Angular Server-Side Rendering: Implementing TransferState

This challenge focuses on a crucial aspect of Angular's server-side rendering (SSR) strategy: preserving application state between the server and the client. Properly implementing TransferState is essential for improving perceived performance and providing a seamless user experience by avoiding redundant data fetching on the client.

Problem Description

Your task is to create a reusable Angular service that leverages TransferState to effectively manage and transfer application data from the server to the client during SSR. This service should allow components to fetch data on the server, serialize it using TransferState, and then deserialize it on the client to prevent a second, identical data fetch.

Key Requirements:

1. Create a TransferStateService: This service should be a singleton and accessible throughout the application.
2. Implement setState(key: string, value: any): A method to store data in TransferState with a unique string key.
3. Implement getState<T>(key: string): T | undefined: A method to retrieve data from TransferState using its key. It should return the stored value or undefined if not found.
4. Handle Server-Side Logic: The service must correctly store data when the application is running on the server (e.g., using a simple mock data fetch).
5. Handle Client-Side Logic: The service must correctly retrieve previously stored data when the application boots on the client.
6. Integration with a Mock Component: Demonstrate the usage of your TransferStateService within a simple Angular component that simulates fetching data.

Expected Behavior:

When the application runs on the server:

• A component requests data.
• The TransferStateService intercepts this request, fetches the data (or simulates fetching), and stores it using setState.
• This stored data is then serialized by Angular's SSR mechanism.

When the application runs on the client:

• The same component requests data.
• Before attempting a real fetch, it checks the TransferStateService using getState.
• If the data is found, it's returned immediately, avoiding an HTTP request.
• If the data is not found (e.g., during a fresh client-side navigation or if TransferState wasn't used on the server), the component proceeds with its normal data fetching logic.

Edge Cases to Consider:

• What happens if getState is called for a key that was never set?
• How does your service behave when running outside of an SSR context (e.g., purely client-side rendering)?

Examples

Example 1: Storing and Retrieving a Simple Object

Scenario: A UserProfileService needs to fetch user data.

Server-Side (Conceptual):

// Inside a server-side data fetching function/resolver
const userId = 123;
const userData = { id: userId, name: 'Alice', email: 'alice@example.com' };
transferStateService.setState('user-data-' + userId, userData);

Client-Side (Conceptual - in a component or service constructor):

// Inside a component's ngOnInit or a service's constructor
const userId = 123;
let userData = transferStateService.getState<UserData>('user-data-' + userId);

if (!userData) {
  // Data not found in TransferState, fetch it normally
  userData = this.userService.fetchUserData(userId); // Assume this makes an HTTP call
}
// Use userData

Example 2: Handling Missing State

Scenario: A component tries to retrieve data that was never set.

Client-Side (Conceptual):

// Inside a component's ngOnInit
const config = transferStateService.getState<AppConfig>('app-config');

if (!config) {
  // app-config was never set on the server, or this is a client-only route
  // Fetch default configuration or handle error
  this.appConfig = this.configService.getDefaultConfig();
} else {
  this.appConfig = config;
}

Constraints

• The solution must be implemented in TypeScript.
• The TransferStateService should be an Angular injectable service.
• The service should not rely on any external libraries beyond Angular's core and @angular/platform-server.
• The solution should be compatible with Angular 14+.
• Performance is important; avoid unnecessary operations that could slow down server or client initialization.

Notes

• You will need to simulate the server-side environment for testing purposes. This can often be done by mocking the isPlatformServer check and the TransferState provider.
• Consider how your service will interact with the Angular TransferState class, which is typically provided by @angular/platform-server.
• Think about how to make your getState method type-safe.
• The goal is to abstract away the direct usage of TransferState behind a simpler service interface.