Dart AppLifcycleState.didChangeLifecycleState( ) 函数在应用程序进入前台或后台时不被调用

当我打开应用程序或在后台运行时，不会调用didChangeAppLifecycleState()，也不会执行print语句.

   import 'package:flutter/widgets.dart';
   import 'package:flutter/material.dart';


   void main(){
    runApp(
    MaterialApp(
      home: Home(),
    )
   );
   }


   class Home extends StatefulWidget {
   @override
    _HomeState createState() => _HomeState();
   }

   class _HomeState extends State<Home> with WidgetsBindingObserver{
   AppLifecycleState state;
   @override
   void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
   }
  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    super.dispose();
  }

  void didChangeAppLifeCycleState(AppLifecycleState appLifecycleState) {
    state = appLifecycleState;
    print(appLifecycleState);
    print(":::::::");
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(
        child: Center(
          child:Text("hi")
        ),
      ),
    );
  }
}
}

didChangeAppLifeCycleState()中的打印语句未执行.