.. _layers:

======
Layers
======

In LVGL "layers" can be interpreted in various ways:

1. The order of widget creation naturally creates a layering of widgets
2. Permanent screen-sized layers can be also used
3. For some draw operations LVGL renders a widget and all its children into a buffer (a.k.a. layer) first

.. _layers_creation:

Order of creation
*****************

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

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

.. image:: /misc/layers.png

.. code:: c

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

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

   lv_obj_t * btn2 = lv_button_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_delete(label2);

.. _layers_order:

Change order
------------

There are four explicit ways to bring an object to the foreground:

- Use :cpp:expr:`lv_obj_move_foreground(obj)` to bring an object to the foreground.
  Similarly, use :cpp:expr:`lv_obj_move_background(obj)` to move it to the background.
- Use :cpp:expr:`lv_obj_move_to_index(obj, idx)` to move an object to a given index in the order of children.

  - ``0``: background
  - ``child_num - 1``: foreground
  - ``< 0``: count from the top, to move forward (up): :cpp:expr:`lv_obj_move_to_index(obj, lv_obj_get_index(obj) - 1)`

- Use :cpp:expr:`lv_obj_swap(obj1, obj2)` to swap the relative layer position of two objects.
- When :cpp:expr:`lv_obj_set_parent(obj, new_parent)` is used, ``obj`` will be on the foreground of the ``new_parent``.

Screen-like layers
******************
.. _layers_top_and_sys:

Top and sys layers
------------------

LVGL uses two special layers named ``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 (:cpp:func:`lv_screen_active`), and
``layer_sys`` is on top of ``layer_top``.

The get these layers use :cpp:func:`lv_layer_top` and :cpp:func:`lv_layer_sys`.

These layers work like any other widget, meaning the can be styles, scrolled,
and any kind of widgets can be created on them.

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 clicks and
acts as a modal.

.. code:: c

   lv_obj_add_flag(lv_layer_top(), LV_OBJ_FLAG_CLICKABLE);

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

.. _layers_bottom:

Bottom layers
-------------

Similarly top and sys. layer bottom layer is also screen size but
it's located below the active screen. It's visible only if the active screen's
background opacity is < 255.

The get the bottom layer use :cpp:func:`lv_layer_bottom`.

Draw layers
***********

Some style properties make LVGL to allocate a buffer and render a widget and its children there first. Later that layer will be merged to the screen or its parent layer after applying some transformations or other modifications.

Simple layer
------------

The following style properties trigger the creation of a "Simple layer":

- ``opa_layered``
- ``bitmap_mask_src``
- ``blend_mode``


In this case widget will be sliced into ``LV_DRAW_SW_LAYER_SIMPLE_BUF_SIZE`` sized chunks.

If there is no memory for a new chunk, LVGL will try allocating layer when another chunk is rendered and freed.


Transformed layer
---------------

When the widget is transformed a larger part of the widget needs to rendered to provide enough data for transformation. LVGL tries to render as small area of the widget as possible, but due to the nature of transformations no slicing is possible in this case.


The following style properties trigger the creation of a "Transform layer":

- ``transform_scale_x``
- ``transform_scale_y``
- ``transform_skew_x``
- ``transform_skew_y``
- ``transform_rotate``

Clip corner
-----------

The ``clip_corner`` style property also makes LVGL to create a 2 layers with radius height for the top and bottom part of the widget.

.. _layers_api:

API
***
