Middleware<T extends Object?> typedef

A middleware function that wraps or intercepts a handler.

A Middleware takes an existing Handler and returns a new Handler that provides additional behavior before or after calling the original handler. This enables composition of cross-cutting concerns like logging, validation, error handling, timing, etc.

Middleware follows the decorator pattern:

• It receives the "next" handler in the chain
• It creates a new handler that wraps the next handler
• When called, it can perform pre-processing, call the next handler, then post-process

Example:

// Timing middleware that logs how long the handler takes
Middleware<String> timingMiddleware = (next) {
  return (input) async {
    final sw = Stopwatch()..start();
    try {
      return await next(input);
    } finally {
      sw.stop();
      print('Took ${sw.elapsedMilliseconds}ms');
    }
  };
};

// Validation middleware that checks input before processing
Middleware<String> validationMiddleware = (next) {
  return (input) async {
    if (input.isEmpty) throw ArgumentError('Input cannot be empty');
    return await next(input);
  };
};