RFP: Async support in Flet Python client

Discussed in https://github.com/flet-dev/flet/discussions/72

Problem

It is currently impossible to call async method from Flet event handlers. They are synchronous functions run in separate threads. This limitation requires to use bulky constructs like:

search_data  = new_event_loop().run_until_complete(get_weather(search_value))

where get_weather(search_value) is async function.

Solution

As in other languages like C# a “proper” async must be implemented from ground up - it’s impossible/ineffective to partially support calling async methods from sync ones. The entire program must be running in an event loop (in Python terms).

Flet Python client must be re-implemented to use asyncio library.

Sample code

Flet client will be providing both sync and async methods, for example async version of “Hello, world” might look like:

import asyncio
import flet as ft

async def main(page: ft.Page):

    async def btn_click(e):
        var result = await some_call_to_rest_api()
        page.controls.append(ft.Text(result))
        await page.update_async()

    await page.add_async(
        ft.Text("Hello!"),
        ft.ElevatedButton("Call async method", on_click=btn_click)
    )

ft.app(target=main)

All async methods will have _async suffix.

WebSockets client will be implemented with websockets library based on asyncio.

Resources:

• https://docs.python.org/3/library/asyncio.html

Flet API

Sync is going to be default style of writing Flet apps. Flet API that needs to be available in both sync and async variants:

• Connection class - have a base Connection class and two implementations: SyncConnection and AsyncConnection:
  • def connect()
  • async def connect_async()
  • def send_command(self, session_id: str, command: Command)
  • async def send_command_async(self, session_id: str, command: Command)
  • def send_commands(self, session_id: str, commands: List[Command])
  • async def send_commands_async(self, session_id: str, commands: List[Command])
  • def close(self)
  • on_event - could be either Callable or Coroutine
  • on_session_created - could be either Callable or Coroutine
  • ReconnectingWebSocket must be created inside SyncConnection.
  • Can we use send_commands only in all cases?
• Page class:
  • TBD

[FeodorFitsner]

We go ahead with Async support in Flet: https://flet.dev/blog/flet-mobile-update#asyncio-support

[TutorExilius]

How wonderful, thanks! 😃