Resistance calculator: app causes Flipper to hang on the `dev` firmware

[instantiator]

When running on the dev firmware, the app causes the Flipper to hang when attempting to show the resistor editing scene.

To reproduce

• Switch ufbt over to the dev SDK
• Use qFlipper to flash your Flipper with the dev firmware
• Launch the app
• Select either of the resistor types from the main menu
• App (and Flipper) hangs indefinitely

NB. Back button + left will restart the Flipper.

[instantiator]

Debugging help and commentary from @jamisonderek

So the issue is:

widget_alloc does...

view_set_input_callback(widget->view, ...)

then later you do...

view_set_input_callback(widget_get_view(app->widget), widget_input_callback);

which does..

void view_set_input_callback(View* view, ViewInputCallback callback) {
    furi_assert(view);
    furi_assert(view->input_callback == NULL); <<<<<

For your issue, gui_widget_view_input_callback calls all of the widget's input handlers. Ideally you would create something new like widget_element_button_action that would invoke a callback when a button was pressed. Unfortunately, widget_add_element is static. 😦

[instantiator]

Working through this, the issue is that:

• widget_alloc calls view_set_input_callback and provides gui_widget_view_input_callback
• gui_widget_view_input_callback works its way through the various elements in the widget, calling the input handler for each
• later, where I attempt to replace that input handler with my own call to view_set_input_callback, to handle the various directional button presses, it fails on furi_assert(view->input_callback == NULL); (here).

Therefore, under circumstances where furi_assert is enforced (eg. the dev firmware?) it's not possible to assign an alternate input callback.

It also doesn't seem possible to modify any of the various structs directly:

• Widget is defined in widget.c
• View is defined in view_i.h (internal only)
• WidgetElement is defined in widget_element_i.h (internal only)

It's also not possible to craft a new element type, and add it to the widget, as:

• widget_add_element is static (ie. private).

So, the only option seems to be to add a button:

• widget_add_button_element is the only element add function with an input callback parameter
  • ButtonCallback is defined in widget_element.h

[instantiator]

An alternative is to modify the firmware to give some more flexbility to developers.

option 1

Modify the firmware to support definition of new elements, and their addition to widgets, ie.

• modify widget_add_element to not be static
• modify Widget and WidgetElement to be accessible
• create a new element that can act on the input

option 2 (simpler)

Alter View to permit direct alteration of the input callback, ie.

• Remove the furi_assert(view->input_callback == NULL); constraint in view_set_input_callback

[instantiator]

Filed the simplest possible PR against the firmware:
flipperdevices/flipperzero-firmware#2623