title: Angular 2 watch component property for changes without @Input or @Output
body: <p>In Angular 1.x I could use .$watch to watch anything I wanted, however in Angular 2 we have ngOnChanges which is really cool and performant but only watches input and output decorators. However from time to time I really need to watch local properties so that I can emit an event when they change. I remember seeing an article explaining how you can explicitly tell Angular 2 to watch a single property but for the life of me I can't find the article anymore and every search I do just tells me to use the input/output decorators with onChanges. How can I create a watcher for a @ViewChild or other local properties?</p>
  
  <p>For example I have a ngForm, that when it's validation state changes from valid to invalid or vise versa I want to emit the state to another component so it can react. However since my ngForm is a @ViewChild and not a @Input or @Output ngOnChanges doesn't detect the change.</p>