Why is it good practice to use `const` constructors in Flutter widgets?

A widget created with const is built only once and Flutter reuses it (canonicalization), instead of recreating it on every build. Also, when the parent widget rebuilds, Flutter can skip rebuilding a const subtree because it knows it didn't change. This reduces work and improves performance.

In Flutter, build is called a lot. Every time, new widgets are created. If a widget doesn't depend on any data that changes, recreating it over and over is a waste.

When you mark a widget as const:

• A single instance is created that Flutter always reuses (two const Text('Hi') are literally the same object in memory).
• When rebuilding the parent, Flutter compares and detects that the const widget is identical to the previous one, so it doesn't rebuild that subtree.

It's one of the easiest and highest-impact optimizations: basically "free", you just have to write const.

import 'package:flutter/material.dart';

class Screen extends StatelessWidget {
  const Screen({super.key});

  @override
  Widget build(BuildContext context) {
    return Column(
      children: const [
        // These widgets never change: mark them const.
        // Flutter creates them once and reuses them on every build.
        Text("Fixed title"),
        SizedBox(height: 8),
        Icon(Icons.star),
      ],
    );
  }
}

If Screen rebuilds 100 times, those three const widgets aren't recreated: the same instance is reused.

Not being able to use const because an intermediate value doesn't allow it, and not noticing. It only takes a single child depending on a runtime variable for the parent's const to become impossible. It's also common to forget const entirely: Flutter's linter (prefer_const_constructors) usually warns you, and it's worth listening to it.

"A const constructor makes the widget get created once and lets Flutter reuse it, instead of recreating it on every build. Also, when the parent rebuilds, Flutter detects that a const subtree didn't change and skips rebuilding it. It's an almost-free optimization: I just write const on the widgets that don't depend on changing data. That's why Flutter's linter insists so much on prefer_const_constructors."