# Container with color optimization

> A container with a color and no other background decoration no longer builds the same child widgets.




:::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.

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


## Summary

A new `ColoredBox` widget has been added to the framework,
and the `Container` widget has been optimized to use it
if a user specifies a `color` instead of a `decoration`.

## Context

It is very common to use the `Container` widget as follows:

```dart
return Container(color: Colors.red);
```

Previously, this code resulted in a widget hierarchy that used a
`BoxDecoration` to actually paint the background color.
The `BoxDecoration` widget covers many cases other than
just painting a background color,
and is not as efficient as the new `ColoredBox` widget,
which only paints a background color.

Widget tests that wanted to assert based on the color
of a container in the widget tree would previously have
to find the `BoxDecoration` to actually get
the color of the container.
Now, they are able to check the `color` property
on the `Container` itself, unless a `BoxDecoration`
was explicitly provided as the `decoration` property.
It is still an error to supply both `color` and
`decoration` to `Container`.

## Migration guide

Tests that assert on the color of a `Container`
or that expected it to create a
`BoxDecoration` need to be modified.

Code before migration:

```dart
testWidgets('Container color', (WidgetTester tester) async {
  await tester.pumpWidget(Container(color: Colors.red));

  final Container container = tester.widgetList<Container>().first;
  expect(container.decoration.color, Colors.red);
  // Or, a test may have specifically looked for the BoxDecoration, e.g.:
  expect(find.byType(BoxDecoration), findsOneWidget);
});
```

Code after migration:

```dart
testWidgets('Container color', (WidgetTester tester) async {
  await tester.pumpWidget(Container(color: Colors.red));

  final Container container = tester.widgetList<Container>().first;
  expect(container.color, Colors.red);
  // If your test needed to work directly with the BoxDecoration, it should
  // instead look for the ColoredBox, e.g.:
  expect(find.byType(BoxDecoration), findsNothing);
  expect(find.byType(ColoredBox), findsOneWidget);
});
```

## Timeline

Landed in version: 1.15.4<br>
In stable release: 1.17

## References

API documentation:

* [`Container`][]
* [`ColoredBox`][]
* [`BoxDecoration`][]

Relevant issues:

* [Issue 9672][]
* [Issue 28753][]

Relevant PR:

* [Colored box and container optimization #50979][]

[`Container`]: https://api.flutter.dev/flutter/widgets/Container-class.html
[`ColoredBox`]: https://api.flutter.dev/flutter/widgets/ColoredBox-class.html
[`BoxDecoration`]: https://api.flutter.dev/flutter/painting/BoxDecoration-class.html
[Issue 9672]: https://github.com/flutter/flutter/issues/9672
[Issue 28753]: https://github.com/flutter/flutter/issues/28753
[Colored box and container optimization #50979]: https://github.com/flutter/flutter/pull/50979