Event

The Event class is a lightweight multicast delegate that supports both synchronous and asynchronous listeners.

This pattern is useful when you want to notify multiple callbacks of a single event, such as in plugin systems, GUI toolkits, or loosely coupled applications. It provides an elegant alternative to more verbose observer implementations or string-based event emitters.

Overview

Add a listener with += (function, method, or coroutine)
Remove a listener with -=
Trigger the event with event(args...) (non-blocking)
Use await event.call_async(...) to await all listeners

Example Usage

from pattern_kit import Event

def on_document_loaded(doc):
    print("Sync listener:", doc)

async def on_document_async(doc):
    print("Async listener:", doc)

on_new_document = Event()
on_new_document += on_document_loaded
on_new_document += on_document_async

# Fire-and-forget (async listeners will run in background)
on_new_document("report.pdf")

# Fully awaited (awaits async listeners)
await on_new_document.call_async("report.pdf")

API Reference

class pattern_kit.behavioral.event.Event[source]

Bases: object

A lightweight event object that acts like a multicast delegate.

Listeners can be added with +=, removed with -=, and all will be called when the event is triggered like a function: event(args…).

Supports both synchronous and asynchronous listeners.

async call_async(*args, **kwargs) → None[source]: Await all async listeners. Sync ones are called immediately.