How to pass an Python array as event user_data

[amirgon]

@embeddedt

I believe @amirgon has recommended that one avoids user_data in MicroPython code. You don’t need it since you can pass anything that’s callable as a callback.

That’s correct.

Here is a previous discussion on the subject:

V8.1 msgbox... activated by multiple buttons

I strongly advise against using user_data argument in Micropython for that purpose. user_data is used internally by the Micropython Bindings. You should keep the user_data argument as None in 99% of the cases. It should only be used in cases where the caller on the C side needs to examine the user_data and extract some information from it, which is never the case on LVGL callbacks. You can easily achieve what you want very nicely in other ways. Remember that in Python, as opposed to C, you…

@kisvegabor

Probably I’m over complicating it and thinking to C-ish, but the full scenario is to store arbitrary data (the a array in the example) in some objects and get them them later.

• Do you want to attach some data to an LVGL object?
• Or do you want to pass context to a callback?

To pass some extra context to an event handler, the simplest way is to use a lambda as explained above. This allows you to pass any number of arguments to the event callback function (arguments which you set when you add the event callback).

@kisvegabor

I wonder if in MP we can do something like:

btn1.new_memer = my_array

If you want to “assign” some values to an LVGL object, the simplest way is to inherit from the LVGL object and add members to your derived objects. For a derived object to keep its identity you need to set user_data to point to the derived object instead of the base object.

For example:

class MyBtn(lv.btn):
    def __init__(self, parent, additional_data):
        super().__init__(parent)
        self.additional_data = additional_data
        self.set_user_data(self)

Then you can use btn.additional_data anywhere, also in an event callbacks. (Online example)

(if you don’t set user_data in MyBtn constructor it will still work, but when calling evt.get_target() you would get the parent lv.btn instead of MyBtn and you will not have access to your data members)

We use this technique today when creating custom widgets in Micropython:

lvgl/lv_binding_micropython/blob/28109f072a05c35de67b21edf20c7300a7ee9ceb/examples/custom_widget_example.py#L154