I think most easy way is using Subject. In bellow example code the child will be notified each time 'tellChild' is called.
Parent.component.ts
import {Subject} from 'rxjs/Subject';
...
export class ParentComp {
changingValue: Subject<boolean> = new Subject();
tellChild(){
this.changingValue.next(true);
}
}
Parent.component.html
<my-comp [changing]="changingValue"></my-comp>
Child.component.ts
...
export class ChildComp implements OnInit{
@Input() changing: Subject<boolean>;
ngOnInit(){
this.changing.subscribe(v => {
console.log('value is changing', v);
});
}
Working sample on Stackblitz