IconData class marked as final

The IconData class is now marked as final, which prevents it from being implemented or extended. This change is part of an effort to generalize the mechanism for tree-shaking assets and native code.

The Flutter team is working on a generalized mechanism to bring tree-shaking of assets and native code to packages. The existing bespoke Icon Tree Shaker is being folded into this general mechanism.

For performance, locality, and understandability, the general mechanism doesn't support recording const instances in complex type hierarchies. Therefore, the IconData class is now marked as final.

Code that implements or extends IconData now fails to compile with the following error:

The class 'IconData' is 'final' and can't be extended or implemented outside of its library.

Instead of implementing IconData, such as with an enum that supports dot shorthand, type safety, and an automated .values list, use a wrapper class with static const instances.

If you used an enum that implements IconData, migrate to a class with static const instances and a custom widget.

Code before migration:

enum AppIcons implements IconData {
  arrowUpward(0xe062),
  arrowDownward(0xe061);
​
  const AppIcons(this.codePoint)
    : fontFamily = 'MaterialIcons',
      fontPackage = null,
      matchTextDirection = false;
​
  @override
  final int codePoint;
  @override
  final String? fontFamily;
  @override
  final String? fontPackage;
  @override
  final bool matchTextDirection;
}
​
Widget build(BuildContext context) {
  // Example usage of AppIcons:
  return Icon(AppIcons.arrowUpward);
}

To maintain dot shorthand support and type safety, use a wrapper class and a custom widget.

Code after migration:

final class AppIconData {
  final IconData iconData;
​
  const AppIconData._(this.iconData);
​
  static const arrowUpward = AppIconData._(
    IconData(0xe062, fontFamily: 'MaterialIcons'),
  );
  static const arrowDownward = AppIconData._(
    IconData(0xe061, fontFamily: 'MaterialIcons'),
  );
​
  static const values = [arrowUpward, arrowDownward];
}
​
class AppIcon extends StatelessWidget {
  const AppIcon(this.icon, {super.key});
  final AppIconData icon;
​
  @override
  Widget build(BuildContext context) {
    return Icon(icon.iconData);
  }
}
​
Widget build(BuildContext context) {
  // Usage preserves dot shorthand if the type can be inferred:
  return const AppIcon(AppIconData.arrowUpward);
  // Or if inferred: const AppIcon(.arrowUpward)
}

If you rely on .values for tools like Widgetbook, you can maintain the values list manually as shown in the previous example or use code generation.

To enable tree-shaking, some IconData parameters are marked with the mustBeConst annotation. If you must use a non-const IconData and are willing to forgo tree-shaking for that icon, add an ignore comment for the lint.

// ignore: non_const_argument_for_const_parameter
Icon(myDynamicIconData);

Landed in version: 3.44.0-0.1.pre
In stable release: 3.44

Relevant PR:

• Mark IconData final and @mustBeConst

Relevant issues:

• Breaking Change: Marking class IconData as final
• Marking IconData's constructor parameters as @mustBeConst