Order of creation¶
By default, LVGL draws old objects on the background and new objects on the foreground.
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*/ lv_obj_del(label2);
Bring to the foreground¶
There are several ways to bring an object to the foreground:
lv_obj_set_top(obj, true). If
objor 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.
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.
lv_obj_set_parent(obj, new_parent)is used,
objwill be on the foreground on the
Top and sys layers¶
LVGL uses two special layers named as
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 (
layer_sys is on top of
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.
layer_sys is also used for a similar purpose on LVGL. For example, it places the mouse cursor above all layers to be sure it's always visible.