ngOnChanges Called after a bound input property changes
ngOnInit Called once the component is initialized
ngDoCheck  Called during every change detection run
ngAfterContentInit  Called after content (ng-content) has been projected into view
ngAfterContentChecked Called every time the projected content has been checked
ngAfterViewInit Called after the component’s view (and child views) has been initialized
ngAfterViewChecked Called every time the view (and child views) have been checked
ngOnDestroy Called once the component is about to be destroyed


  ngOnChanges(changes: SimpleChanges) {

   }
   previousValue: any	Declared in constructor.
 currentValue: any	Declared in constructor.
 firstChange: boolean