# Web-specific golden comparisons are no longer supported

> The `WebGoldenComparator` class and `webGoldenComparator` top-level instance are deprecated, pending removal, and the web rendering backends (including both CanvasKit and skWasm) now use `GoldenFileComparator` and `goldenFileComparator` directly.




:::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 `flutter_test` package and `flutter` tool will no longer use the
[`webGoldenComparator`][] top-level variable, and instead use the original
[`goldenFileComparator`][] top-level variable (like the non-web platforms).

For _users_ of `flutter_test`, these changes will be made automatically.

## Background

Originally, [`WebGoldenComparator`][class-WebGoldenComparator] was added for
the HTML-backend of Flutter web, as it was not possible to create an encoded
PNG (byte buffer), and a new API was needed. As [the HTML backend is being
deprecated and removed][Issue 145954], this separate API is no longer necessary.

## Migration guide

For most users, no changes are required (other than migrating off the HTML
backend, which is not covered here), the `flutter` tool will automatically
configure [`goldenFileComparator`][] and use it (when using a non-HTML web
backend).

For users that implement a custom [`WebGoldenComparator`][], you will
migrate the implementation to [`GoldenFileComparator`][]. Fortunately the
Canvas Kit and SkWasm backends already required similar methods (`compareButes`
and `updateBytes`).

For example:

```dart
// Before
class MyWebGoldenComparator extends WebGoldenComparator {
  @override
  Future<bool> compare(double width, double height, Uri golden) {
    // will be removed in the migration
  }

  @override
  Future<bool> update(double width, double height, Uri golden) {
    // will be removed in the migration
  }

  @override
  Future<bool> compareBytes(Uint8List bytes, Uri golden) {
    // will be renamed "compare"
  }

  @override
  Future<bool> updateBytes(Uint8List bytes, Uri golden) {
    // will be renamed "update" and the parameter orders swapped
  }
}

// After
class MyGenericGoldenComparator extends GoldenFileComparator {
  @override
  Future<bool> compare(Uint8List bytes, Uri golden) {
    // used to be "compareBytes"
  }

  @override
  Future<bool> update(Uri golden, Uint8List bytes) {
    // used to be "updateBytes"
  }
}
```

## Timeline

Landed in version: 3.29.0-0.0.pre<br>
In stable release: 3.29

## References

Relevant Issues:

- [Issue 145954][], where the HTML renderer was deprecated.
- [Issue 160261][], where it was proposed to consolidate `GoldenFileComparator` and `WebGoldenComparator`.

Relevant PRs:

- [PR 161196][], where `WebGoldenComparator` was deprecated and the `flutter` CLI started using `goldenFileComparator`.

[Issue 145954]: https://github.com/flutter/flutter/issues/145954
[Issue 160261]: https://github.com/flutter/flutter/issues/160261
[PR 161196]: https://github.com/flutter/flutter/pull/161196
[class-WebGoldenComparator]: https://api.flutter-io.cn/flutter/flutter_test/WebGoldenComparator-class.html
[`webGoldenComparator`]: https://api.flutter-io.cn/flutter/flutter_test/webGoldenComparator.html
[`goldenFileComparator`]: https://api.flutter-io.cn/flutter/flutter_test/goldenFileComparator.html