31. カスタムプロバイダ — useValue / useFactory / useClass

// カスタムプロバイダ(useFactory)の本質を Map で再現
// 「トークン → ファクトリ関数で値を生成。依存も注入する」
type Provider =
  | { provide: string; useValue: unknown }
  | { provide: string; useFactory: (...deps: any[]) => unknown; inject?: string[] };

class MiniContainer {
  private resolved = new Map<string, unknown>();

  constructor(providers: Provider[]) {
    for (const p of providers) {
      if ("useValue" in p) {
        this.resolved.set(p.provide, p.useValue); // 固定値
      } else {
        // inject で指定したトークンを解決してファクトリに渡す
        const deps = (p.inject ?? []).map((token) => this.get(token));
        this.resolved.set(p.provide, p.useFactory(...deps));
      }
    }
  }

  get<T>(token: string): T {
    if (!this.resolved.has(token)) throw new Error("未登録: " + token);
    return this.resolved.get(token) as T;
  }
}

const container = new MiniContainer([
  { provide: "CONFIG", useValue: { dbName: "app" } }, // useValue
  {
    provide: "DB_CLIENT", // useFactory: CONFIG を読んで値を組み立てる
    useFactory: (config: { dbName: string }) =>
      ({ connectedTo: config.dbName }),
    inject: ["CONFIG"],
  },
]);

const config = container.get<{ dbName: string }>("CONFIG");
console.log("CONFIG.dbName: " + config.dbName);

const client = container.get<{ connectedTo: string }>("DB_CLIENT");
console.log("DB_CLIENT.connectedTo: " + client.connectedTo);