Add prototype of class-based handlers

tests/test_dispatcher/test_handler/test_message.py

@@ -0,0 +1,28 @@
+import datetime
+from typing import Any
+
+import pytest
+
+from aiogram.api.types import Chat, Message, User
+from aiogram.dispatcher.handler.message import MessageHandler
+
+
+class MyHandler(MessageHandler):
+    async def handle(self) -> Any:
+        return self.event.text
+
+
+class TestClassBasedMessageHandler:
+    @pytest.mark.asyncio
+    async def test_message_handler(self):
+        event = Message(
+            message_id=42,
+            date=datetime.datetime.now(),
+            text="test",
+            chat=Chat(id=42, type="private"),
+            from_user=User(id=42, is_bot=False, first_name="Test"),
+        )
+        handler = MyHandler(event=event)
+
+        assert handler.from_user == event.from_user
+        assert handler.chat == event.chat