feat(obj): add API to set/get object ID.

[XuNeo]

Description of the feature or fix

Add macro LV_USE_OBJ_ID_ASSIGN(default ON) to automatically assign ID to object after it's created.
Add API lv_obj_[get|set]_id.
Add API lv_obj_get_child_by_id . It could be useful for binding language.

Notes

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[kisvegabor]

Suggested change

	/* Automatically assign an ID when obj is created */
	#define LV_USE_OBJ_ID_ASSIGN 1
	# if LV_USE_OBJ_ID != 0
	/* Automatically assign an ID when obj is created. */
	#define LV_OBJ_ID_AUTO_ASSIGN 1

[XuNeo]

I found out we can use #define LV_OBJ_ID_AUTO_ASSIGN LV_USE_OBJ_ID So it's automatically enabled and always defined.

Do you think it's acceptable?

[liamHowatt]

I do think kisvegabor's suggestion matches the existing pattern in the config file. Configs that depend on another config are inside an #if ... #endif and indented.

[XuNeo]

The generated conf.h looks like this.

/* Automatically assign an ID when obj is created */
#ifndef LV_OBJ_ID_AUTO_ASSIGN
    #ifdef CONFIG_LV_OBJ_ID_AUTO_ASSIGN
        #define LV_OBJ_ID_AUTO_ASSIGN CONFIG_LV_OBJ_ID_AUTO_ASSIGN
    #else
        #define LV_OBJ_ID_AUTO_ASSIGN   LV_USE_OBJ_ID
    #endif
#endif

So LV_OBJ_ID_AUTO_ASSIGN is always defined, and default to 1 if LV_USE_OBJ_ID is enabled.

This is what we want(always define the macro with correct value).

[liamHowatt]

I understand that you want to use LV_OBJ_ID_AUTO_ASSIGN as a sort of compound macro that, when 1, means LV_USE_OBJ_ID && (!defined(LV_OBJ_ID_AUTO_ASSIGN) || (defined(LV_OBJ_ID_AUTO_ASSIGN) && LV_OBJ_ID_AUTO_ASSIGN != 0)).

Until now, using defines like LV_DEMO_MUSIC_LARGE when LV_USE_DEMO_MUSIC=0 is "undefined behavior". Code that uses LV_DEMO_MUSIC_LARGE first checks that LV_USE_DEMO_MUSIC == 1 before considering the value of LV_DEMO_MUSIC_LARGE. In your changes, you use LV_OBJ_ID_AUTO_ASSIGN as a shortcut.

It makes sense, but I cannot say whether it's okay to break the old pattern for the first time. @kisvegabor?

[XuNeo]

You are correct. This breaks the old pattern.
For system using Kconfig, it normally uses #ifdef instead of #if .

LVGL follow the latter style, thus it makes sense to discuss whether we should always define those macros, either enabled as 1 or disabled as 0.

I can follow the old pattern in this PR and raise another discussion if that's more appropriate.

[kisvegabor]

@liamHowatt just pointed out this unresolved thread.

If we use #if it would really make more sense to not add guards around the child configs. Let's keep it as it is in this feature, and discuss it a new issue.

[kisvegabor]

@liamHowatt please review and merge it if all look good.