# Customizing tabs alignment using the new TabBar.tabAlignment property

> Introducing the TabBar.tabAlignment property.




:::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

Using `TabBar.tabAlignment` to customize the alignment of tabs in a `TabBar`.

## Context

The `TabBar.tabAlignment` property sets where a Material 3 `TabBar` places tabs.
The `TabAlignment` enum has the following values:

* `TabAlignment.start`: Aligns the tabs to the start of the scrollable `TabBar`.
* `TabAlignment.startOffset`: Aligns the tabs to the start of the
   scrollable `TabBar` with an offset of `52.0` pixels.
* `TabAlignment.center`: Aligns the tabs to the center of the `TabBar`.
* `TabAlignment.fill`: Aligns the tabs to the start and stretches the tabs
   to fill the fixed `TabBar`.

The scrollable `TabBar` supports the following alignments:

* `TabAlignment.start`
* `TabAlignment.startOffset`
* `TabAlignment.center`

The fixed `TabBar` supports the following alignments:

* `TabAlignment.fill`
* `TabAlignment.center`

When you set `ThemeData.useMaterial3` to `true`,
a scrollable `TabBar` aligns tabs as `TabAlignment.startOffset` by default.
To change this alignment, set the
`TabBar.tabAlignment` property for widget level customization.
Or, set the `TabBarThemeData.tabAlignment` property for app level customization.

## Description of change

When you set `TabBar.isScrollable` and `ThemeData.useMaterial3` to `true`,
the tabs in a scrollable `TabBar` defaults to `TabAlignment.startOffset`.
This aligns the tabs to the start of the
scrollable `TabBar` with an offset of `52.0` pixels.
This changes the previous behavior.
The tabs were aligned to the start of the scrollable `TabBar`
when more tabs needed to display than the width allowed.

## Migration guide

A Material 3 scrollable `TabBar` uses `TabAlignment.startOffset` as
the default tab alignment.
This aligns the tabs to the start of the
scrollable `TabBar` with an offset of `52.0` pixels.

To align the tabs to the start of the
scrollable `TabBar`, set `TabBar.tabAlignment` to `TabAlignment.start`.
This change also removed the `52.0` pixel offset.
The following code snippets show how to use `TabBar.tabAlignment` to
align tabs to the start of the scrollable `TabBar`:

Code before migration:

```dart
TabBar(
  isScrollable: true,
  tabs: List<Tab>.generate(
    count,
    (int index) => Tab(text: 'Tab $index'),
  ).toList(),
);
```

Code after migration:

```dart
TabBar(
  tabAlignment: TabAlignment.start,
  isScrollable: true,
  tabs: List<Tab>.generate(
    count,
    (int index) => Tab(text: 'Tab $index'),
  ).toList(),
);
```

## Timeline

Landed in version: 3.13.0-17.0.pre<br>
In stable release: 3.16

## References

API documentation:

* [`TabBar`][]
* [`TabBar.tabAlignment`][]
* [`TabAlignment`][]

Relevant PRs:

* [Introduce `TabBar.tabAlignment`][]
* [Fix Material 3 Scrollable `TabBar`][]

[`TabBar`]: https://api.flutter-io.cn/flutter/material/TabBar-class.html
[`TabBar.tabAlignment`]: https://api.flutter-io.cn/flutter/material/TabBar/tabAlignment.html
[`TabAlignment`]: https://api.flutter-io.cn/flutter/material/TabAlignment.html

[Introduce `TabBar.tabAlignment`]: https://github.com/flutter/flutter/pull/125036
[Fix Material 3 Scrollable `TabBar`]: https://github.com/flutter/flutter/pull/131409