Flutter Mobile Development · レッスン

セマンティクス、スクリーンリーダー、アクセシブルウィジェット

Semantics APIでウィジェットに情報を付加し、TalkBackやVoiceOverのユーザーを支援します。

「セマンティクス、スクリーンリーダー、アクセシブルウィジェット」はCoddyKit上の無料Flutter Mobile Developmentレッスンです。これはレッスン4/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはFlutter Mobile Development学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Flutter Mobile Developmentコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

Why Accessibility Matters

Millions of people rely on screen readers to use mobile apps. On Android the screen reader is TalkBack; on iOS it is VoiceOver. When enabled, the user swipes between elements and the phone speaks a description of each one out loud.

Flutter does not draw native widgets, so the OS cannot inspect your UI directly. Instead, Flutter builds a parallel semantics tree describing each element (its label, role, and state) and hands it to the platform's accessibility services.

Good news: many built-in widgets (Text, ElevatedButton, Checkbox) already populate this tree automatically.
Your job: fix the cases where the automatic description is missing, wrong, or confusing.

The Semantics Tree

Every visible widget can contribute a SemanticsNode to a tree that mirrors your widget tree. Each node carries properties such as label, value, hint, and flags like isButton or isChecked.

You rarely build this tree by hand. Instead you annotate widgets using the Semantics widget or the convenience properties exposed by existing widgets (for example Image's semanticLabel).

The screen reader reads nodes in roughly top-to-bottom, left-to-right order.
You can merge several widgets into one spoken node, or exclude purely decorative widgets entirely.

The Semantics Widget

The core tool is the Semantics widget. Wrap any widget with it and supply the properties the screen reader should announce.

Here an icon-only button has no visible text, so without a label TalkBack would just say "button". Adding label makes it meaningful.

label — what the element is ("Delete item").
button: true — marks the node as tappable so the reader says "Delete item, button".

Semantics(
  label: 'Delete item',
  button: true,
  child: IconButton(
    icon: const Icon(Icons.delete),
    onPressed: _deleteItem,
  ),
)

Labeling Images and Icons

Images and icons are the most common accessibility gap: visually they convey meaning, but they carry no text. Provide a semanticLabel so the description is spoken.

If an image is purely decorative (adds no information), set excludeFromSemantics: true so the reader skips it instead of announcing a filename or "image".

Column(
  children: [
    // Meaningful image: describe it
    Image.asset(
      'assets/profile.png',
      semanticLabel: 'Profile photo of Ada Lovelace',
    ),
    // Decorative divider image: skip it
    Image.asset(
      'assets/flourish.png',
      excludeFromSemantics: true,
    ),
  ],
)

label, value, and hint

Three properties handle most situations, and they are not interchangeable:

label — the identity of the control ("Volume").
value — its current state ("70 percent").
hint — what happens on interaction ("Adjust to change volume").

A screen reader typically announces them as: label, value, hint. Keeping state in value (not baked into label) lets the reader re-announce only the changing part when the user interacts.

Semantics(
  label: 'Volume',
  value: '$volumePercent percent',
  hint: 'Swipe up or down to adjust',
  slider: true,
  child: VolumeSlider(value: volumePercent),
)

Merging Semantics

A card might contain an avatar, a name, and a subtitle as three separate widgets. By default the screen reader stops on each one, forcing three swipes for one logical item.

Wrap them in MergeSemantics to combine their descriptions into a single node. The reader then announces them together: "Ada Lovelace, Online".

Use merging when several widgets form one conceptual element. Do not merge unrelated controls, since merging also hides their individual interactions.

MergeSemantics(
  child: Row(
    children: const [
      CircleAvatar(child: Text('A')),
      SizedBox(width: 8),
      Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [
          Text('Ada Lovelace'),
          Text('Online'),
        ],
      ),
    ],
  ),
)

Excluding Decorative Widgets

Sometimes a widget produces noise: a redundant decorative icon next to text, or a background graphic. Wrap it in ExcludeSemantics to drop it (and its subtree) from the semantics tree.

Here the chevron icon is purely visual; the row's text already tells the user everything, so we exclude the icon to avoid "chevron right" clutter.

Row(
  children: [
    const Text('Account settings'),
    const Spacer(),
    ExcludeSemantics(
      child: const Icon(Icons.chevron_right),
    ),
  ],
)

Live Announcements

Some changes are not tied to a focused widget — for example "Message sent" after a network call. To speak these immediately, use SemanticsService.announce.

Pass the message and the current TextDirection (so right-to-left languages are handled correctly). Use this sparingly; too many announcements overwhelm the user.

import 'package:flutter/semantics.dart';

Future<void> _onSendComplete(BuildContext context) async {
  await SemanticsService.announce(
    'Message sent',
    Directionality.of(context),
  );
}

Touch Target Size

Accessibility is not only about screen readers. People with motor difficulties need targets large enough to tap reliably. The recommended minimum is roughly 48 x 48 logical pixels (matching Material's kMinInteractiveDimension).

Wrapping a small control with a tight SizedBox can shrink its hit area. Prefer IconButton (which enforces a minimum) or expand the target explicitly.

// Guarantee a comfortable tap target
ConstrainedBox(
  constraints: const BoxConstraints(
    minWidth: kMinInteractiveDimension,
    minHeight: kMinInteractiveDimension,
  ),
  child: InkWell(
    onTap: _toggleFavorite,
    child: const Icon(Icons.favorite_border),
  ),
)

Building Accessible Labels in Dart

Labels are just strings, so you often build them with plain Dart logic before passing them to Semantics. Keeping this logic in a pure function makes it easy to test without any UI.

The snippet below is standalone Dart: it formats a button's accessible label from its state. Notice how the label describes identity while a separate state word communicates the current value clearly.

String favoriteLabel({required bool isFavorited, required String title}) {
  final action = isFavorited ? 'Remove from favorites' : 'Add to favorites';
  return '$action: $title';
}

void main() {
  print(favoriteLabel(isFavorited: false, title: 'Dart Basics'));
  print(favoriteLabel(isFavorited: true, title: 'Dart Basics'));
}

Testing Your Semantics

You do not have to guess what the screen reader will say. Flutter offers several ways to verify:

Turn on TalkBack or VoiceOver on a real device and swipe through your screen.
Use the Accessibility Inspector in DevTools to view the live semantics tree.
Write widget tests with the SemanticsTester / matchesSemantics matcher to assert labels and flags.

Flutter also ships accessibility guideline checks you can run in tests, such as minimum tap-target size and text contrast.

testWidgets('delete button is labeled', (tester) async {
  await tester.pumpWidget(const MyApp());
  expect(
    tester.getSemantics(find.byIcon(Icons.delete)),
    matchesSemantics(label: 'Delete item', isButton: true),
  );
});

Quick Check

You have an icon-only IconButton showing a trash can. With TalkBack enabled, users only hear "button" and cannot tell what it does. What is the most appropriate fix?

Recap

You learned how to make Flutter widgets usable with TalkBack and VoiceOver:

Flutter builds a semantics tree from your widgets; built-ins fill it in, but icons and images often need help.
Use the Semantics widget with label (identity), value (state), and hint (interaction).
Give images a semanticLabel, or set excludeFromSemantics: true when decorative.
MergeSemantics combines several widgets into one spoken node; ExcludeSemantics removes noise.
Announce dynamic events with SemanticsService.announce, and keep tap targets at least 48 x 48.
Verify with TalkBack/VoiceOver, the DevTools Accessibility Inspector, and matchesSemantics in widget tests.

Accessible apps reach more users and are simply better engineered.

よくある質問

「セマンティクス、スクリーンリーダー、アクセシブルウィジェット」レッスンは無料ですか？

はい。「セマンティクス、スクリーンリーダー、アクセシブルウィジェット」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Flutter Mobile Developmentコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Flutter Mobile Developmentコースには全4レッスンが含まれています。

「セマンティクス、スクリーンリーダー、アクセシブルウィジェット」で何を学びますか？

Semantics APIでウィジェットに情報を付加し、TalkBackやVoiceOverのユーザーを支援します。ブラウザで直接実行するハンズオンコードでFlutter Mobile Developmentを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Flutter Mobile Developmentを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのFlutter Mobile Developmentは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。これはレッスン4/4です。

「セマンティクス、スクリーンリーダー、アクセシブルウィジェット」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このFlutter Mobile Developmentレッスンでコードを書いて実行できますか？

はい。すべてのFlutter Mobile Developmentレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

このコースのすべてのレッスン

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