lvgl/docs/overview/object.md

```eval_rst
.. include:: /header.rst 
:github_url: |github_link_base|/overview/object.md
```
# Objects

In LVGL the **basic building blocks** of a user interface are the objects, also called *Widgets*.
For example a [Button](/widgets/core/btn), [Label](/widgets/core/label), [Image](/widgets/core/img), [List](/widgets/extra/list), [Chart](/widgets/extra/chart) or [Text area](/widgets/core/textarea).

You can see all the [Object types](/widgets/index) here.

All objects are referenced using an `lv_obj_t` pointer as a handle. This pointer can later be used to set or get the attributes of the object.

## Attributes

### Basic attributes

All object types share some basic attributes:
- Position
- Size
- Parent
- Styles
- Event handlers
- Etc

You can set/get these attributes with `lv_obj_set_...` and `lv_obj_get_...` functions. For example:

```c
/*Set basic object attributes*/
lv_obj_set_size(btn1, 100, 50);	  /*Set a button's size*/
lv_obj_set_pos(btn1, 20,30);      /*Set a button's position*/
```

To see all the available functions visit the [Base object's documentation](/widgets/obj).

### Specific attributes

The object types have special attributes too. For example, a slider has
- Minimum and maximum values
- Current value

For these special attributes, every object type may have unique API functions. For example for a slider:

```c
/*Set slider specific attributes*/
lv_slider_set_range(slider1, 0, 100);	   				/*Set the min. and max. values*/
lv_slider_set_value(slider1, 40, LV_ANIM_ON);		/*Set the current value (position)*/
```

The API of the widgets is described in their [Documentation](/widgets/index) but you can also check the respective header files (e.g. *widgets/lv_slider.h*)

## Working mechanisms

### Parent-child structure

A parent object can be considered as the container of its children. Every object has exactly one parent object (except screens), but a parent can have any number of children.
There is no limitation for the type of the parent but, there are typical parent (e.g. button) and typical child (e.g. label) objects.

### Moving together

If the position of the parent changes the children will move with the parent.
Therefore all positions are relative to the parent.

![](/misc/par_child1.png "Objects are moving together 1")

```c
lv_obj_t * parent = lv_obj_create(lv_scr_act());   /*Create a parent object on the current screen*/
lv_obj_set_size(parent, 100, 80);	                 /*Set the size of the parent*/

lv_obj_t * obj1 = lv_obj_create(parent);	         /*Create an object on the previously created parent object*/
lv_obj_set_pos(obj1, 10, 10);	                     /*Set the position of the new object*/
```

Modify the position of the parent:

![](/misc/par_child2.png "Graphical objects are moving together 2")  

```c
lv_obj_set_pos(parent, 50, 50);	/*Move the parent. The child will move with it.*/
```

(For simplicity the adjusting of colors of the objects is not shown in the example.)

### Visibility only on the parent

If a child is partially or fully out of its parent then the parts outside will not be visible.

![](/misc/par_child3.png "A graphical object is visible on its parent")  

```c
lv_obj_set_x(obj1, -30);	/*Move the child a little bit off the parent*/
```

### Create and delete objects

In LVGL objects can be created and deleted dynamically in run time. It means only the currently created (existing) objects consume RAM.

This allows for the creation of a screen just when a button is clicked to open it, and for deletion of screens when a new screen is loaded.

UIs can be created based on the current environment of the device. For example one can create meters, charts, bars and sliders based on the currently attached sensors.
 
Every widget has its own **create** function with a prototype like this:
```c
lv_obj_t * lv_<widget>_create(lv_obj_t * parent, <other paramaters if any>);
```

In most of the cases the create functions have only a *parent* parameter that tells on which object create the new widget.

The return value is a pointer to the created object with `lv_obj_t *` type.


There is a common **delete** function for all object types. It deletes the object and all of its children.

```c
void lv_obj_del(lv_obj_t * obj);
```

`lv_obj_del` will delete the object immediately.
If for any reason you can't delete the object immediately you can use `lv_obj_del_async(obj)` that will perform the deletion on the next call of `lv_timer_handler()`.
This is useful e.g. if you want to delete the parent of an object in the child's `LV_EVENT_DELETE` handler.

You can remove all the children of an object (but not the object itself) using `lv_obj_clean(obj)`.

You can use `lv_obj_del_delayed(obj, 1000)` to delete an object after some time. The delay is expressed in millliseconds. 


## Screens

### Create screens
The screens are special objects which have no parent object. So they can be created like:
```c
lv_obj_t * scr1 = lv_obj_create(NULL);
```

Screens can be created with any object type. For example, a [Base object](/widgets/obj) or an image to make a wallpaper.

### Get the active screen
There is always an active screen on each display. By default, the library creates and loads a "Base object" as a screen for each display.

To get the currently active screen use the `lv_scr_act()` function. 

### Load screens

To load a new screen, use `lv_scr_load(scr1)`.

### Layers
There are two automatically generated layers:
- top layer
- system layer

They are independent of the screens and they will be shown on every screen. The *top layer* is above every object on the screen and the *system layer* is above the *top layer* too.
You can add any pop-up windows to the *top layer* freely. But, the *system layer* is restricted to system-level things (e.g. mouse cursor will be placed here in `lv_indev_set_cursor()`).

The `lv_layer_top()` and `lv_layer_sys()` functions return pointers to the top and system layers respectively.

Read the [Layer overview](/overview/layer) section to learn more about layers.


#### Load screen with animation

A new screen can be loaded with animation too using `lv_scr_load_anim(scr, transition_type, time, delay, auto_del)`. The following transition types exist:
- `LV_SCR_LOAD_ANIM_NONE`: switch immediately after `delay` milliseconds
- `LV_SCR_LOAD_ANIM_OVER_LEFT/RIGHT/TOP/BOTTOM` move the new screen over the current towards the given direction
- `LV_SCR_LOAD_ANIM_MOVE_LEFT/RIGHT/TOP/BOTTOM` move both the current and new screens towards the given direction
- `LV_SCR_LOAD_ANIM_FADE_ON` fade the new screen over the old screen

Setting `auto_del` to `true` will automatically delete the old screen when the animation is finished.

The new screen will become active (returned by `lv_scr_act()`) when the animations starts after `delay` time.

### Handling multiple displays
Screens are created on the currently selected *default display*.
The *default display* is the last registered display with `lv_disp_drv_register` or you can explicitly select a new default display using `lv_disp_set_default(disp)`.

`lv_scr_act()`, `lv_scr_load()` and `lv_scr_load_anim()` operate on the default screen.

Visit [Multi-display support](/overview/display) to learn more.

## Parts

The widgets are built from multiple parts. For example a [Base object](/widgets/obj) uses the main and scrollbar parts but a [Slider](/widgets/core/slider) uses the main, the indicator and the knob parts. 
Parts are similar to *pseudo elements* in CSS.

The following predefined parts exist in LVGL:
- `LV_PART_MAIN` A background like rectangle*/``
- `LV_PART_SCROLLBAR`  The scrollbar(s)
- `LV_PART_INDICATOR` Indicator, e.g. for slider, bar, switch, or the tick box of the checkbox
- `LV_PART_KNOB` Like a handle to grab to adjust the value*/
- `LV_PART_SELECTED` Indicate the currently selected option or section
- `LV_PART_ITEMS` Used if the widget has multiple similar elements (e.g. tabel cells)*/
- `LV_PART_TICKS` Ticks on scales e.g. for a chart or meter
- `LV_PART_CURSOR` Mark a specific place e.g. text area's or chart's cursor
- `LV_PART_CUSTOM_FIRST` Custom parts can be added from here.

The main purpose of parts to allow styling the "components" of the widgets. 
Therefore the parts are described in more detail in the [Style overview](/overview/style) section.

## States
The object can be in a combination of the following states:
- `LV_STATE_DEFAULT` Normal, released state
- `LV_STATE_CHECKED` Toggled or checked state
- `LV_STATE_FOCUSED` Focused via keypad or encoder or clicked via touchpad/mouse 
- `LV_STATE_FOCUS_KEY` Focused via keypad or encoder but not via touchpad/mouse 
- `LV_STATE_EDITED` Edit by an encoder
- `LV_STATE_HOVERED` Hovered by mouse (not supported now)
- `LV_STATE_PRESSED` Being pressed
- `LV_STATE_SCROLLED` Being scrolled
- `LV_STATE_DISABLED` Disabled state
- `LV_STATE_USER_1` Custom state
- `LV_STATE_USER_2` Custom state
- `LV_STATE_USER_3` Custom state
- `LV_STATE_USER_4` Custom state

The states are usually automatically changed by the library as the user presses, releases, focuses etc an object. 
However, the states can be changed manually too. 
To set or clear given state (but leave the other states untouched) use `lv_obj_add/clear_state(obj, LV_STATE_...)`
In both cases ORed state values can be used as well. E.g. `lv_obj_add_state(obj, part, LV_STATE_PRESSED | LV_PRESSED_CHECKED)`.

To learn more about the states read the related section of the [Style overview](/overview/style).