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73 lines
3.9 KiB
Markdown
73 lines
3.9 KiB
Markdown
# Events and Messages
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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.
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## Messages
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Events are a particular kind of *message* which is 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.
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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.
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## Message Queue
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Every Textual app and widget 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 first order that was added.
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Textual processes messages in the same way. Messages are picked off the message 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.
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This processing of messages is done within an asyncio Task which is started when you mount the widget. The task monitors an asyncio queue for new messages. When a message arrives, the task dispatches it to the appropriate handler method. Once all messages have been processed the task goes back to waiting for messages.
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If you aren't yet familiar with asyncio, you can consider this part to be black box and trust that Textual will get events to your handler methods.
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By way of an example, let's consider what happens if you were to type "Text" in to a text input widget. When you hit the ++t++ key it is translated in to a [key][textual.events.Key] event and sent to the widget's message queue. Ditto for ++e++, ++x++, and ++t++.
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The widget's task will pick the first key event from the queue (for the ++t++ key) and call the `on_key` handler to update the display.
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<div class="excalidraw">
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--8<-- "docs/images/events/queue.excalidraw.svg"
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</div>
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When the `on_key` method returns, Textual will get the next event off 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.
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!!! note
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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.
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<div class="excalidraw">
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--8<-- "docs/images/events/queue2.excalidraw.svg"
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</div>
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## Creating Messages
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## Handlers
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### Naming
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Let's explore how Textual decides what method to call for a given event.
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- Start with `"on_"`.
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- Add the messages namespace (if any) converted from CamelCase to snake_case plus an underscore `"_"`
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- Add the name of the class converted from CamelCase to snake_case.
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### Default behaviors
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You may be familiar with using Python's [super](https://docs.python.org/3/library/functions.html#super) function to call a function defined in a base class. You will not have to do this for Textual event handlers as Textual will automatically call any handler methods defined in the base class *after* the current handler has run. This allows textual to run any default behavior for the given event.
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For instance if a widget defines an `on_key` handler it will run when the user hits a key. Textual will also run `Widget.on_key`, which allows Textual to respond to any key bindings. This is generally desirable, but you can prevent Textual from running the base class handler by calling [prevent_default()][textual.message.Message.prevent_default] on the event object.
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For the case of key events, you may want to prevent the default behavior for keys that you handle by calling `event.prevent_default()`, but allow the base class to handle all other keys.
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### Bubbling
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<hr>
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TODO: events docs
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- What are events
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- Handling events
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- Auto calling base classes
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- Event bubbling
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- Posting / emitting events
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