Events such as keydown and keyup are not triggered by an ui.input when not assigned directly upon creation

[Alyxion]

Description

During the development of a Typeahead component which attaches itself visually to the bottom of an ui.input field to (later) show search result data from a DB, i stumbled over a potential bug when modifying certain event_listeners such as keydown and keyup via the element.on method "post creation".

Situation:

• An ui.input field is created
• A focus handler is set up via .on("focus", handle_focus) and a blur_handler is setup via on("blur", handle_blur)
• In handle_focus the.on("keydown", handle_keydown) is assigned to track cursor key presses
• In handle_blur the event listener is removed again.
  • As NiceGUI doesn't officially support removal of event listeners added via .on yet there is a helper class involved which "cleans up again" / can unassign event listeners, but that just as side note and has no impact on this report / test as you can see in the example code attached below.
• Expected behavior: handle_keydown will be called when a key is pressed.
• Actual behavior: It is not

If a "keypress" or "keyup" listener is assigned later and not directly upon creation of the input field, so e.g. delayed by a timer or an event, then the handlers will never be called. If they are assigned directly upon creation, so before everything goes through the websocket from Python to JS, then they work though.

If I assign the same events directly via JavaScript to the DOM element w/ help of the new getHtmlElement function though then everything works perfectly fine. Thus there seems to be some synchronization kind of issue for certain events. A delayed assigned blur event just for example works like a charm.

from nicegui import ui

@ui.page('/')
async def index():
    log_area = ui.scroll_area().style('height: 200px; width: 400px')

    def log_key(e):
        with log_area:
            ui.label(f"Key pressed: {e.args['key']}").classes('font-mono text-xs').style('line-height: 1; margin: 0; padding: 0;')
        log_area.scroll_to(percent=1.0)

    with ui.row():
        input1 = ui.input()
        ui.run_javascript(f'''
            getHtmlElement({input1.id}).addEventListener("keydown", () => console.log("keydown on input1 via js"));
            ''')
        input1.on('keydown', log_key)   # will be triggered
        input1.on('keydown', js_handler='() => console.log("keydown on input1")')  # will be triggered
        input2 = ui.input()

    def attach_event():
        input2.on('keydown', log_key)  # will NOT be triggered
        input2.on('keydown', js_handler='() => console.log("keydown on input2")')  # will NOT be triggered
        input2.on('blur', lambda e: print('blur'))  # will be triggered
        ui.run_javascript(f'''
            getHtmlElement({input2.id}).addEventListener("keydown", () => console.log("keydown on input2 via js"));
            ''')  # will be triggered    

    # setup timer which attaches the event
    ui.timer(1.0, attach_event, once=True, immediate=False)
    
ui.run()

[falkoschindler]

Interesting observation, @Alyxion!

This handler does not work:

i = ui.input()
ui.timer(1.0, lambda: i.on('keydown', ui.notify), once=True)

But button click handlers can be assigned this way. So it's not a general event registration problem.

b = ui.button()
ui.timer(1.0, lambda: b.on('click', ui.notify), once=True)

And as far as I can tell, the event listeners are registered correctly and sent to the client. But there is no websocket message sent to the server when a key is pressed. Very strange!

[falkoschindler]

Does anyone have an idea how to approach this issue? I wonder if it is caused by Vue or Quasar and the way they handle UI updates. But other elements, even similar ones like ui.editor and ui.codemirror work as expected. What is so special about ui.input?

[falkoschindler]

This works:

@ui.page('/')
async def page():
    i = ui.input()
    await asyncio.sleep(1.0)
    i.on('keydown', ui.notify)

But this doesn't:

@ui.page('/')
async def page():
    i = ui.input()
    await asyncio.sleep(1.0)
    i.on('keydown', ui.notify)

So it really depends on whether the event handler is part of the initial HTTP response, or sent via an update message later.

[falkoschindler]

I've got the feeling that the reason for this problem is related to #4087, where we noticed that q-input behaves differently than other Quasar element. It doesn't preserve its id attribute like other elements. And apparently Vue doesn't manage to update event handlers once the q-input tag has been translated into HTML.

Maybe this problem needs to be addressed upstream. A quick look into https://github.com/quasarframework/quasar/issues/ and https://github.com/quasarframework/quasar/discussions/ didn't yield any insights though.