Skip to main content

Middleware

Steward's router has support for Middleware functions that are executed before the route handler that they are attached to. The middleware function typedef is like so:

typedef MiddlewareFunc = Future<Response> Function(Request) Function(
    Future<Response> Function(Request) nextHandler);

While this may look complex, the actual implementation of a custom middleware is fairly straightforward. We can look to the middlewares that Steward ships for an example of how we can structure our own middleware. Let's take a look at the RequestLogger middleware:

/// An extremely simple Middleware function that prints the incoming request URI
/// helpful for debugging and in understanding how middleware functions are structured
Future<Response> Function(Request) RequestLogger(
    Future<Response> Function(Request) next) {
  return (Request request) {
    print('Incoming Request: ${request.request.uri}');
    return next(request);
  };
}

This RequestLogger returns a function that takes in a request and then returns the result of calling next with that request. Generally, your middleware will always return a function and that function with return next(request). This keeps the middleware chain going, but should you want to terminate it early (for example, if a user is unauthorized), you can always return a Response instead of returning next(request).

Out of the Box Middleware​

Steward ships with some middleware out of the box. Namely, you have:

  • RequestLogger - used for logging incoming requests to STDOUT
  • CORSMiddleware - used for configuring CORS headers

Middleware ships under a different module and should be imported like so: import 'package:steward/middleware/cors_middleware.dart';