> ## Documentation Index
> Fetch the complete documentation index at: https://docs.langbot.app/llms.txt
> Use this file to discover all available pages before exploring further.

# Component: Event Listener

LangBot's pipeline generates events during runtime for plugins to hook into and process. Each plugin can only have one event listener (components/event\_listener/default.yaml), but can register any number of events within it.

## Adding Event Listener Component

Execute the command `lbp comp EventListener` in the plugin directory. Creating an event listener component doesn't require any configuration.

```bash theme={null}
➜  HelloPlugin > lbp comp EventListener
Generating component EventListener...
Component EventListener generated successfully.
组件 EventListener 生成成功。
```

This will generate `default.yaml` and `default.py` files in the `components/event_listener/` directory. The `.yaml` file defines the basic information for the event listener, and the `.py` file is the event listener handler:

```bash theme={null}
➜  HelloPlugin > tree
...
├── components
│   ├── __init__.py
│   └── event_listener
│       ├── __init__.py
│       ├── default.py
│       └── default.yaml
...
```

## Manifest File: Event Listener

Since each plugin can only have one event listener, no modifications are needed in the manifest file.

## Plugin Processing

The following code is generated by default (components/event\_listener/default.py). You need to register and implement event processing logic in the `initialize` method of the `DefaultEventListener` class.

```python theme={null}
# Auto generated by LangBot Plugin SDK.
# Please refer to https://docs.langbot.app/en/plugin/dev/tutor.html for more details.
from __future__ import annotations

from langbot_plugin.api.definition.components.common.event_listener import EventListener
from langbot_plugin.api.entities import events, context


class DefaultEventListener(EventListener):

    async def initialize(self):
        await super().initialize()

        "Fill with your code here"
```

Adding listeners for specific events:

```python theme={null}
...
class DefaultEventListener(EventListener):

    async def initialize(self):
        await super().initialize()
        
        @self.handler(events.PersonMessageReceived)
        async def handler(event_context: context.EventContext):
            print("Hello LangBot Plugin!")
            print(event_context)
            
            await event_context.reply(
                platform_message.MessageChain([
                    platform_message.Plain(text=f"Hello from Nahida Plugin!"),
                ])
            )
```

This code registers a listener for the `PersonMessageReceived` (receiving any message from private chat) event, prints the event context (EventContext) information when the event is triggered, and calls the event context API to reply with a message.

EventContext stores common context information for this event trigger, while EventContext.event is an object of PersonMessageReceived (or other corresponding event types), storing detailed information about the event.

```python theme={null}
# Summary of EventContext definition
class EventContext(pydantic.BaseModel):
    """Event context, stores information for this event execution"""
    ...

    event: pydantic.SerializeAsAny[BaseEventModel]
    """Event object, specific type is the type specified when registering the handler, can be found in events.py"""
    
    ...
```

## Event Registration

Event registration is implemented through the `@self.handler` decorator, with the decorator parameter being the event type. All monitorable events are defined in `langbot_plugin.api.entities.builtin.events`.

## What's Next

You have learned the basic information about event listeners. Next, you can:

* Check the `List of Monitorable Events` and `Event Context APIs` in the documentation [Pipeline Events](/en/plugin/dev/apis/pipeline-events)
* Check out [Plugin Common APIs](/en/plugin/dev/apis/common)