As discussed on #557, #785, #316 and #935, callbacks must have a well defined structure that would allow an automatic script (for [micro]python bindings) find the related user_data of each callback.
A user_data is needed when more than one callback instance can be present for a function callback, and we need to correlate the right callback object of a higher level language with the C callback functions that was just fired.

A convention that we agreed upon was:

• The callback will receive as the first parameter the pointer to the structure that contains the user_data
• The user_data will be named according to the callback name (<callback>_user_data)

I checked and this convention solves some cases, but not all cases.

Here is a list of callbacks which currently do not follow this convention:

void lv_indev_feedback_t(struct _lv_indev_t *, uint8_t)
int32_t lv_anim_path_t(const struct _lv_anim_t *)
void lv_anim_fp_t(void *, int32_t)
void lv_anim_cb_t(void *)
void lv_group_style_mod_func_t(struct _lv_group_t *, lv_style_t *)
void lv_group_focus_cb_t(struct _lv_group_t *)
lv_res_t lv_img_decoder_info_f_t(const void *src, lv_img_header_t *header)
const uint8_t *lv_img_decoder_open_f_t(const void *src, const lv_style_t *style)
lv_res_t lv_img_decoder_read_line_f_t(lv_coord_t x, lv_coord_t y, lv_coord_t len, uint8_t *buf)
void lv_img_decoder_close_f_t(void)
lv_res_t lv_tileview_action_t(lv_obj_t *, lv_coord_t, lv_coord_t)
void lv_spinbox_value_changed_cb_t(lv_obj_t *spinbox, int32_t new_value)
void mem_blend_cb(lv_color_t *dest, const lv_color_t *src, uint32_t length, lv_opa_t opa)
void mem_fill_cb(lv_color_t *dest_buf, const lv_area_t *dest_area, const lv_area_t *fill_area, lv_color_t color)
void task(void *)
const uint8_t *get_bitmap(const struct _lv_font_struct *, uint32_t)
int16_t get_width(const struct _lv_font_struct *, uint32_t)
void cb(lv_obj_t *) // used on lv_obj_animate
void f(lv_log_level_t, const char *, uint32_t, const char *)

I would like to ask your opinion:

• Which functions do you think are important for [micro]python bindings? My opinion is that we should support as much as possible, including images, animations etc.
• Do we need user_data for each of these functions? In some cases (like logging) a single global callback is enough so we don't need a user_data per callback (or we can manage it globally, internally in the binding code)
• Where do you think we can add the missing user_data on each of these callbacks? Can we use the existing convention (and pass a struct as first parameter) or do we need some new convention (like passing the user_data itself to the callback)?