feat: allow default language to be changed

Author: GeekyEggo

src/i18n/provider.ts

@@ -1,20 +1,21 @@
+import { EventEmitter } from "../event-emitter.js";
 import type { JsonObject } from "../json.js";
 import { freeze, get } from "../objects.js";
 import { defaultLanguage, type Language } from "./language.js";
 
 /**
  * Internalization provider, responsible for managing localizations and translating resources.
  */
-export class I18nProvider<T extends string> {
+export class I18nProvider<T extends string> extends EventEmitter<I18nProviderEventMap<T>> {
 	/**
-	 * Map of localized resources, indexed by their language.
+	 * Backing field for the default language.
 	 */
-	readonly #translations: Map<Language<T>, JsonObject | null> = new Map();
+	#language: Language<T>;
 
 	/**
-	 * Default language of the provider.
+	 * Map of localized resources, indexed by their language.
 	 */
-	readonly #language: Language<T>;
+	readonly #translations: Map<Language<T>, JsonObject | null> = new Map();
 
 	/**
 	 * Function responsible for providing localized resources for a given language.
@@ -27,18 +28,39 @@ export class I18nProvider<T extends string> {
 	 * @param readTranslations Function responsible for providing localized resources for a given language.
 	 */
 	constructor(language: Language<T>, readTranslations: TranslationsReader<T>) {
+		super();
+
 		this.#language = language;
 		this.#readTranslations = readTranslations;
 	}
 
+	/**
+	 * The default language of the provider.
+	 * @returns The language.
+	 */
+	public get language(): Language<T> {
+		return this.#language;
+	}
+
+	/**
+	 * The default language of the provider.
+	 * @param value The language.
+	 */
+	public set language(value: Language<T>) {
+		if (this.#language !== value) {
+			this.#language = value;
+			this.emit("languageChange", value);
+		}
+	}
+
 	/**
 	 * Translates the specified {@link key}, as defined within the resources for the {@link language}.
 	 * When the key is not found, the default language is checked. Alias of {@link I18nProvider.translate}.
 	 * @param key Key of the translation.
 	 * @param language Optional language to get the translation for; otherwise the default language.
 	 * @returns The translation; otherwise the key.
 	 */
-	public t(key: string, language: Language<T> = this.#language): string {
+	public t(key: string, language: Language<T> = this.language): string {
 		return this.translate(key, language);
 	}
 
@@ -49,7 +71,7 @@ export class I18nProvider<T extends string> {
 	 * @param language Optional language to get the translation for; otherwise the default language.
 	 * @returns The translation; otherwise the key.
 	 */
-	public translate(key: string, language: Language<T> = this.#language): string {
+	public translate(key: string, language: Language<T> = this.language): string {
 		// Determine the languages to search for.
 		const languages = new Set<Language<T>>([
 			language,
@@ -88,6 +110,16 @@ export class I18nProvider<T extends string> {
 	}
 }
 
+/**
+ * Events that can occur as part of the {@link I18nProvider}.
+ */
+type I18nProviderEventMap<T extends string> = {
+	/**
+	 * Occurs when the language changes.
+	 */
+	languageChange: [language: Language<T>];
+};
+
 /**
  * Function responsible for providing localized resources for a given language.
  * @param language The language whose resources should be retrieved.