PubSub

If you build a chat app using Flet you need somehow to pass user messages between sessions. When a user sends a message it should be broadcasted to all other app sessions and displayed on their pages.

Flet provides a simple built-in PubSub mechanism for asynchronous communication between page sessions.

Flet PubSub allows broadcasting messages to all app sessions or sending only to specific "topic" (or "channel") subscribers.

A typical PubSub usage would be:

• subscribe to broadcast messages or subscribe to a topic on app session start.
• send broadcast message or send to a topic on some event, like "Send" button click.
• unsubscribe from broadcast messages or unsubscribe from a topic on some event, like "Leave" button click.
• unsubscribe from everything on page.on_close.

This is an example of a simple chat application:

import flet as ft

def main(page: ft.Page):
    page.title = "Flet Chat"

    # subscribe to broadcast messages
    def on_message(msg):
        messages.controls.append(ft.Text(msg))
        page.update()

    page.pubsub.subscribe(on_message)

    def send_click(e):
        page.pubsub.send_all(f"{user.value}: {message.value}")
        # clean up the form
        message.value = ""
        page.update()

    messages = ft.Column()
    user = ft.TextField(hint_text="Your name", width=150)
    message = ft.TextField(hint_text="Your message...", expand=True)  # fill all the space
    send = ft.ElevatedButton("Send", on_click=send_click)
    page.add(messages, ft.Row(controls=[user, message, send]))

ft.app(target=main, view=ft.AppView.WEB_BROWSER)