####################
Dependency injection
####################

Dependency injection is a programming technique that makes a class independent of its dependencies.
It achieves that by decoupling the usage of an object from its creation.
This helps you to follow `SOLID's <https://en.wikipedia.org/wiki/SOLID>`_ dependency
inversion and single responsibility principles.


How it works in aiogram
=======================

For each update :class:`aiogram.dispatcher.dispatcher.Dispatcher` passes handling context data.
Filters and middleware can also make changes to the context.

To access contextual data you should specify corresponding keyword parameter in handler or filter.
For example, to get :class:`aiogram.fsm.context.FSMContext` we do it like that:

.. code-block:: python

    @router.message(ProfileCompletion.add_photo, F.photo)
    async def add_photo(
        message: types.Message, bot: Bot, state: FSMContext
    ) -> Any:
        ... # do something with photo


Injecting own dependencies
==========================

Aiogram provides several ways to complement / modify contextual data.

The first and easiest way is to simply specify the named arguments in
:class:`aiogram.dispatcher.dispatcher.Dispatcher` initialization, polling start methods
or :class:`aiogram.webhook.aiohttp_server.SimpleRequestHandler` initialization if you use webhooks.

.. code-block:: python

    async def main() -> None:
        dp = Dispatcher(..., foo=42)
        return await dp.start_polling(
            bot, bar="Bazz"
        )

Analogy for webhook:

.. code-block:: python

    async def main() -> None:
        dp = Dispatcher(..., foo=42)
        handler = SimpleRequestHandler(dispatcher=dp, bot=bot, bar="Bazz")
        ... # starting webhook

:class:`aiogram.dispatcher.dispatcher.Dispatcher`'s workflow data also can be supplemented
by setting values as in a dictionary:

.. code-block:: python

    dp = Dispatcher(...)
    dp["eggs"] = Spam()

The middlewares updates the context quite often.
You can read more about them on this page:

- :ref:`Middlewares <middlewares>`

The last way is to return a dictionary from the filter:

.. literalinclude:: ../../examples/context_addition_from_filter.py

...or using :ref:`MagicFilter <magic-filters>` with :code:`.as_(...)` method.


Using type hints
================

.. note::

    Type-hinting middleware data is optional and is not required for the correct operation of the dispatcher.
    However, it is recommended to use it to improve the readability of the code.

You can use type hints to specify the type of the context data in the middlewares, filters and handlers.

The default middleware data typed dict can be found in :class:`aiogram.dispatcher.middlewares.data.MiddlewareData`.

In case when you have extended the context data, you can use the :class:`aiogram.dispatcher.middlewares.data.MiddlewareData` as a base class and specify the type hints for the new fields.

.. warning::

    If you using type checking tools like mypy, you can experience warnings about that this type hint against Liskov substitution principle in due stricter type is not a subclass of :code:`dict[str, Any]`.
    This is a known issue and it is not a bug. You can ignore this warning or use :code:`# type: ignore` comment.

Example of using type hints:

.. code-block:: python

    from aiogram.dispatcher.middlewares.data import MiddlewareData


    class MyMiddlewareData(MiddlewareData, total=False):
        my_custom_value: int


    class MyMessageMiddleware(BaseMiddleware):
        async def __call__(
            self,
            handler: Callable[[Message, MyMiddlewareData], Awaitable[Any]],
            event: Message,
            data: MyMiddlewareData,
        ) -> Any:
            bot = data["bot"]  # <-- IDE will show you that data has `bot` key and its type is `Bot`

            data["my_custom_value"] = bot.id * 42  # <-- IDE will show you that you can set `my_custom_value` key with int value and warn you if you try to set it with other type
            return await handler(event, data)


Available context data type helpers
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. autoclass:: aiogram.dispatcher.middlewares.data.MiddlewareData
    :members:
    :undoc-members:
    :member-order: bysource


.. autoclass:: aiogram.dispatcher.middlewares.data.I18nData
    :members:
    :undoc-members:
    :member-order: bysource