---
title: Introduction to the Event System
description: The Nexios event system provides a powerful way to implement loosely coupled, event-driven architectures in your applications. It allows components to communicate without direct dependencies, making your code more maintainable and flexible.
head:
  - - meta
    - property: og:title
      content: Introduction to the Event System
  - - meta
    - property: og:description
      content: The Nexios event system provides a powerful way to implement loosely coupled, event-driven architectures in your applications. It allows components to communicate without direct dependencies, making your code more maintainable and flexible.
---
# Introduction to the Event System

The Nexios event system provides a powerful way to implement loosely coupled, event-driven architectures in your applications. It allows components to communicate without direct dependencies, making your code more maintainable and flexible.

##  Basic Event Usage

```python
from nexios import NexiosApp

app = NexiosApp()

@app.events.on("user.created")
async def handle_user_created(user):
    print(f"User created: {user['name']}")

# Trigger the event
app.events.emit("user.created", {"name": "Bob"})
```
At its core, Nexios events implement the publish-subscribe (pub-sub) pattern .

##  Subscribing to Events

To subscribe to an event, use the `on` method:

```python{3}
from nexios import NexiosApp
app = NexiosApp()
@app.events.on("user.created")
async def handle_user_created(user):
    print(f"User created: {user['name']}")
```

This allows you to register a function that will be called when the "user.created" event is emitted.

## Emitting Events

To emit an event, use the `emit` method:

```python{3}
@app.post("/users")
async def create_user(req, res):
    await app.events.emit("user.created", {"name": "Bob"})
    ...
```
This Endpoint will emit the "user.created" event when a new user is created
And the `handle_user_created` function will be called with the user data

## Managing Event Instances

Each event is associated with a specific event emitter instance. This means that you can create multiple event emitters and manage their events independently

the `NexiosApp` and `Router` class provides a default event emitter instance called `events` but you can also create your own event emitters


```python{3}
from nexios import NexiosApp
from nexios.events import EventEmitter

app = NexiosApp()

emitter = EventEmitter("custom")

emitter.on("user.created")
async def handle_user_created(user):
    print(f"User created: {user['name']}")

emitter.emit("user.created", {"name": "Bob"})
```
## Removing Event Listeners

You can remove event listeners when they're no longer needed:

```python
# Define handler
async def temporary_handler(data):
    print(f"Processing data: {data}")

# Add handler
app.events.on("data.received", temporary_handler)

# Later, remove the handler
app.events.off("data.received", temporary_handler)

# Or remove all handlers for an event
app.events.off("data.received")
```

## Priority Listeners

You can set a priority for event listeners. The higher the priority, the earlier the listener is called. By default, listeners are called in the order they are added

```python
from nexios.events import EventPriority
events.on("data.received", temporary_handler, priority=EventPriority.LOW)
events.on("data.received", temporary_handler, priority=EventPriority.MEDIUM)
events.on("data.received", temporary_handler, priority=EventPriority.HIGH)
```

## One-time Listeners

```python
@emitter.once('first.login')  # Special decorator
def first_login(user):
    print(f" Welcome {user}")

emitter.emit('first.login', 'Alice')  # Fires
emitter.emit('first.login', 'Alice')  # Doesn't fire
```

## Namespaces

For more complex applications, you can create separate event namespaces:

```python
from nexios.events import EventEmitter
app = EventEmitter()
ui = app.namespace('ui')  # Creates 'ui' namespace

@ui.on('button.click')  # Actually listens to 'ui:button.click'
def handle_click(btn):
    print(f"{btn} clicked!")

# All these work:
ui.emit('button.click', 'submit')
app.emit('ui:button.click', 'submit')  # Same as above

```

## sync Events
Nexios Supports Async Events

```python
from nexios.events import AsyncEventEmitter

events = AsyncEventEmitter()
@events.on('user.created')
async def handle_user_created(user):
    print(f"User created: {user['name']}")
```