Using const widgets helps to avoid unnecessary rebuilds because the widgets are only built once and reused when the state changes.

import 'package:flutter/material.dart';
class MyWidget extends StatefulWidget {
 @override
 _MyWidgetState createState() => _MyWidgetState();
}
class _MyWidgetState extends State<MyWidget> {
 bool _isSelected = false;
 @override
 Widget build(BuildContext context) {
 return Column(
      children: [
 const SizedBox(height: 20), // const widget
 const Text('Click the button to change the state'), // const widget
 const SizedBox(height: 20), // const widget
 ElevatedButton(
          key: UniqueKey(), // unique key
          onPressed: () {
            setState(() {
              _isSelected = !_isSelected;
 });
 },
          child: const Text('Toggle State'),
 ),
 const SizedBox(height: 20), // const widget
 AnimatedBuilder(
          animation: _isSelected
 ? const AlwaysStoppedAnimation(1)
 : const AlwaysStoppedAnimation(0),
          builder: (context, child) {
 return Opacity(
              opacity: _isSelected ? 1 : 0, // shouldRebuild
              child: Container(
                color: Colors.red,
                height: 100,
                width: 100,
 ),
 );
 },
 ),
 ],
 );
 }
}

With this technique you can help to reduce widget rebuilds in your Flutter app and improve its performance.