Update a var from app.component.ts

Hi all, I have home.ts and app.component.ts

in home.ts I have like following

testVar = ‘’;
this.testVar = ‘1’;

then I put testVar on home.html, if the method updateTestVar is called from home.ts directly the UI can update testVar value.

But if I called the method from app.component.ts via the home ref, the method updateTestVar is called, but the UI does not update, anyone can help , thanks.

What does “the home ref” mean?