[中文]

Widget ID

LVGL provides an optional field in lv_obj_t to store the Widget ID. Widget ID can be used in many cases, for example, to identify the Widget. Or we can store a program backtrace to where the Widget is created.

Usage

Enable this feature by setting LV_USE_OBJ_ID to 1 in lv_conf.h.

Enable LV_OBJ_ID_AUTO_ASSIGN to automatically assign an ID to Widget when it's created. It's done by calling function lv_obj_assign_id() from lv_obj_constructor().

You can either use your own ID generator by defining the function lv_obj_assign_id() or you can utilize the built-in one. To use the builtin ID generator, set LV_USE_OBJ_ID_BUILTIN to 1.

You can directly access the ID by lv_obj_get_id(widget) or use API lv_obj_stringify_id(widget, buf, len) to get a string representation of the ID.

Use custom ID generator

Set LV_USE_OBJ_ID_BUILTIN to 0 in lv_conf.h.

Below APIs needed to be implemented and linked to lvgl.

void lv_obj_set_id(lv_obj_t * widget, void * id);
void lv_obj_assign_id(const lv_obj_class_t * class_p, lv_obj_t * widget);
void lv_obj_free_id(lv_obj_t * widget);
const char * lv_obj_stringify_id(lv_obj_t * widget, char * buf, uint32_t len);
int lv_obj_id_compare(void * id1, void * id2);

lv_obj_assign_id() is called when a Widget is created. The Widget final class is passed from parameter class_p. Note it may be different than obj->class_p which is the class currently being constructed.

lv_obj_free_id() is called when Widget is deconstructed. Free any resource allocated in lv_obj_assign_id().

lv_obj_stringify_id() converts id to a string representation. The string is stored in buf.

Dump Widget Tree

Use API lv_obj_dump_tree(lv_obj_t * widget, int depth) to dump the Widget Tree. It will walk through all children and print the Widget ID together with Widget address.

This is useful to debug UI crash. From log we can rebuilt UI the moment before crash. For example, if the obj is stored to a timer->user_data, but obj is deleted when timer expired. Timer callback will crash because of accessing wild pointer. From the dump log we can clearly see that the obj does not exist.

Find child by ID

Use API lv_obj_t * lv_obj_get_child_by_id(const lv_obj_t * widget, void * id) to find a child by ID. It will walk through all children and return the first child with the given ID. .. Autogenerated

lv_obj.h