Flutter Mobile Development · レッスン

StatelessとStatefulのウィジェット

StatelessWidgetとStatefulWidgetの基本的な違いを理解し、動的なUIの構築でそれぞれをいつ、どのように使うかを学びます。

「StatelessとStatefulのウィジェット」はCoddyKit上の無料Flutter Mobile Developmentレッスンです。これはレッスン1/4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。

Widgets: The Building Blocks

Flutter UIs are built entirely from widgets. Think of them as LEGO bricks! Everything you see on screen – text, buttons, even layout containers — is a widget.

Widgets describe how your app's UI should look given its current configuration and state. There are two main types of widgets you'll encounter:

StatelessWidget
StatefulWidget

StatelessWidget: Static UI

A StatelessWidget is a widget that does not change its appearance or behavior over time. Once it's built, it stays the same.

It doesn't have any internal "state" to manage.
Its properties are immutable (cannot be changed after creation).
It's perfect for displaying static content like text, icons, or images that don't need to react to user input or data changes.

Simple StatelessWidget

Here's a basic StatelessWidget that displays a welcome message. Notice how its content is fixed.

Run this code to see a static "Hello, CoddyKit!" message.

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Stateless Demo'),
        ),
        body: const Center(
          child: Text(
            'Hello, CoddyKit!',
            style: TextStyle(fontSize: 24),
          ),
        ),
      ),
    );
  }
}

When to Go Stateless

Use a StatelessWidget when:

The widget's appearance depends only on its own configuration parameters (passed in its constructor).
It doesn't need to respond to user interactions (like taps) or internal data changes.
Examples: Text, Icon, Image, AppBar, Padding, Row, Column.

These widgets are efficient because Flutter doesn't need to rebuild them if their parent changes, unless their properties explicitly change.

StatefulWidget: Dynamic UI

A StatefulWidget is a widget that can change its appearance and behavior over time, reacting to user input or data changes.

It has internal "state" that can be modified.
When its state changes, the widget rebuilds to reflect the new state.
It's perfect for interactive elements like checkboxes, sliders, or dynamic lists.

Updating UI with setState()

To make a StatefulWidget dynamic, you modify its internal state. When the state changes, you must call the setState() method.

Calling setState() tells Flutter that the internal state of this widget has changed, and it needs to rebuild the UI to reflect the new state.

Without setState(), your state might change, but the UI won't update!

Interactive StatefulWidget

Let's build a simple counter app using a StatefulWidget. Tap the button to increment the number.

Observe how setState() is used to update the _counter variable and rebuild the UI.

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Stateful Demo'),
        ),
        body: const Center(
          child: CounterWidget(),
        ),
      ),
    );
  }
}

class CounterWidget extends StatefulWidget {
  const CounterWidget({super.key});

  @override
  State<CounterWidget> createState() => _CounterWidgetState();
}

class _CounterWidgetState extends State<CounterWidget> {
  int _counter = 0; // This is the mutable state

  void _incrementCounter() {
    setState(() {
      _counter++; // Update state
    });
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        Text(
          'Count: $_counter',
          style: const TextStyle(fontSize: 32),
        ),
        const SizedBox(height: 20),
        ElevatedButton(
          onPressed: _incrementCounter,
          child: const Text('Increment'),
        ),
      ],
    );
  }
}

StatefulWidget's Journey

StatefulWidgets go through a lifecycle. Key moments include:

createState(): Creates the mutable state object.
initState(): Called once when the widget is inserted into the widget tree. Good for one-time setup.
build(): Called every time the widget needs to display its UI, typically after setState().
dispose(): Called when the widget is removed from the tree. Good for cleaning up resources.

Understanding these helps manage complex interactions.

Choosing the Right Widget

Here's a quick comparison to help you decide:

StatelessWidget:
- Immutable properties.
- Doesn't change after creation.
- No internal state.
- Efficient for static content.
StatefulWidget:
- Mutable state (managed by a `State` object).
- Can change dynamically.
- Responds to user interaction/data.
- Uses setState() to trigger rebuilds.

Most widgets start as stateless; only make them stateful if they truly need to change dynamically.

Widget Type Challenge

You are building a Flutter app. Which type of widget would you primarily use for a simple icon that, when tapped, changes its color?

Recap: Dynamic vs. Static

Great job! You've learned the fundamental difference between StatelessWidget and StatefulWidget.

StatelessWidgets are for static UI elements that don't change after they're built.
StatefulWidgets are for dynamic UI elements that need to react to interactions or data changes, using setState() to update their appearance.

Choosing the right widget type is crucial for building efficient and maintainable Flutter applications. In the next lesson, we'll dive into basic layout widgets!

よくある質問

「StatelessとStatefulのウィジェット」レッスンは無料ですか？

はい。「StatelessとStatefulのウィジェット」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。

「StatelessとStatefulのウィジェット」で何を学びますか？

StatelessWidgetとStatefulWidgetの基本的な違いを理解し、動的なUIの構築でそれぞれをいつ、どのように使うかを学びます。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1/4です。

「StatelessとStatefulのウィジェット」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。

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