Order of creation

By default, LVGL draws new objects on top of old objects.

For example, assume we added a button to a parent object named button1 and then another button named button2. Then button1 (with its child object(s)) will be in the background and can be covered by button2 and its children.


/*Create a screen*/
lv_obj_t * scr = lv_obj_create(NULL, NULL);
lv_scr_load(scr);          /*Load the screen*/

/*Create 2 buttons*/
lv_obj_t * btn1 = lv_btn_create(scr, NULL);         /*Create a button on the screen*/
lv_btn_set_fit(btn1, true, true);                   /*Enable to automatically set the size according to the content*/
lv_obj_set_pos(btn1, 60, 40);              	   /*Set the position of the button*/

lv_obj_t * btn2 = lv_btn_create(scr, btn1);         /*Copy the first button*/
lv_obj_set_pos(btn2, 180, 80);                    /*Set the position of the button*/

/*Add labels to the buttons*/
lv_obj_t * label1 = lv_label_create(btn1, NULL);	/*Create a label on the first button*/
lv_label_set_text(label1, "Button 1");          	/*Set the text of the label*/

lv_obj_t * label2 = lv_label_create(btn2, NULL);  	/*Create a label on the second button*/
lv_label_set_text(label2, "Button 2");            	/*Set the text of the label*/

/*Delete the second label*/

Bring to the foreground

There are several ways to bring an object to the foreground:

  • Use lv_obj_set_top(obj, true). If obj or any of its children is clicked, then LVGL will automatically bring the object to the foreground. It works similarly to a typical GUI on a PC. When a window in the background is clicked, it will come to the foreground automatically.

  • Use lv_obj_move_foreground(obj) to explicitly tell the library to bring an object to the foreground. Similarly, use lv_obj_move_background(obj) to move to the background.

  • When lv_obj_set_parent(obj, new_parent) is used, obj will be on the foreground on the new_parent.

Top and sys layers

LVGL uses two special layers named as layer_top and layer_sys. Both are visible and common on all screens of a display. They are not, however, shared among multiple physical displays. The layer_top is always on top of the default screen (lv_scr_act()), and layer_sys is on top of layer_top.

The layer_top can be used by the user to create some content visible everywhere. For example, a menu bar, a pop-up, etc. If the click attribute is enabled, then layer_top will absorb all user click and acts as a modal.

lv_obj_set_click(lv_layer_top(), true);

The layer_sys is also used for similar purposes on LVGL. For example, it places the mouse cursor above all layers to be sure it's always visible.