Layers

When the term "layer" is used in LVGL documentation, it may refer to one of several things:

  1. For Widgets, the Order of Creation creates a natural layering (Z-order) of Widgets sharing the same parent.

  2. In the context of pixel rendering (drawing), there are Draw Layers.

  3. Permanent Screen Layers are part of each Display (lv_display) object.

#1 is covered below. #2 and #3 are covered in Draw Layers and Screen Layers respectively.

Order of Creation

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

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

../_images/layers.png 
/* Create a screen */
lv_obj_t * scr = lv_obj_create(NULL);
lv_screen_load(scr);          /* Load the screen */

/* Create 2 buttons */
lv_obj_t * btn1 = lv_button_create(scr);     /* Create the first button on the screen */
lv_obj_set_pos(btn1, 60, 40);                /* Set the position of the first button */

lv_obj_t * btn2 = lv_button_create(scr);     /* Create the second button on the screen */
lv_obj_set_pos(btn2, 180, 80);               /* Set the position of the second button */

/* Add labels to the buttons */
lv_obj_t * label1 = lv_label_create(btn1);   /* 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);   /* 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_delete(label2);

Changing Order

There are three explicit ways to change a Widget's layer position (Z-order) among Widgets that share the same parent:

  • Use lv_obj_move_to_index(widget, idx) to move a Widget to a new index.

    0:

    Background

    child_count - 1:

    Foreground

    < 0:

    Count down from the top (-1 = topmost).

    lv_obj_get_index(widget) - 1:

    Move down 1 layer.

    lv_obj_get_index(widget) + 1:

    Move up 1 layer.

  • Use lv_obj_swap(widget1, widget2) to swap the layer positions (Z-orders) of the two Widgets.

  • When lv_obj_set_parent(widget, new_parent) is used, widget will become the foremost child of new_parent.

API

lv_obj_tree.h