mediatify

A lightweight mediator implementation for TypeScript, inspired by ASP.NET Core MediatR, following the Command Query Responsibility Segregation (CQRS) pattern. This package helps to decouple request dispatching from request handling, allowing for better separation of concerns.

Features

- CQRS pattern: Supports Commands and Queries in a single IRequest interface.
- Handler Registration: Handlers are automatically registered by scanning project directories for annotated classes (supports .ts, .js, .mjs, .cjs out of the box).
- Pipeline Behavior: Supports pipeline behaviors (like logging, validation, etc.) that can be applied around requests.
- Singleton Mediator: A singleton mediator ensures all handlers are registered once and reused throughout the app.
- Testing Friendly: The mediator can now be reset between tests so you can control which handlers are available per test case.
- Asynchronous Support: Fully supports async/await for request handling and pipeline behaviors.

Installation

You can install the package via npm:

``bash
npm install mediatify
`

`bash
yarn add mediatify
`

Usage

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#### Step 1: Define the Command (Request, Response)

`ts
// core/useCase/createUser/CreateUserCommand.ts
export class CreateUserCommand implements IRequest {
constructor(public username: string, public email: string) {}
}

`

`ts
// core/useCase/createUser/CreateUserCommandResponse.ts
export class CreateUserCommandResponse {
constructor(public result: User) {}
}
`

This CreateUserCommand will be used to create a new user, and the CreateUserCommandResponse contains the user model as a response.

#### Step 2: Define the Command Handler

`ts
// ore/useCase/createUser/CreateUserCommandHandler.ts
export class CreateUserCommandHandler implements IRequestHandler {

constructor(@Inject(HttpClient) private httpClient: HttpClient) {}

async handle(request: CreateUserCommand): Promise {
// Simulate user creation logic
const response = await this.httpClient.post(API_URL, request);
return new CreateUserResponse(response.data);
}
}
`

The CreateUserCommandHandler is responsible for handling the CreateUserCommand and returning a new user object.

#### Step 3: Define a Pipeline (Optional)

Pipelines allow you to run custom logic before or after a request is handled:

`ts
// pipelines/LoggingPipeline.ts
import { IPipeline } from "mediatify";

export class LoggingPipeline implements IPipeline {
async process(request: TRequest, next: () => Promise): Promise {
console.log('Handling request:', request);
const response = await next();
console.log('response:' , response);
return response;
}
}
`

#### Step 4: Register Handlers and Pipelines

Handlers are automatically registered by scanning the project directories. Below is an example of registering the CreateUserCommandHandler and using the pipeline:

`ts
async function main() {
const mediator = Mediator.getInstance();
await mediator.registerHandlers("../core/useCases/");

mediator.registerPipeline(new LoggingPipeline());


// Send a request
const request = new CreateUserCommand("faraj", "farajshuaib@gmail.com");

const createUserCommandResponse = await mediator.send(request);


console.log(createUserCommandResponse.result);

}

`

API

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#### Methods

- reset() Resets the registered handlers and pipelines. Helpful inside unit tests.
- registerHandler(requestType: string, handler: IRequestHandler) Registers a handler for a specific request type.
- registerPipeline(pipeline: IPipeline) Registers a pipeline behavior to be applied to all requests.
- send, TResponse>(request: TRequest): Promise Sends a request and invokes the corresponding handler, passing through the pipeline behaviors if registered.
- registerHandlers(pathOrOptions?: string | RegisterHandlersOptions, options?: RegisterHandlersOptions) Scans a directory for annotated handlers and registers them automatically.

#### registerHandlers Options

registerHandlers can accept either a directory path or an options object (or both).
The available options are:

| Option | Description |
| --- | --- |
| baseDir | Overrides the base directory used to resolve a relative handlers path (defaults to process.cwd() with a fallback to the package directory). |
| pattern | Custom glob pattern for handler discovery. If omitted the mediator looks for .ts, .js, .mjs and .cjs files. |
| extensions | Explicit list of extensions that should be scanned. |
| ignore | Glob patterns that should be ignored while scanning. |
| handlerFactory | Function that receives the handler constructor and returns an instance. Useful for wiring your own dependency injection container. |
| onDuplicate | How to behave when the same request type is discovered twice. Accepts "replace" (default), "skip" or "error". |

`ts
const mediator = Mediator.getInstance();
await mediator.registerHandlers("./build/useCases", {
baseDir: __dirname,
onDuplicate: "skip",
handlerFactory: (HandlerClass) => container.resolve(HandlerClass),
});
`

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Handlers are registered automatically by scanning the project files using the @Handler decorator. Both TypeScript source files and already-compiled JavaScript files will be discovered by default.

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A pipeline is a function that processes a request before and/or after the handler is invoked. Pipelines are executed in the order they are registered and can now safely be reused across requests without worrying about registration order being mutated.

`ts
interface IPipeline {
process(request: TRequest, next: () => Promise): Promise;
}
`

Inspired by

This package is heavily inspired by the MediatR library in ASP.NET Core, which implements the mediator pattern in .NET.

Contributing

Contributions are welcome! Please follow these steps to contribute:

1. Fork the repository.
2. Create a new branch (git checkout -b feature-branch).
3. Make your changes.
4. Run the tests (yarn test).
5. Commit your changes (git commit -am 'Add new feature').
6. Push to the branch (git push origin feature-branch`).
7. Create a new Pull Request.
8. Get your changes reviewed.

License

This project is licensed under the MIT License.