Previous value of the element

[me21]

Question

Hello everyone,

Is there a way to obtain the previous value of the element inside the on_change handler?
I would like to keep two date inputs in sync with each other by default, so that whenever the first is changed by the user, the second one is changed as well; but if the user explicitly changes the second one's value, they are no longer in sync.

[falkoschindler]

Hi @me21,

What is wrong with this straightforward approach:

a = ui.date()
b = ui.date().bind_value(a)  # two-way sync

or

a = ui.date()
b = ui.date().bind_value_from(a)  # one-way sync

Apart from that, I created PR #4456 to add a previous_value field to ValueChangeEventArguments. Backward-compatibility is still a minor issue, but in general it is a rather trivial change.

[me21]

Thank you.
I would like to break the binding when the user manually changes the second date. Is it possible to achieve?

[falkoschindler]

Out of the top of my head I don't see a way to do that with bindings. With change handlers it seems to be possible though:

sync = {'active': True}

def handle_change_a():
    if sync['active']:
        b.value = a.value
        sync['active'] = True  # after handle_change_b

def handle_change_b():
    sync['active'] = False

a = ui.date(on_change=handle_change_a)
b = ui.date(on_change=handle_change_b)

[falkoschindler]

PR #4456 has been merged and will be released as part of version 3.0.