OBJ ID

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

Usage

Enable this feature by setting LV_USE_OBJ_ID to 1 in lv_conf.h. Use the builtin obj ID generator by setting LV_USE_OBJ_ID_BUILTIN to 1. Otherwise provide your own custom implementation.

The ID is automatically generated and assigned to obj->id during obj's construction by calling API lv_obj_assign_id(obj) from lv_obj_constructor.

You can directly access the ID by obj->id or use API lv_obj_stringify_id(obj, 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_assign_id(const lv_obj_class_t * class_p, lv_obj_t * obj);
void lv_obj_free_id(lv_obj_t * obj);
const char * lv_obj_stringify_id(lv_obj_t * obj, char * buf, uint32_t len);

:c:func:lv_obj_assign_id is called when an object is created. The object final class is passed from parameter class_p. Note it may be different than obj->class_p which is the class currently being constructed.

:c:func:lv_obj_free_id is called when object is deconstructed. Free any resource allocated in lv_obj_assign_id.

:c:func:lv_obj_stringify_id converts id to a string representation. The string is stored in buf.

Dump obj tree

Use API lv_obj_dump_tree(lv_obj_t * obj, int depth) to dump the object tree. It will walk through all children and print the object ID together with object 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.