Issue with ipyleaflet rendering

[thedbcooper]

Hi! I am having issues with rendering leaflet maps.

In Jupyter notebooks, adding a layer to an existing map works as expected. In Shiny, the LayersControl disappears.

Example Code:

requires-python = ">=3.12.12"
dependencies = [
    "geopandas>=1.1.1",
    "ipyleaflet>=0.20.0",
    "requests>=2.32.5",
    "shiny>=1.5.0",
    "shinywidgets>=0.7.0",
]

from shiny import App, ui, reactive
from shinywidgets import output_widget, render_widget
from ipyleaflet import Map, LayersControl, Marker

app_ui = ui.page_fluid(
    ui.h2("Ipyleaflet Layer Fix"),
    ui.input_action_button("add_layer", "Add Marker Layer"),
    output_widget("map")
)

def server(input, output, session):
    # 1. Initialize the map
    m = Map(center=(50, 0), zoom=5)

    # Add the initial LayersControl
    control = LayersControl(position='topright')
    m.add_control(control)

    @render_widget
    def map():
        return m

    # 2. Use an Effect to update the map (instead of re-rendering it)
    @reactive.Effect
    @reactive.event(input.add_layer)
    def _():
        # Add your new layer(s)
        new_marker = Marker(location=(50, 0), draggable=True)
        m.add_layer(new_marker)

app = App(app_ui, server)

[cpsievert]

This smells like a bug that should get fixed, but a workaround in the meantime would be create any Marker()s outside of the @reactive.Effect, like this:

 new_marker = Marker(location=(50, 0), draggable=True)
@reactive.Effect
@reactive.event(input.add_layer)
def _():
    m.add_layer(new_marker)

If you're willing, I'd very much a appreciate if you report this over at https://github.com/posit-dev/py-shinywidgets/issues/new