Flutter Mobile Development · レッスン

ブランドトークン向けカスタムThemeExtension

ThemeExtensionクラスを作成し、独自のデザイントークンを定義して利用します。

「ブランドトークン向けカスタムThemeExtension」はCoddyKit上の無料Flutter Mobile Developmentレッスンです。これはレッスン3/4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 Brand Tokens Need a Home

Material 3 gives you a rich ColorScheme, but real products carry extra design decisions that don't map to any built-in slot: a brand gradient, a success color, a promotional accent, a custom card radius.

Hardcoding these as global constants breaks down the moment you support light and dark modes, because a single constant can't change with the active ThemeData.

You want these values to live inside the theme.
You want them to interpolate smoothly during theme animations.
You want to read them with the same Theme.of(context) ergonomics as everything else.

Flutter's answer is ThemeExtension.

What a ThemeExtension Is

ThemeExtension<T> is an abstract class you subclass to attach your own typed bundle of values to a ThemeData.

The generic parameter T is your own class. This is what lets Flutter store and later retrieve your extension by its exact type instead of by a string key.

It is type-safe: no casting from a Map.
It is theme-aware: it ships inside ThemeData, so light and dark can hold different instances.
It supports animation: Flutter calls lerp to blend two instances when themes change.

You must implement two methods: copyWith and lerp.

Declaring the Extension Class

Start by subclassing ThemeExtension with your class as the type argument. Make every field final so instances are immutable.

Here we model a small brand palette: a promotional accent, a success color, and a brand gradient.

import 'package:flutter/material.dart';

class BrandColors extends ThemeExtension<BrandColors> {
  const BrandColors({
    required this.accent,
    required this.success,
    required this.brandGradient,
  });

  final Color accent;
  final Color success;
  final Gradient brandGradient;

  @override
  ThemeExtension<BrandColors> copyWith({
    Color? accent,
    Color? success,
    Gradient? brandGradient,
  }) {
    return BrandColors(
      accent: accent ?? this.accent,
      success: success ?? this.success,
      brandGradient: brandGradient ?? this.brandGradient,
    );
  }
}

Implementing copyWith

copyWith returns a new instance with some fields replaced. The pattern is always the same: each parameter is nullable, and you fall back to this.field when the caller passes null.

It keeps the class immutable — you never mutate, you clone with changes.
It is what consumers use to tweak a single token without rebuilding the whole object.

Note the return type is ThemeExtension<BrandColors>, matching the abstract signature, even though you construct a concrete BrandColors.

Implementing lerp for Smooth Transitions

lerp (linear interpolation) blends this toward another instance by a factor t between 0.0 and 1.0. Flutter calls it during theme animations so your custom tokens fade as smoothly as the built-in ones.

Use the static helpers each type provides: Color.lerp and Gradient.lerp. Guard against the other being a different extension type by returning this.

@override
ThemeExtension<BrandColors> lerp(
  covariant ThemeExtension<BrandColors>? other,
  double t,
) {
  if (other is! BrandColors) {
    return this;
  }
  return BrandColors(
    accent: Color.lerp(accent, other.accent, t)!,
    success: Color.lerp(success, other.success, t)!,
    brandGradient: Gradient.lerp(brandGradient, other.brandGradient, t)!,
  );
}

Defining Light and Dark Instances

Because the extension lives inside ThemeData, you create one instance tuned for light mode and another for dark mode. Expose them as static const (or static final when a value isn't const-constructible) on the class for easy reference.

This is exactly the win over global constants: the same token name resolves to different values depending on the active theme.

class BrandColors extends ThemeExtension<BrandColors> {
  // ...constructor, fields, copyWith, lerp as before...

  static const light = BrandColors(
    accent: Color(0xFFFF6D00),
    success: Color(0xFF2E7D32),
    brandGradient: LinearGradient(
      colors: [Color(0xFFFF6D00), Color(0xFFFFAB40)],
    ),
  );

  static const dark = BrandColors(
    accent: Color(0xFFFFAB40),
    success: Color(0xFF66BB6A),
    brandGradient: LinearGradient(
      colors: [Color(0xFFFFAB40), Color(0xFFFFD180)],
    ),
  );
}

Registering the Extension on ThemeData

Attach instances through the extensions parameter of ThemeData. It takes an Iterable of extensions; Flutter indexes them by runtime type.

Give your light ThemeData the light instance and your dark ThemeData the dark instance so they switch automatically with the platform brightness.

MaterialApp(
  theme: ThemeData(
    colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
    extensions: const [BrandColors.light],
  ),
  darkTheme: ThemeData(
    colorScheme: ColorScheme.fromSeed(
      seedColor: Colors.deepPurple,
      brightness: Brightness.dark,
    ),
    extensions: const [BrandColors.dark],
  ),
  home: const HomePage(),
);

Consuming the Extension in a Widget

Read your extension with Theme.of(context).extension<BrandColors>(). The generic type argument is the lookup key, returning a nullable BrandColors?.

Once you have the instance, every token is a plain typed field — no casts, full autocomplete.

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

  @override
  Widget build(BuildContext context) {
    final brand = Theme.of(context).extension<BrandColors>()!;
    return Container(
      decoration: BoxDecoration(
        gradient: brand.brandGradient,
        borderRadius: BorderRadius.circular(16),
      ),
      padding: const EdgeInsets.all(16),
      child: Text(
        'Limited offer',
        style: TextStyle(color: brand.accent),
      ),
    );
  }
}

A Clean Consumption Extension

Calling Theme.of(context).extension<BrandColors>()! everywhere is noisy. A common idiom is to add a small BuildContext extension that hides the lookup behind a getter.

It centralizes the non-null assertion in one place.
Call sites become a tidy context.brand.accent.

extension BrandThemeX on BuildContext {
  BrandColors get brand =>
      Theme.of(this).extension<BrandColors>()!;
}

// Usage inside any build method:
// final color = context.brand.success;

Pure-Dart Mental Model of lerp

You can reason about lerp without Flutter. Interpolation is just a + (b - a) * t applied per channel. The snippet below blends two integers the same way Color.lerp blends each ARGB channel.

Running this shows how t = 0 yields the start, t = 1 yields the end, and t = 0.5 yields the midpoint — the exact behavior your theme animation relies on.

int lerpInt(int a, int b, double t) {
  return (a + (b - a) * t).round();
}

void main() {
  const start = 0; // think: red channel of color A
  const end = 200; // red channel of color B
  for (final t in [0.0, 0.25, 0.5, 0.75, 1.0]) {
    print('t=$t -> ${lerpInt(start, end, t)}');
  }
}

Multiple Extensions and Common Pitfalls

You can register several extensions side by side — for example BrandColors and a separate BrandShapes for radii and spacing. Flutter keys each by its own type, so they never collide.

Forgetting to register: extension<BrandColors>() returns null if you never added it to ThemeData.extensions; the ! then throws.
Skipping lerp: if lerp just returns this, theme transitions snap instead of fading.
Wrong type argument: the type in extension<T>() must match the registered class exactly.

ThemeData(
  extensions: const [
    BrandColors.light,
    BrandShapes.standard,
  ],
);

// Look each one up independently:
// final brand = Theme.of(context).extension<BrandColors>()!;
// final shapes = Theme.of(context).extension<BrandShapes>()!;

Quick Check

You added BrandColors to your light and dark ThemeData and now animate between them. The brand accent color jumps abruptly instead of fading smoothly. Which mistake most likely causes this?

Recap

You now own a full custom theming workflow in Flutter:

Subclass ThemeExtension<T> with final fields for your brand tokens.
Implement copyWith (nullable params, fall back to this.field) and lerp (use Color.lerp, Gradient.lerp, guard the type).
Build distinct light and dark instances and register them via ThemeData.extensions.
Consume with Theme.of(context).extension<BrandColors>(), ideally wrapped in a context.brand getter.

The payoff: brand-specific design tokens that are type-safe, mode-aware, and animate smoothly — just like Material 3's built-in ColorScheme.

よくある質問

「ブランドトークン向けカスタムThemeExtension」レッスンは無料ですか？

はい。「ブランドトークン向けカスタムThemeExtension」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。

「ブランドトークン向けカスタムThemeExtension」で何を学びますか？

ThemeExtensionクラスを作成し、独自のデザイントークンを定義して利用します。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3/4です。

「ブランドトークン向けカスタムThemeExtension」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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