Access page child component in app.component.ts

#1

Case:
handle hardware back button in app.component
in some pages, there’s
If exist custom back button, call “back” function inside that component
Else, use pop in nav controller if stack length > 1

Idea:

  1. Tried ViewChild in app.component; however, no reference found

  2. Look into (ionic-angular)Nav.last().getContent()
    Can see element in ion-content but not those in ion-header/ion-navbar

  3. Look into current view controller
    But no sign of child component of the page

Question:
In app.component, how to access current page element?

Thanks all