I'm using angular 7 and I have a form with two input fields, while the first one is always required, the second one should be required only if a checkbox is checked.
I'm trying to use a FormGroup with a custom validator:
<form [formGroup]="exampleForm">
<mat-form-field>
<input matInput placeholder="first" formControlName="first">
</mat-form-field>
<mat-checkbox [(ngModel)]=" checked" [ngModelOptions]="{standalone:true}">Make second input field required</mat-checkbox>
<mat-form-field>
<input matInput placeholder="second" formControlName="second">
</mat-form-field>
</form>
exampleForm: FormGroup;
checked: boolean;
ngOnInit() {
this.exampleForm = new FormGroup({
'second': new FormControl('', [this.validateIfChecked()]),
'first': new FormControl('example', [Validators.required])
});
}
validateIfChecked(): ValidatorFn {
return (control: AbstractControl): {
[key: string]: any
} | null => {
if (this.checked) {
return control.value ? null : {
'err': true
};
}
return null;
}
}
The problem is that the validation is performed only when the text in the two input fields is updated, while if I check/uncheck the checkbox the state doesn't change and to force the validation I have to change the text in the second textbox.
Here you can see an example on stackblitz: if you check the checkbox, the status doesn't change.
How can can I force the validation when the checkbox status changes?
You need to use crossfield validation. Include the checkbox in your formgroup
<form [formGroup]="exampleForm">
<mat-form-field>
<input matInput placeholder="first" formControlName="first">
</mat-form-field>
<mat-checkbox formControlName="checked">Make second input field required</mat-checkbox>
<mat-form-field>
<input matInput placeholder="second" formControlName="second">
</mat-form-field>
</form>
ngOnInit() {
this.exampleForm = new FormGroup({
'second': new FormControl(''),
'checked': new FormControl(false),
'first': new FormControl('example')
}, [this.validateIfChecked()]);
}
validateIfChecked(): ValidatorFn {
return (form: FormGroup): ValidationErrors | null => {
const checked = form.get('checked');
const second= form.get('second');
if (checked && !second) {
return {
'err': true
};
}
return null;
}
}
In this case, if 'checked' is true then 'second' is required
if in doubt https://angular.io/guide/form-validation#cross-field-validation
If you actually don't want to include the checkbox value to your form, you can make a separate form control, which is not included in your form. Based on the checkbox value you can clear validators or add required validator:
checked = new FormControl(false);
// ...
this.checked.valueChanges.subscribe((bool: boolean) => {
bool ? this.exampleForm.get('second').setValidators(Validators.required) :
this.exampleForm.get('second').clearValidators();
this.exampleForm.get('second').updateValueAndValidity();
});
And the relevant template:
<mat-checkbox [formControl]="checked">Make second input field required</mat-checkbox>
Your forked StackBlitz
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