Blog: $watch in Angular2 (https://thingsofgeek.com/2016/07/04/watch-with-angular2.html)

example.component.ts

import { Component, Input } from '@angular/core';

@Component({
  selector: 'example-component',
})
export class ExampleComponent {

  public internalVal = null;

  constructor() {}
  
  @Input('externalVal')
  set updateInternalVal(externalVal) {
    this.internalVal = externalVal;
  }
}

example.directive.js

var module = angular.module("myApp");

module.directive('exampleDirective', function () {
  return {
    template: '<div>{{internalVar}}</div>',
    scope: {
      externalVar: "="
    },
    controller: function(scope, element) {
      scope.$watch('externalVar', function(newVal, oldVal) {
        if (newVal !== oldVal) {
          scope.internalVar = newVal;
        }
      });
    }
});