Flutter Mobile Development · レッスン

複数形、性別、ICUメッセージフォーマット

ICUメッセージ構文を使い、複数形、選択肢、パラメーター付き文字列を処理します。

「複数形、性別、ICUメッセージフォーマット」はCoddyKit上の無料Flutter Mobile Developmentレッスンです。これはレッスン2/4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 ICU Messages Matter

Translating an app is more than swapping words. Different languages pluralize, gender, and order words differently. A naive string like "$count items" reads wrong as "1 items", and many languages have several plural forms.

Flutter solves this with ICU message syntax (International Components for Unicode), the same standard used across the industry. You write one message with rules for plurals, select/gender, and placeholders, and the right form is chosen at runtime per locale.

plural — pick a form based on a number
select — pick a branch based on a string (e.g. gender)
placeholders — inject typed values like names, dates, numbers

The ARB File: Where Messages Live

Flutter's flutter_localizations + intl toolchain reads ARB files (Application Resource Bundle). Each locale has its own file: app_en.arb, app_tr.arb, etc.

An ARB entry has a key, a value (the ICU message), and an optional @key metadata object describing placeholders.

Keys become generated Dart getters/methods.
The @key block declares each placeholder's type and optional format.

{
  "appTitle": "My Shop",
  "@appTitle": {
    "description": "The title shown in the app bar"
  },
  "welcome": "Welcome, {name}!",
  "@welcome": {
    "placeholders": {
      "name": { "type": "String" }
    }
  }
}

Simple Placeholders

A placeholder is written in curly braces inside the message: {name}. The generated Dart code becomes a method whose parameter matches the placeholder name.

After enabling generate: true in pubspec.yaml and running flutter gen-l10n, you call it through AppLocalizations.

// app_en.arb
//   "welcome": "Welcome, {name}!"

import 'package:flutter/material.dart';
import 'package:flutter_gen/gen_l10n/app_localizations.dart';

Widget build(BuildContext context) {
  final l10n = AppLocalizations.of(context)!;
  return Text(l10n.welcome('Aylin')); // -> "Welcome, Aylin!"
}

Pluralization with ICU

The plural form picks a branch based on a number argument. Syntax: {count, plural, ...categories...}.

ICU plural categories are =0, =1, zero, one, two, few, many, other. The other branch is mandatory — it is the fallback when no other category matches. Use # to print the number itself.

=0, =1 match the exact value.
one, few, many are language-defined categories (English only uses one and other).

{
  "itemsInCart": "{count, plural, =0{Your cart is empty} =1{1 item in cart} other{# items in cart}}",
  "@itemsInCart": {
    "placeholders": {
      "count": { "type": "int" }
    }
  }
}

Calling a Plural Message

The generated method takes the count argument. The framework selects the correct branch for the active locale automatically.

Notice that English only ever needs one/other, but a locale like Polish or Arabic will use few/many in its own ARB file — your Dart call site does not change.

final l10n = AppLocalizations.of(context)!;

Text(l10n.itemsInCart(0));  // "Your cart is empty"
Text(l10n.itemsInCart(1));  // "1 item in cart"
Text(l10n.itemsInCart(7));  // "7 items in cart"

Why '=1' and 'one' Are Different

A common confusion: =1 is an exact value match, while one is a language plural category. They are not the same.

=1 matches only the literal number 1.
one matches whatever the locale's CLDR rules call the "one" category. In English that is just 1, but in some locales the one category also covers values like 21, 31, etc.

Best practice: provide other always; add one for natural singular/plural; use =0/=1 only when you need special wording ("empty", "no messages") that differs from the grammatical plural.

Gender and the 'select' Form

The select form branches on a string value rather than a number. The classic use is grammatical gender, which changes pronouns and verb agreement in many languages.

Syntax: {gender, select, male{...} female{...} other{...}}. Like plural, other is required as the fallback (and handles unknown/non-binary values gracefully).

{
  "sharedPhoto": "{gender, select, male{He shared a photo} female{She shared a photo} other{They shared a photo}}",
  "@sharedPhoto": {
    "placeholders": {
      "gender": { "type": "String" }
    }
  }
}

Demonstrating select() in Pure Dart

You don't need Flutter to understand the intl primitives. The Intl.select function mirrors what the generated code does: it maps a key to a branch with an other fallback.

This standalone program shows the selection logic, including the fallback when an unexpected value arrives.

import 'package:intl/intl.dart';

String sharedPhoto(String gender) {
  return Intl.select(gender, {
    'male': 'He shared a photo',
    'female': 'She shared a photo',
    'other': 'They shared a photo',
  }, name: 'sharedPhoto', args: [gender]);
}

void main() {
  print(sharedPhoto('male'));    // He shared a photo
  print(sharedPhoto('female'));  // She shared a photo
  print(sharedPhoto('unknown')); // They shared a photo (fallback)
}

Combining Gender and Plurals (Nesting)

ICU lets you nest a plural inside a select (or vice versa) to handle messages that vary by both gender and count. Place one block inside a branch of the other.

Keep nesting shallow — deeply nested messages are painful for translators. If it gets complex, consider splitting into separate keys.

{
  "likes": "{gender, select, female{She got {count, plural, =0{no likes} =1{1 like} other{# likes}}} male{He got {count, plural, =0{no likes} =1{1 like} other{# likes}}} other{They got {count, plural, =0{no likes} =1{1 like} other{# likes}}}}",
  "@likes": {
    "placeholders": {
      "gender": { "type": "String" },
      "count": { "type": "int" }
    }
  }
}

Formatted Placeholders: Numbers and Dates

Placeholders can be formatted per locale using the format field in metadata. Numbers use formats like decimalPattern or currency; dates use DateFormat skeletons such as yMMMd.

This means 1234.5 renders as 1,234.5 in en-US but 1.234,5 in many European locales — automatically.

{
  "price": "Total: {amount}",
  "@price": {
    "placeholders": {
      "amount": {
        "type": "double",
        "format": "currency",
        "optionalParameters": { "symbol": "\u20ac", "decimalDigits": 2 }
      }
    }
  },
  "lastSeen": "Last seen {when}",
  "@lastSeen": {
    "placeholders": {
      "when": { "type": "DateTime", "format": "yMMMd" }
    }
  }
}

Formatting Numbers and Dates in Plain Dart

The same intl formatters power those ARB format fields. Here is a runnable program using NumberFormat and DateFormat directly, the way the generated localization code does under the hood.

import 'package:intl/intl.dart';

void main() {
  final amount = 1234.5;
  final usd = NumberFormat.currency(locale: 'en_US', symbol: '\$');
  final eur = NumberFormat.currency(locale: 'de_DE', symbol: '\u20ac');

  print(usd.format(amount)); // \$1,234.50
  print(eur.format(amount)); // 1.234,50 \u20ac

  final date = DateTime(2026, 6, 10);
  print(DateFormat.yMMMd('en_US').format(date)); // Jun 10, 2026
  print(DateFormat.yMMMd('tr_TR').format(date)); // 10 Haz 2026
}

Quick Check: Choosing the Right Form

You need a message that says "no messages", "1 message", or "N messages" depending on a count, and you want it to also work correctly in locales with few/many categories. Which approach is correct?

Recap

You now know how to localize dynamic text in Flutter with ICU message syntax:

ARB files hold messages per locale; @key metadata declares placeholder types and formats.
Placeholders {name} inject typed values into messages.
plural {count, plural, =0{} =1{} other{}} picks a form by number; other is mandatory and # prints the value.
=1 vs one: exact-value match versus language plural category — use exact matches only for special wording.
select {gender, select, male{} female{} other{}} branches on a string; great for gender.
Nesting combines gender and count when needed.
format metadata + NumberFormat/DateFormat render numbers and dates per locale.

Write messages so translators control wording per language while your Dart call sites stay unchanged.

よくある質問

「複数形、性別、ICUメッセージフォーマット」レッスンは無料ですか？

はい。「複数形、性別、ICUメッセージフォーマット」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。

「複数形、性別、ICUメッセージフォーマット」で何を学びますか？

ICUメッセージ構文を使い、複数形、選択肢、パラメーター付き文字列を処理します。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2/4です。

「複数形、性別、ICUメッセージフォーマット」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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