Handling Updates
================

Calling `API methods`_ sequentially is cool, but how to react when, for example, a new message arrives? This page deals
with updates and how to handle such events in Pyrogram. Let's have a look at how they work.

First, let's define what are these updates. As hinted already, updates are simply events that happen in your Telegram
account (incoming messages, new members join, button presses, etc...), which are meant to notify you about a new
specific state that changed. These updates are handled by registering one or more callback functions in your app using
`Handlers <../pyrogram/Handlers.html>`_.

Each handler deals with a specific event and once a matching update arrives from Telegram, your registered callback
function will be called and its body executed.

Registering an Handler
----------------------

To explain how handlers work let's have a look at the most used one, the
:obj:`MessageHandler <pyrogram.MessageHandler>`, which will be in charge for handling :obj:`Message <pyrogram.Message>`
updates coming from all around your chats. Every other handler shares the same setup logic; you should not have troubles
settings them up once you learn from this section.

Using add_handler()
-------------------

The :meth:`add_handler() <pyrogram.Client.add_handler>` method takes any handler instance that wraps around your defined
callback function and registers it in your Client. Here's a full example that prints out the content of a message as
soon as it arrives:

.. code-block:: python

    from pyrogram import Client, MessageHandler


    def my_function(client, message):
        print(message)


    app = Client("my_account")

    my_handler = MessageHandler(my_function)
    app.add_handler(my_handler)

    app.run()

Let's examine these four new pieces. First one: a callback function we defined which accepts two arguments -
*(client, message)*. This will be the function that gets executed every time a new message arrives and Pyrogram will
call that function by passing the client instance and the new message instance as argument.

.. code-block:: python

    def my_function(client, message):
        print(message)

Second one: the :obj:`MessageHandler <pyrogram.MessageHandler>`. This object tells Pyrogram the function we defined
above must only handle updates that are in form of a :obj:`Message <pyrogram.Message>`:

.. code-block:: python

    my_handler = MessageHandler(my_function)

Third: the method :meth:`add_handler() <pyrogram.Client.add_handler>`. This method is used to actually register the
handler and let Pyrogram know it needs to be taken into consideration when new updates arrive and the dispatching phase
begins.

.. code-block:: python

    app.add_handler(my_handler)

Last one, the :meth:`run() <pyrogram.Client.run>` method. What this does is simply calling
:meth:`start() <pyrogram.Client.start>` and a special method :meth:`idle() <pyrogram.Client.idle>` that keeps your main
scripts alive until you press ``CTRL+C``; the client will be automatically stopped after that.

.. code-block:: python

    app.run()

Using Decorators
----------------

All of the above will become quite verbose, especially in case you have lots of handlers to register. A much nicer way
to do so is by decorating your callback function with the :meth:`on_message() <pyrogram.Client.on_message>` decorator.

.. code-block:: python

    from pyrogram import Client

    app = Client("my_account")


    @app.on_message()
    def my_handler(client, message):
        print(message)


    app.run()


.. note::

    Due to how these decorators work in Pyrogram, they will wrap your defined callback function in a tuple consisting of
    ``(handler, group)``; this will be the value held by your function identifier (e.g.: *my_function* from the example
    above).

    In case, for some reason, you want to get your own function back after it has been decorated, you need to access
    ``my_function[0].callback``, that is, the *callback* field of the *handler* object which is the first element in the
    tuple, accessed by bracket notation *[0]*.

.. _API methods: usage.html