textual/docs/guide/events.md

Events and Messages

We've used event handler methods in many of the examples in this guide. This chapter explores events and messages (see below) in more detail.

Messages

Events are a particular kind of message sent by Textual in response to input and other state changes. Events are reserved for use by Textual but you can also create custom messages for the purpose of coordinating between widgets in your app.

More on that later, but for now keep in mind that events are also messages, and anything that is true of messages is true of events.

Message Queue

Every [App][textual.app.App] and [Widget][textual.widget.Widget] object contains a message queue. You can think of a message queue as orders at a restaurant. The chef takes an order and makes the dish. Orders that arrive while the chef is cooking are placed in a line. When the chef has finished a dish they pick up the next order in the line.

Textual processes messages in the same way. Messages are picked off a queue and processed (cooked) by a handler method. This guarantees messages and events are processed even if your code can not handle them right way.

This processing of messages is done within an asyncio Task which is started when you mount the widget. The task monitors a queue for new messages and dispatches them to the appropriate handler when they arrive.

!!! tip

The FastAPI docs have an [excellent introduction](https://fastapi.tiangolo.com/async/) to Python async programming.

By way of an example, let's consider what happens if you were to type "Text" in to a TextInput widget. When you hit the ++t++ key, Textual creates a [key][textual.events.Key] event and sends it to the widget's message queue. Ditto for ++e++, ++x++, and ++t++.

The widget's task will pick the first message from the queue (a key event for the ++t++ key) and call the on_key method with the event as the first argument. In other words it will call TextInput.on_key(event), which updates the display to show the new letter.

--8<-- "docs/images/events/queue.excalidraw.svg"

When the on_key method returns, Textual will get the next event from the the queue and repeat the process for the remaining keys. At some point the queue will be empty and the widget is said to be in an idle state.

!!! note

This example illustrates a point, but a typical app will be fast enough to have processed a key before the next event arrives. So it is unlikely you will have so many key events in the message queue.

--8<-- "docs/images/events/queue2.excalidraw.svg"

Default behaviors

You may be familiar with Python's super function to call a function defined in a base class. You will not have to use this in event handlers as Textual will automatically call handler methods defined in a widget's base class(es).

For instance, let's say we are building the classic game of Pong and we have written a Paddle widget which extends [Static][textual.widgets.Static]. When a [Key][textual.events.Key] event arrives, Textual calls Paddle.on_key (to respond to ++left++ and ++right++ keys), then Static.on_key, and finally Widget.on_key.

Preventing default behaviors

If you don't want this behavior you can call [prevent_default()][textual.message.Message.prevent_default] on the event object. This tells Textual not to call any more handlers on base classes.

!!! warning

You won't need `prevent_default` very often. Be sure to know what your base classes do before calling it, or you risk disabling some core features builtin to Textual.

Bubbling

Messages have a bubble attribute. If this is set to True then events will be sent to a widget's parent after processing. Input events typically bubble so that a widget will have the opportunity to respond to input events if they aren't handled by their children.

The following diagram shows an (abbreviated) DOM for a UI with a container and two buttons. With the "No" button focused, it will receive the key event first.

--8<-- "docs/images/events/bubble1.excalidraw.svg"

After Textual calls Button.on_key the event bubbles to the button's parent and will call Container.on_key (if it exists).

--8<-- "docs/images/events/bubble2.excalidraw.svg"

As before, the event bubbles to it's parent (the App class).

--8<-- "docs/images/events/bubble3.excalidraw.svg"

The App class is always the root of the DOM, so there is no where for the event to bubble to.

Stopping bubbling

Event handlers may stop this bubble behavior by calling the [stop()][textual.message.Message.stop] method on the event or message. You might want to do this if a widget has responded to the event in an authoritative way. For instance when a text input widget responds to a key event it stops the bubbling so that the key doesn't also invoke a key binding.

Custom messages

You can create custom messages for your application that may be used in the same way as events (recall that events are simply messages reserved for use by Textual).

The most common reason to do this is if you are building a custom widget and you need to inform a parent widget about a state change.

Let's look at an example which defines a custom message. The following example creates color buttons which—when clicked—send a custom message.

=== "custom01.py"

```python title="custom01.py" hl_lines="10-15 27-29 42-43"
--8<-- "docs/examples/events/custom01.py"
```

=== "Output"

```{.textual path="docs/examples/events/custom01.py"}
```

Note the custom message class which extends [Message][textual.message.Message]. The constructor stores a [color][textual.color.Color] object which handler methods will be able to inspect.

The message class is defined within the widget class itself. This is not strictly required but recommended, for these reasons:

• It reduces the amount of imports. If you import ColorButton, you have access to the message class via ColorButton.Selected.
• It creates a namespace for the handler. So rather than on_selected, the handler name becomes on_color_button_selected. This makes it less likely that your chosen name will clash with another message.

Sending events

In the previous example we used [emit()][textual.message_pump.MessagePump.emit] to send an event to it's parent. We could also have used [emit_no_wait()][textual.message_pump.MessagePump.emit_no_wait] for non async code. Sending messages in this way allows you to write custom widgets without needing to know in what context they will be used.

There are other ways of sending (posting) messages, which you may need to use less frequently.

• [post_message][textual.message_pump.MessagePump.post_message] To post a message to a particular event.
• [post_message_no_wait][textual.message_pump.MessagePump.post_message_no_wait] The non-async version of post_message.

Message handlers

Most of the logic in a Textual app will be written in message handlers. Let's explore handlers in more detail.

Handler naming

Textual uses the following scheme to map messages classes on to a Python method.

• Start with "on_".
• Add the messages namespace (if any) converted from CamelCase to snake_case plus an underscore "_"
• Add the name of the class converted from CamelCase to snake_case.

--8<-- "docs/images/events/naming.excalidraw.svg"

Handler arguments

Message handler methods can be written with or without a positional argument. If you add a positional argument, Textual will call the handler with the event object. The following handler (taken from custom01.py above) contains a message parameter. The body of the code makes use of the message to set a preset color.

    def on_color_button_selected(self, message: ColorButton.Selected) -> None:
        self.screen.styles.animate("background", message.color, duration=0.5)

If the body of your handler doesn't require any information in the message you can omit it from the method signature. If we just want to play a bell noise when the button is clicked, we could write our handler like this:

    def on_color_button_selected(self) -> None:
        self.app.bell()

This pattern is a convenience that saves writing out a parameter that may not be used.

Async handlers

Method handlers may be coroutines. If you prefix your handlers with the async keyword, Textual will await them. This lets your handler use the await keyword for asynchronous APIs.

If your event handlers are coroutines it will allow multiple events to be processed concurrently, but bear in mind an individual widget (or app) will not be able to pick up a new message from its message queue until the handler has returned. This is rarely a problem in practice; as long has handlers return within a few milliseconds the UI will remain responsive. But slow handlers might make your app hard to use.

!!! info

To re-use the chef analogy, if an order comes in for beef wellington (which takes a while to cook), orders may start to pile up and customers may have to wait for their meal. The solution would be to have another chef work on the wellington while the first chef picks up new orders.

Network access is a common cause of slow handlers. If you try to retrieve a file from the internet, the message handler may take anything up to a few seconds to return, which would prevent the widget or app from updating during that time. The solution is to launch a new asyncio task to do the network task in the background.

Let's look at an example which looks up word definitions from an api as you type.

!!! note

You will need to install [httpx](https://www.python-httpx.org/) with `pip install httpx` to run this example.

=== "dictionary.py"

```python title="dictionary.py" hl_lines="26"
--8<-- "docs/examples/events/dictionary.py"
```

=== "dictionary.css"

```python title="dictionary.css" 
--8<-- "docs/examples/events/dictionary.css"
```

=== "Output"

```{.textual path="docs/examples/events/dictionary.py" press="tab,t,e,x,t,_,_,_,_,_,_,_,_,_,_,_"}
```

Note the highlighted line in the above code which calls asyncio.create_task to run coroutine in the background. Without this you would find typing in to the text box to be unresponsive.