nextjs-router-source-example-9.tsx

// packages/next/client/router.ts

// Router 싱글톤을 저장하는 객체
// https://github.com/vercel/next.js/blob/v12.3.4/packages/next/client/router.ts#L20
const singletonRouter: SingletonRouterBase = {
  router: null, // holds the actual router instance
  readyCallbacks: [],
  ready(cb: () => void) {
    if (this.router) return cb()
    if (typeof window !== 'undefined') {
      this.readyCallbacks.push(cb)
    }
  },
}

// createRouter 코드 - Router 싱글톤 초기화
// https://github.com/vercel/next.js/blob/v12.3.4/packages/next/client/router.ts#L146
/**
 * Create a router and assign it as the singleton instance.
 * router을 생성하고 싱글톤 인스턴스로서 할당한다.
 * 
 * This is used in client side when we are initializing the app.
 * client-side에서 앱을 초기화할 때 사용된다.
 * 
 * This should **not** be used inside the server.
 * 서버에서 사용되면 **안된다**.
 * @internal
 */
export function createRouter(
  ...args: ConstructorParameters<typeof Router>
): Router {
  singletonRouter.router = new Router(...args)
  singletonRouter.readyCallbacks.forEach((cb) => cb())
  singletonRouter.readyCallbacks = []

  return singletonRouter.router
}