(),
  }));

  data = toSignal(
    toObservable(this.params).pipe(
      switchMap((params) => this.fetchData(params))
    )
  );

  private fetchData({ id, from, to }: { id: string; from: Date; to: Date }) {
    return of(`Example data for id [${id}] from [${from}] to [${to}]`).pipe(
      delay(1000)
    );
  }
}

Any modification to an input will result in triggering a new fetch operation.

See a working demonstration on StackBlitz

Answer 3

Answer №2

While the accepted answer does indeed work, I find it to be a somewhat makeshift solution. Signals in Angular are meant to reduce reliance on RXJS, and there is talk of potentially making RXJS optional in the future. This could make Angular more accessible to new developers.

Instead, I suggest considering the writable-signal approach. It may require some caution to avoid circular dependencies, but in many cases, including the one you described, it should suffice.

As of now (June 2024), Angular signals still have some rough edges, and there doesn't seem to be a fully satisfactory solution for this common scenario. However, I am optimistic that this will improve in the coming years.

Answer 4

While the accepted answer does indeed work, I find it to be a somewhat makeshift solution. Signals in Angular are meant to reduce reliance on RXJS, and there is talk of potentially making RXJS optional in the future. This could make Angular more accessible to new developers.

Instead, I suggest considering the writable-signal approach. It may require some caution to avoid circular dependencies, but in many cases, including the one you described, it should suffice.

As of now (June 2024), Angular signals still have some rough edges, and there doesn't seem to be a fully satisfactory solution for this common scenario. However, I am optimistic that this will improve in the coming years.

Answer 5

Answer №3

When you need to take action upon a change in signal value, it is recommended to utilize the effect functionality from @angular/core.

The bi-directional conversions between observables may seem like a workaround, but for your specific scenario, it can be efficiently implemented as suggested.

In this situation, the ideal approach is to create an effect that monitors the changes in your input signal values, triggers the necessary requests, and updates the signal with the retrieved data:

effect(
  () => {
    this.fetch(
      this.dataSeriesId(),
      this.fromDate(),
      this.toDate()
    ).subscribe((res) => {
      this.data.set(res);
    });
  },
  { allowSignalWrites: true }
);

For a complete working example, refer to this link: https://stackblitz.com/edit/stackblitz-starters-r6qwly?file=src%2Fmain.ts

Additionally, in cases where the timing of fetching data may cause incorrect results, ensuring to cancel the old subscription whenever parameters change is essential:

let dataLoadSub: Subscription | null = null;
effect(
  () => {
    dataLoadSub?.unsubscribe();
    dataLoadSub = this.fetch(
      this.dataSeriesId(),
      this.fromDate(),
      this.toDate()
    ).subscribe((res) => {
      this.data.set(res);
    });
  },
  { allowSignalWrites: true }
);

Answer 6

When you need to take action upon a change in signal value, it is recommended to utilize the effect functionality from @angular/core.

The bi-directional conversions between observables may seem like a workaround, but for your specific scenario, it can be efficiently implemented as suggested.

In this situation, the ideal approach is to create an effect that monitors the changes in your input signal values, triggers the necessary requests, and updates the signal with the retrieved data:

effect(
  () => {
    this.fetch(
      this.dataSeriesId(),
      this.fromDate(),
      this.toDate()
    ).subscribe((res) => {
      this.data.set(res);
    });
  },
  { allowSignalWrites: true }
);

For a complete working example, refer to this link: https://stackblitz.com/edit/stackblitz-starters-r6qwly?file=src%2Fmain.ts

Additionally, in cases where the timing of fetching data may cause incorrect results, ensuring to cancel the old subscription whenever parameters change is essential:

let dataLoadSub: Subscription | null = null;
effect(
  () => {
    dataLoadSub?.unsubscribe();
    dataLoadSub = this.fetch(
      this.dataSeriesId(),
      this.fromDate(),
      this.toDate()
    ).subscribe((res) => {
      this.data.set(res);
    });
  },
  { allowSignalWrites: true }
);

Angular Signals: How can we effectively prompt a data fetch when the input Signals undergo a change in value?

Answer №1

Answer №2

Answer №3

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