Angular validation directive for template-driven forms

greaterThanValidator.directive.ts

import { Directive, Input, OnChanges, SimpleChanges } from '@angular/core';
import { NG_VALIDATORS, Validator, AbstractControl, ValidationErrors } from '@angular/forms';

@Directive({
	selector: '[greaterThan]',
	providers: [
		{
			provide: NG_VALIDATORS,
			useExisting: GreaterThanValidator,
			multi: true,
		},
	],
})
export class GreaterThanValidator implements Validator, OnChanges {
	@Input() public greaterThan: number;

	private _onChange: () => void;

	validate(control: AbstractControl): ValidationErrors | null {
		if (this.greaterThan <= control.value) {
			return null;
		}
		return { 'greaterThan': true };
	}

	registerOnValidatorChange(fn: () => void): void {
		this._onChange = fn;
	}

	ngOnChanges(changes: SimpleChanges): void {
		if ('greaterThan' in changes) {
			if (this._onChange) {
				this._onChange();
			}
		}
	}
}