How do you reduce widget rebuild?

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.