Conditional required validator directive in Angular 2

Question

I need to make certain form fields required or not based on the value of other fields. The built-in RequiredValidator directive doesn't seem to support this, so I created my own directive:

@Directive({
  selector: '[myRequired][ngControl]',
  providers: [new Provider(NG_VALIDATORS, { useExisting: forwardRef(() => MyRequiredValidator), multi: true })]
})
class MyRequiredValidator {
  @Input('myRequired') required: boolean;

  validate(control: AbstractControl): { [key: string]: any } {
    return this.required && !control.value
      ? { myRequired: true }
      : null;
  }
}

Sample usage:

<form>
  <p><label><input type="checkbox" [(ngModel)]="isNameRequired"> Is Name Required?</label></p>
  <p><label>Name: <input type="text" [myRequired]="isNameRequired" #nameControl="ngForm" ngControl="name" [(ngModel)]="name"></label></p>
  <p *ngIf="nameControl.control?.hasError('myRequired')">This field is required.</p>
</form>

This works fine if the user first toggles the check box and then types or erases text in the text box. However, if the user toggles the check box while the text box is blank, then the validation message doesn't update appropriately.

How can I modify MyRequiredValidator to trigger validation when its required property is changed?

Note: I'm looking for a solution that only involves changes to MyRequiredValidator. I want to avoid adding any logic to the App component.

Plunker: https://plnkr.co/edit/ExBdzh6nVHrcm51rQ5Fi?p=preview

Answer 1

I would use something like that:

@Directive({
  selector: '[myRequired][ngControl]',
  providers: [new Provider(NG_VALIDATORS, { useExisting: forwardRef(() => MyRequiredValidator), multi: true })]
})
class MyRequiredValidator {
  @Input('myRequired') required: boolean;

  ngOnChanges() {
    // Called when required is updated
    if (this.control) {
      this.control.updateValueAndValidity();
    }
  }

  validate(control: AbstractControl): { [key: string]: any } {
    this.control = control;
    return this.required && !control.value
      ? { myRequired: true }
      : null;
  }
}

See this plunkr: https://plnkr.co/edit/14jDdUj1rdzAaLEBaB9G?p=preview.