Add registerFactory for requests

lib/src/request/handler/request_handler.dart

@@ -3,6 +3,10 @@ import 'dart:async';
 import 'package:dart_mediator/src/request/request.dart';
 import 'package:meta/meta.dart';
 
+/// Factory to create a [RequestHandler].
+typedef RequestHandlerFactory<TResponse, TRequest extends Request<TResponse>>
+    = RequestHandler<TResponse, TRequest> Function();
+
 /// Handler for [TRequest].
 @internal
 abstract interface class RequestHandler<TResponse,

lib/src/request/handler/request_handler_store.dart

@@ -3,8 +3,9 @@ import 'package:dart_mediator/src/request/handler/request_handler.dart';
 
 class RequestHandlerStore {
   final _handlers = <Type, RequestHandler>{};
+  final _handlerFactories = <Type, RequestHandlerFactory>{};
 
-  /// Registers the [handler] to a given [TEvent].
+  /// Registers the [handler] to a given [TRequest].
   void register<TResponse, TRequest extends Request<TResponse>>(
     RequestHandler<TResponse, TRequest> handler,
   ) {
@@ -13,9 +14,31 @@ class RequestHandlerStore {
       'register<$TResponse, $TRequest> was called with an already registered handler',
     );
 
+    assert(
+      !_handlerFactories.containsKey(TRequest),
+      'register<$TRequest> was called with an already registered factory',
+    );
+
     _handlers[TRequest] = handler;
   }
 
+  /// Registers the [factory] to a given [TRequest].
+  void registerFactory<TResponse, TRequest extends Request<TResponse>>(
+    RequestHandlerFactory<TResponse, TRequest> factory,
+  ) {
+    assert(
+      !_handlers.containsKey(TRequest),
+      'registerFactory<$TResponse, $TRequest> was called with an already registered handler',
+    );
+
+    assert(
+      !_handlerFactories.containsKey(TRequest),
+      'registerFactory<$TRequest> was called with an already registered factory',
+    );
+
+    _handlerFactories[TRequest] = factory;
+  }
+
   /// Unregisters the given [handler].
   void unregister<TResponse, TRequest extends Request<TResponse>>(
     RequestHandler<TResponse, TRequest> handler,
@@ -31,7 +54,7 @@ class RequestHandlerStore {
   /// Returns the registered [RequestHandler]'s for [TRequest].
   RequestHandler<TResponse, TRequest>
       getHandlerFor<TResponse, TRequest extends Request<TResponse>>() {
-    final handler = _handlers[TRequest];
+    final handler = _handlers[TRequest] ?? _handlerFactories[TRequest]?.call();
 
     assert(
       handler != null,

lib/src/request/request_manager.dart

@@ -43,6 +43,13 @@ class RequestManager {
     _requestHandlerStore.register(handler);
   }
 
+  /// Registers the request [factory] for the given [TRequest].
+  void registerFactory<TResponse, TRequest extends Request<TResponse>>(
+    RequestHandlerFactory<TResponse, TRequest> factory,
+  ) {
+    _requestHandlerStore.registerFactory(factory);
+  }
+
   /// Sends a [request] to a single [RequestHandler].
   ///
   /// Make sure the [RequestHandler] is [register]ed before calling this method.

test/unit/request/request_handler/request_handler_store_test.dart

@@ -30,6 +30,17 @@ void main() {
       });
     });
 
+    group('registerFactory', () {
+      test('it registers the handler', () {
+        expect(
+          () => requestHandlerStore.registerFactory<int, MockRequest<int>>(
+            () => MockRequestHandler<int, MockRequest<int>>(),
+          ),
+          returnsNormally,
+        );
+      });
+    });
+
     group('unregister', () {
       final mockRequestHandler = MockRequestHandler<int, MockRequest<int>>();
 
@@ -83,6 +94,18 @@ void main() {
           correctHandler,
         );
       });
+
+      test('it returns the request handler factory', () {
+        final mockHandler = MockRequestHandler<int, MockRequest<int>>();
+        correctHandlerFactory() => mockHandler;
+
+        requestHandlerStore.registerFactory(correctHandlerFactory);
+
+        expect(
+          requestHandlerStore.getHandlerFor<int, MockRequest<int>>(),
+          mockHandler,
+        );
+      });
     });
   });
 }

test/unit/request/requests_manager_test.dart

@@ -48,6 +48,22 @@ void main() {
       });
     });
 
+    group('registerFactory', () {
+      test('it registers the handler', () {
+        mockRequestHandlerFactory() =>
+            MockRequestHandler<String, MockRequest<String>>();
+
+        requestsManager.registerFactory<String, MockRequest<String>>(
+          mockRequestHandlerFactory,
+        );
+
+        verify(
+          () => mockRequestHandlerStore
+              .registerFactory(mockRequestHandlerFactory),
+        );
+      });
+    });
+
     group('registerFunction', () {
       test('it registers the handler', () {
         requestsManager.registerFunction<String, MockRequest<String>>(