# IconData class marked as final

> The IconData class is now marked as final, preventing it from being extended or implemented.




:::important
These breaking change docs are accurate, as of the release
under which they are published. Over time, the
workarounds described here might become inaccurate.
We don't, in general, keep these breaking change docs up
to date as of each release.

这些破坏性改动文档的准确性仅限于其发布时对应的版本。
随着时间的推移，文档中描述的方案可能会逐渐失效。
通常情况下，我们不会在每次发布新版本时都对这些文档进行同步更新。

The [breaking change index file](/release/breaking-changes)
lists the docs created for each release.

[破坏性改动列表](/release/breaking-changes) 
列出了每个版本的文档。

:::


## Summary

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.

[`final`]: https://dart.cn/language/class-modifiers#final

## Background

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:

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

## Migration guide

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.

### Migrating custom icon types

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

Code before migration:

```dart
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:

```dart
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.

### Ignore the `mustBeConst` lint

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.

```dart
// ignore: non_const_argument_for_const_parameter
Icon(myDynamicIconData);
```

## Timeline

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

## References

Relevant PR:

* [Mark `IconData` `final` and `@mustBeConst`][pr-181345]

Relevant issues:

* [Breaking Change: Marking `class IconData` as `final`][issue-181342]
* [Marking `IconData`'s constructor parameters as `@mustBeConst`][issue-181344]

[pr-181345]: https://github.com/flutter/flutter/pull/181345
[issue-181342]: https://github.com/flutter/flutter/issues/181342
[issue-181344]: https://github.com/flutter/flutter/issues/181344