Flutter - Why is child widget's initState() is not called on every rebuild of Parent widget?

Question

I have a Parent widget, which holds some state, a counter in this case.

This state is passed on to a Child widget using its constructor.

Now, how I understand it, the Child should get re-built every-time Parent's state changes, since it's inside the build() function of Parent, and build() gets called every-time the state changes.

This concept lead me to believe that the INIT STATE! message would be printed every time the counter changes. But it is not the case!

I essentially want a "hook" that gets fired only once, whenever the Child's constructor arguments (message) change.

Can someone please explain why this is the case, and what is the right way to have the aforementioned "hook"?

class Child extends StatefulWidget {   final String message;    const Child(this.message, {Key key}) : super(key: key);    @override   _ChildState createState() => _ChildState(); }  class _ChildState extends State<Child> {   @override   void initState() {     print("INIT STATE!");     super.initState();   }    @override   Widget build(BuildContext context) {     return Center(       child: Text(widget.message),     );   } }  class Parent extends StatefulWidget {   @override   _ParentState createState() => _ParentState(); }  class _ParentState extends State<Parent> {   int _counter = 0;    @override   void initState() {     Timer.periodic(Duration(seconds: 1), (_) {       if (mounted) {         setState(() {           _counter += 1;         });       }     });     super.initState();   }    @override   Widget build(BuildContext context) {     return Child(_counter.toString());   } } 

Answer 1

Ok, looks like this was clearly mentioned in the State class' docs.

The correct way to do this is to override the didUpdateWidget method in the State subclass.