In this tutorial we are going to learn how we can also implement custom form field validation in Angular 2 template driven forms, by creating our own custom validation directive. We are going to see how to write such directive and how its a best practive to extract the validation function to a separate function, so that it can also be used for model driven validation.

We are going to learn how we can configure the template driven custom validator directive into the Angular 2 Forms mechanism, by plugging the directive into the dependency injection system using NG_VALIDATORS and forwardRef.

import {Validator, NG_VALIDATORS, FormControl} from "@angular/forms";import {validateDuration} from "./validateDuration";import {Directive, forwardRef} from "@angular/core";export const MIN_LENGTH_VALIDATOR = {  provide: NG_VALIDATORS,  multi: true,  useExisting: forwardRef(() => DurationValidator)};// target and duration with ngModel@Directive({  selector: ‘[duration][ngModel]‘,  providers: [MIN_LENGTH_VALIDATOR]})export class DurationValidator implements Validator {  validate(c: FormControl): {[key: string]:any} {    return validateDuration(c);  }}

use:

 Duration: <input type="number" name="duration" [(ngModel)]="duration" duration required>

[Angular2 Form] Angular 2 Template Driven Form Custom Validator