Correct Checkbox Setting After Handling CheckboxChanged in Angular with Signals

When using Angular's new signals API, handling checkbox states can be tricky. I encountered an issue where a checkbox's checked state did not persist after triggering (change). This happened both with Angular Material's mat-checkbox and the default <input type="checkbox"> element.

In this post, I will explain the issue and the simple fix I used that ensures the checkbox remains in the correct state. This fix is a practical solution, but there may be other approaches depending on your implementation.

The Issue

Here's an example of the checkbox setup:

<mat-checkbox
  class="feature-checkbox"
  [checked]="featureEnabled()"
  (change)="onFeatureEnabledChange($event)"
  [indeterminate]="isFeatureLoading()"
  data-cy="feature-checkbox"
></mat-checkbox>

The function featureEnabled() is a signal that determines whether the checkbox should be checked. However, after checking the box, the state did not persist as expected. Here you see this effect in action:

Here, you see that the checkbox changes to checked, even though the state below it stated (the featureEnabled()) will say false.

Even if I'd remove the [checked] (and [indeterminate]) property, this error still occurs.

Why does this happen?

When (change) is triggered, the checked property updates internally, but the UI does not reflect the new state unless explicitly updated.

The Fix

To ensure the checkbox state stays in sync, modify the event handler to explicitly set event.source.checked:

public onFeatureEnabledChange(event: MatCheckboxChange) {
    const checked = event.checked;
    this.store.dispatch(updateFeatureEnabled({ featureEnabled: checked }));
    event.source.checked = this.featureEnabled();
}

Explanation

• event.checked captures the checkbox's new state.
• updateFeatureEnabled dispatches an action to update the store.
• event.source.checked = this.featureEnabled(); ensures the UI reflects the updated signal value.

Conclusion

If you're handling checkboxes with Angular signals and you encounter the issue of the checkbox not updating correctly after failing from the state, make sure to explicitly update the UI state after a change event. This small fix prevents state inconsistencies and ensures the correct behavior in your application.

Have you encountered similar issues or know of other solutions? Please let everyone know in the comments