# Nullable CupertinoThemeData.brightness

> CupertinoThemeData.brightness is now nullable, and it returns the value specified by the user (defaults to null) as is.




:::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

[`CupertinoThemeData.brightness`] is now nullable.

## Context

[`CupertinoThemeData.brightness`][] is now used to
override `MediaQuery.platformBrightness` for Cupertino widgets.
Before this change, the [`CupertinoThemeData.brightness`][]
getter returned `Brightness.light` when it was set to null.

## Description of change

Previously [`CupertinoThemeData.brightness`][]
was implemented as a getter:

```dart
Brightness get brightness => _brightness ?? Brightness.light;
final Brightness _brightness;
```

It is now a stored property:

```dart
final Brightness brightness;
```

## Migration guide

Generally [`CupertinoThemeData.brightness`][]
is rarely useful outside of the Flutter framework.
To retrieve the brightness for Cupertino widgets,
now use [`CupertinoTheme.brightnessOf`][] instead.

With this change, it is now possible to override
`CupertinoThemeData.brightness` in a `CupertinoThemeData`
subclass to change the brightness override. For example:

```dart
class AlwaysDarkCupertinoThemeData extends CupertinoThemeData {
  Brightness brightness => Brightness.dark;
}
```

When a `CupertinoTheme` uses the above `CupertinoThemeData`,
dark mode is enabled for all its Cupertino descendants
that are affected by this `CupertinoTheme`.

## Timeline

Landed in version: 1.16.3<br>
In stable release: 1.17

## References

Design doc:

* [Make `CupertinoThemeData.brightness nullable`][]

API documentation:

* [`CupertinoThemeData.brightness`][]

Relevant issue:

* [Issue 47255][]

Relevant PR:

* [Let material `ThemeData` dictate brightness if `cupertinoOverrideTheme.brightness` is null][]


[`CupertinoTheme.brightnessOf`]: https://api.flutter-io.cn/flutter/cupertino/CupertinoTheme/brightnessOf.html
[`CupertinoThemeData.brightness`]: https://api.flutter-io.cn/flutter/cupertino/NoDefaultCupertinoThemeData/brightness.html
[Issue 47255]: https://github.com/flutter/flutter/issues/47255
[Let material `ThemeData` dictate brightness if `cupertinoOverrideTheme.brightness` is null]: https://github.com/flutter/flutter/pull/47249
[Make `CupertinoThemeData.brightness nullable`]: /go/nullable-cupertinothemedata-brightness