const Constructors and Keys

Helping the framework match widgets

Marking a widget const (when all its arguments are constants) creates a single canonical instance; Flutter can see it is identical to the previous one and skip rebuilding that subtree. Keys control identity: by default Flutter matches old and new widgets in a list by type and position. When items are reordered, inserted or removed, stateful children can end up attached to the wrong data. Giving each item a ValueKey(item.id) (or ObjectKey) lets Flutter match by identity instead. A GlobalKey is unique across the app and gives access to a widget's State or context, for example GlobalKey<FormState>; use it sparingly because it is more expensive.

Keys in a reorderable list

Stable identity for stateful rows.

class TodoList extends StatelessWidget {
  const TodoList({super.key, required this.todos});
  final List<Todo> todos;

  @override
  Widget build(BuildContext context) {
    return ListView(
      children: [
        for (final todo in todos)
          TodoTile(key: ValueKey(todo.id), todo: todo), // match by id, not position
        const Divider(), // const: identical instance every build
      ],
    );
  }
}

Let the linter add const

The prefer_const_constructors and related lints (in package:flutter_lints) point out where const can be added; dart fix --apply can add many automatically.

त्वरित जाँच: Why add a ValueKey to items in a list that can be reordered?

• Keys are required on every widget
• To make the list scroll faster
• To translate the items
• So Flutter matches each widget and its state to the right item by identity