Store callback Output value in persistence storage

[petar-qb]

Fixes #2678

This PR:

• Solves Component properties set through dynamic callbacks cannot be persisted #2678
• Solves the question from the Dash Plotly forum.

TLDR:
This PR ensures that the callback output value is written (instead of being pruned) within the browser's persistence storage for its dcc components that has persistence=True set.

---

For the following code (also posted in the Dash Plotly forum question that's linked above):

from dash import Dash, dcc, html, Input, Output

app = Dash(__name__)

app.layout = html.Div([
    html.Button("Select All", id="select_all_button"),
    dcc.Checklist(
        id='checkbox',
        options=[1, 2, 3],
        value=[],
        persistence=True,
        persistence_type='session'
    ),
])


@app.callback(
    Output('checkbox', 'value'),
    Input('select_all_button', 'n_clicks'),
    prevent_initial_call=True,
)
def select_all_options(n_clicks):
    return [1, 2, 3]


if __name__ == '__main__':
    app.run(debug=True)

Here's how it works from the plotly/dash::dev:

Screen.Recording.2025-01-30.at.11.07.24.mov

Here's how it works from the petar-qb/dash::feature/store-output-value-in-persistence-storage:

Screen.Recording.2025-01-30.at.11.09.25.mov

---

Contributor Checklist

• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follows
  • this GitHub #PR number updates the dash docs
  • here is the show and tell thread in Plotly Dash community

[T4rk1n]

Looks good, but there is a failing test rdps011. I am not sure if it's because the behavior changed to persist value on callbacks or if it's a regression.

Maybe @alexcjohnson can provide more insights on this test?

[T4rk1n]

What did this do? It's inclusion/removal doesn't seem to have an effect.

[petar-qb]

The issue with this code is that it clears the persisted value before the new selection is written via recordUiEdit, once the callback is triggered. For example, if the storage initially contains [[1, 2], []] and callback_select_all (which selects [1, 2, 3]) is triggered:

1. The removed code clears the persistence storage prematurely which causes the original value (originalVal) to be lost, which is problematic.
2. Then recordUiEdit writes [[1, 2, 3], [1, 2]] into storage.

This is incorrect because what should be persisted is [[1, 2, 3], []].

I might need to handle this differently. What do you think?