Passing data between pages via a dcc.Store

[stevej2608]

Hi, I'm not sure if this is a Pages plugin issue or dcc.Store. I have a two page demo. One page accepts an email, the associated callback outputs the email to a store and forces a redirect:

login.py

email = dcc.Input(id='email', name='email', type='email', placeholder="Enter email")
button = html.Button('Sign In', type='submit', id='btn')
redirect = dcc.Location(id='redirect', refresh=True)

layout = html.Div([email, button, redirect])

@callback(Output('redirect', 'href'), Output('store', 'data'), State('email', 'value'), Input('btn', 'n_clicks'))
def email_cb(email, clicks):
    _redirect = NO_UPDATE
    _store = NO_UPDATE
    if clicks:
        _redirect = '/user'
        _store = {'email' : email}

    return _redirect, _store

The user page has a callback that accepts the stored email value and updates the page:

user.py

@callback(Output('user', 'children'), Input('store', 'data'))
def _login_cb(data):
    print('_login_cb')
    ctx = callback_context
    if not ctx.triggered:
        print('  NOT triggered')
    if data:
        print('  Valid store data available')
        return f"Hello {data['email']}"
    else:
        return NO_UPDATE

The page updates with the email correctly. The problem is ctx.triggered is false even when valid store data is available. The print message output is:

_login_cb
  NOT triggered
  Valid store data available

app.py

store = dcc.Store('store', storage_type='session')

app = dash.Dash(
    __name__, plugins=[dl.plugins.pages], external_stylesheets=[dbc.themes.BOOTSTRAP]
)

app.layout = dbc.Container(
    [store, dl.plugins.page_container],
    fluid=True,
)

Any ideas?

[AnnMarieW]

Hi @stevej2608 I don't see anything obviously wrong. Would you be able to share a link to your demo and I can take it for a spin?

[stevej2608]

Thanks for taking a look. Example code is here:

/docs/demos/multi_page_store

[AnnMarieW]

Hi @stevej2608 ,

This is a great question and thanks for the example. I can verify what you reported - An update to dcc.Store on one page does not trigger a callback when it's used as an input to a callback on another page.

This is not just a dash-labs pages plugin thing. It's the same with the "old" way of creating a multi-page dash app. I even checked the "old old" way in dash <v2 -- using @app.callback instead of @callback in the pages folder.

I don't think this is a bug though. It's intuitive to expect that it would trigger the callback since it's an Input (and not State). However, when the dcc.Store is updated on one page, the component from the other page that would trigger this callback does not exist in the browser - so it's not possible for that callback to be triggered.

Based on your example, you are trying to get authentication working. You may find this example helpful #90 (comment)