1. ngOnChanges is called before ngOnInit but after constructor()
2. ngOnChanges is called because of @Input() obj, obj‘s reference changes. If you mutate one the object‘s prop, it won‘t trigger ngOnChanges, you have to create new object.
[Angular] Two things about OnChanges Lifecycle hook
原文：https://www.cnblogs.com/Answer1215/p/10335507.html