Create mesop CLI to support Bazel-agnostic hot reload

[wwwillchen]

After supporting pip install mesop (#41), one of the issues with running Mesop on pip is that it doesn't support hot reload. Right now, Mesop's hot reload mechanism relies on ibazel which injects the livereload script and sends a rebuild event.

Questions:

• How do we trigger a rebuild? Whenever an app file changes. This isn't going to be as good as ibazel where there's a proper dependency graph. We will do a simple heuristic if any file within a directory or sub-directory of the main module is changed, then we will reload everything. This is the same heuristic used inside

  mesop/mesop/cli/execute_module.py

  Line 44 in 541dcd4

  def get_app_modules(

• How do we send an event? Ideally, we will use websockets, otherwise we can do some sort of polling on the client-side.

Tasks:

• Create a new CLI module (?) which supports mesop run foo.py
• Make sure pip package points to CLI module (probably modify setup.py)
• Create a new build event communication channel from server to client (e.g. websocket or polling)
• Update https://github.com/google/mesop/blob/main/mesop/web/src/services/hot_reload_watcher.ts to support both ibazel and CLI mode

Prior Art

Streamlit

• https://github.com/streamlit/streamlit/blob/develop/lib/streamlit/watcher/__init__.py
• Uses Watchdog

Gradio

• https://github.com/gradio-app/gradio/blob/main/gradio/cli/commands/reload.py
• Doesn't seem to be heavily relying on third-party python modules

Home-grown

Basically create a while loop which checks file modified time.

    last_modified = os.path.getmtime(module_path)
    while True:
        # Get the current modification time
        current_modified = os.path.getmtime(module_path)

        # Compare the current modification time with the last modification time
        if current_modified != last_modified:
            # Update the last modification time
            last_modified = current_modified
            module = read_module(module_path)
            if module is not None:
                try:
                    runtime.set_module(module)
                    runtime.reload_all_sessions()
                except Exception as e:
                    runtime.log(
                        pb.ServerLog(
                            message=f"Hot reload error: Module '{module_path}' has {e}.\n${traceback.format_exc()}",
                            level=pb.ServerLog.ERROR,
                        )
                    )

        # Wait for 1 second before checking again
        await asyncio.sleep(1)