[feature request] improvements to triggering events (such as a button click)

[sebovzeoueb]

I think it would be nice to be able to raise an input event without incrementing a number. To start with it's not an intuitive system, and it would allow for passing values with the event.

It would make lists of items simpler to implement and I think being able to raise arbitrary events from code would be a good feature. It would differ from the current reactive value method in that you could trigger an event for the same value multiple times, currently a change in value is required to trigger a reactive update.

Consider the following test app I made while getting help on Discord:

from shiny import module, reactive, ui, render, Inputs, Outputs, Session, App
from shiny._utils import rand_hex

# ---
# MOCK DATABASE
# ---

# imagine this is a database query result
items = [
    "item_1",
    "item_2",
    "item_3"
]

# this is a function that runs the query
def get_items():
    return items

# this function deletes items based on the query
def delete_item(index):
    del items[index]

# ---
# ITEM COMPONENT
# ---

@module.ui
def item_ui(item):
    return [
        ui.markdown(item),
        ui.input_action_button("delete_item", "Delete")
    ]

@module.server
def item_server(input: Inputs, output: Outputs, session: Session, index, on_delete_callback):

    @reactive.effect
    @reactive.event(input.delete_item, ignore_init=True)
    def on_delete():
        delete_item(index)
        on_delete_callback()

# ---
# MAIN
# ---

app_ui = ui.page_auto(
    ui.output_ui("item_list"),
    ui.input_action_button("refresh", "Refresh")
)

def app_server(input: Inputs, output: Outputs, session: Session):

    items = reactive.value([])
    delete_trigger = reactive.value(0)

    def on_delete_callback():
        delete_trigger.set(delete_trigger.get() + 1)

    # this needs to trigger when an item is deleted
    # currently we can manually trigger to show that backend is working as intended
    @reactive.effect
    @reactive.event(input.refresh, delete_trigger, ignore_none=False)
    def on_change_items():
        # we create a random ID to sync UI and server for each item
        items.set([{"item": item, "id": rand_hex(4)} for item in get_items()])

    @reactive.effect
    def item_servers():
        for index, item in enumerate(items.get()):
            item_server(item["id"], index, on_delete_callback)

    @render.ui
    def item_list():
        return [item_ui(item["id"], item["item"]) for item in items.get()]
    
app = App(app_ui, app_server)

Handling the list of items is needlessly unwieldy here. With my proposal you could do this:

from shiny import reactive, ui, render, Inputs, Outputs, Session, App
from shiny._utils import rand_hex

# ---
# MOCK DATABASE
# ---

# imagine this is a database query result
items = [
    "item_1",
    "item_2",
    "item_3"
]

# this is a function that runs the query
def get_items():
    return items

# this function deletes items based on the query
def delete_item(index):
    del items[index]

# ---
# MAIN
# ---

app_ui = ui.page_auto(
    ui.output_ui("item_list"),
    ui.input_action_button("refresh", "Refresh")
)

def app_server(input: Inputs, output: Outputs, session: Session):

    items = reactive.value([])

    # this needs to trigger when an item is deleted
    # currently we can manually trigger to show that backend is working as intended
    @reactive.effect
    @reactive.event(input.refresh, input.delete_item, ignore_none=False)
    def on_change_items():
        # we create a random ID to sync UI and server for each item
        items.set([{"item": item, "id": rand_hex(4)} for item in get_items()])

    @render.ui
    def item_list():
        return [[
            ui.markdown(item["item"]),
            ui.input_action_button("delete_item", "Delete", value=index)  
        ] for index, item in enumerate(items.get())]
    
    @reactive.effect
    @reactive.event(input.delete_item, ignore_init=True)
    def on_delete():
        # the input will be set to the index to delete by the button click
        delete_item(input.delete_item())
    
app = App(app_ui, app_server)

quite a lot less code and all contained within the UI and server.

In addition to being able to pass values to button events, you could add a function like raise_event("delete_item", i) to be able to trigger on_delete from somewhere else in the server code without needing a reactive.value