ngOnChanges gets called before ngOnInit and whenever a component's bound input is changed FROM THE PARENT COMPONENT. Remember that ngOnChanges is specific to bound inputs on the component. This means if you don't have any @Input properties on a child, ngOnChanges will never get called.

How often is ngOnChanges called?

If you need to initialize the directive based on those properties, set them when ngOnInit() runs. The ngOnChanges() method is your first opportunity to access those properties. Angular calls ngOnChanges() before ngOnInit() , but also many times after that. It only calls ngOnInit() once.

What triggers ngOnChanges?

ngOnChanges triggers following the modification of @Input bound class members. Data bound by the @Input() decorator come from an external source. When the external source alters that data in a detectable manner, it passes through the @Input property again. ... It also fires upon initialization of input data.