Base object (lv_obj)

Overview

The 'Base Object' implements the basic properties of widgets on a screen, such as:

  • coordinates

  • parent object

  • children

  • contains the styles

  • attributes like Clickable, Scrollable, etc.

In object-oriented thinking, it is the base class from which all other objects in LVGL are inherited.

The functions and functionalities of the Base object can be used with other widgets too. For example lv_obj_set_width(slider, 100)

The Base object can be directly used as a simple widget: it's nothing more than a rectangle. In HTML terms, think of it as a <div>.

Coordinates

Only a small subset of coordinate settings is described here. To see all the features of LVGL (padding, coordinates in styles, layouts, etc) visit the Coordinates page.

Size

The object size can be modified on individual axes with lv_obj_set_width(obj, new_width) and lv_obj_set_height(obj, new_height), or both axes can be modified at the same time with lv_obj_set_size(obj, new_width, new_height).

Position

You can set the position relative to the parent with lv_obj_set_x(obj, new_x) and lv_obj_set_y(obj, new_y), or both axes at the same time with lv_obj_set_pos(obj, new_x, new_y).

Alignment

You can align the object on its parent with lv_obj_set_align(obj, LV_ALIGN_...). After this every x and y setting will be relative to the set alignment mode. For example, this will shift the object by 10;20 px from the center of its parent:

lv_obj_set_align(obj, LV_ALIGN_CENTER);
lv_obj_set_pos(obj, 10, 20);

//Or in one function
lv_obj_align(obj, LV_ALIGN_CENTER, 10, 20);

To align one object to another use: lv_obj_align_to(obj_to_align, obj_referece, LV_ALIGN_..., x, y)

For example, to align a text below an image: lv_obj_align_to(text, image, LV_ALIGN_OUT_BOTTOM_MID, 0, 10).

The following align types exist: ../_images/align.png

Parents and children

You can set a new parent for an object with lv_obj_set_parent(obj, new_parent). To get the current parent, use lv_obj_get_parent(obj).

To get a specific child of a parent use lv_obj_get_child(parent, idx). Some examples for idx:

  • 0 get the child created first

  • 1 get the child created second

  • -1 get the child created last

The children can be iterated lke this:

uint32_t i;
for(i = 0; i < lv_obj_get_child_cnt(parent); i++) {
  lv_obj_t * child = lv_obj_get_child(parent, i);
  /*Do something with child*/
}

lv_obj_get_index(obj) returns the index of the object in its parent. It is equivalent to the number of younger children in the parent.

You can bring an object to the foreground or send it to the background with lv_obj_move_foreground(obj) and lv_obj_move_background(obj).

You can change the index of an object in its parent using lv_obj_move_to_index(obj, index).

You can swap the position of two objects with lv_obj_swap(obj1, obj2).

Display and Screens

At the highest level of the LVGL object hierarchy is the display which represents the driver for a display device (physical display or simulator). A display can have one or more screens associated with it. Each screen contains a hierarchy of objects for graphical widgets representing a layout that covers the entire display.

When you have created a screen like lv_obj_t * screen = lv_obj_create(NULL), you can make it active with lv_scr_load(screen). The lv_scr_act() function gives you a pointer to the active screen.

If you have multiple displays, it's important to know that the screen functions operate on the most recently created display or the one explicitly selected with lv_disp_set_default.

To get an object's screen use the lv_obj_get_screen(obj) function.

Events

To set an event callback for an object, use lv_obj_add_event_cb(obj, event_cb, LV_EVENT_..., user_data),

To manually send an event to an object, use lv_event_send(obj, LV_EVENT_..., param)

Read the Event overview to learn more about events.

Styles

Be sure to read the Style overview. Here only the most essential functions are described.

A new style can be added to an object with the lv_obj_add_style(obj, &new_style, selector) function. selector is an ORed combination of part and state(s). E.g. LV_PART_SCROLLBAR | LV_STATE_PRESSED.

The base objects use LV_PART_MAIN style properties and LV_PART_SCROLLBAR with the typical backgroud style properties.

Flags

There are some attributes which can be enabled/disabled by lv_obj_add/clear_flag(obj, LV_OBJ_FLAG_...):

  • LV_OBJ_FLAG_HIDDEN Make the object hidden. (Like it wasn't there at all)

  • LV_OBJ_FLAG_CLICKABLE Make the object clickable by input devices

  • LV_OBJ_FLAG_CLICK_FOCUSABLE Add focused state to the object when clicked

  • LV_OBJ_FLAG_CHECKABLE Toggle checked state when the object is clicked

  • LV_OBJ_FLAG_SCROLLABLE Make the object scrollable

  • LV_OBJ_FLAG_SCROLL_ELASTIC Allow scrolling inside but with slower speed

  • LV_OBJ_FLAG_SCROLL_MOMENTUM Make the object scroll further when "thrown"

  • LV_OBJ_FLAG_SCROLL_ONE Allow scrolling only one snappable children

  • LV_OBJ_FLAG_SCROLL_CHAIN Allow propagating the scroll to a parent

  • LV_OBJ_FLAG_SCROLL_ON_FOCUS Automatically scroll object to make it visible when focused

  • LV_OBJ_FLAG_SNAPPABLE If scroll snap is enabled on the parent it can snap to this object

  • LV_OBJ_FLAG_PRESS_LOCK Keep the object pressed even if the press slid from the object

  • LV_OBJ_FLAG_EVENT_BUBBLE Propagate the events to the parent too

  • LV_OBJ_FLAG_GESTURE_BUBBLE Propagate the gestures to the parent

  • LV_OBJ_FLAG_ADV_HITTEST Allow performing more accurate hit (click) test. E.g. accounting for rounded corners

  • LV_OBJ_FLAG_IGNORE_LAYOUT Make the object positionable by the layouts

  • LV_OBJ_FLAG_FLOATING Do not scroll the object when the parent scrolls and ignore layout

  • LV_OBJ_FLAG_LAYOUT_1 Custom flag, free to use by layouts

  • LV_OBJ_FLAG_LAYOUT_2 Custom flag, free to use by layouts

  • LV_OBJ_FLAG_WIDGET_1 Custom flag, free to use by widget

  • LV_OBJ_FLAG_WIDGET_2 Custom flag, free to use by widget

  • LV_OBJ_FLAG_USER_1 Custom flag, free to use by user

  • LV_OBJ_FLAG_USER_2 Custom flag, free to use by user

  • LV_OBJ_FLAG_USER_3 Custom flag, free to use by user

  • LV_OBJ_FLAG_USER_4 Custom flag, free to use by user

Some examples:

/*Hide on object*/
lv_obj_add_flag(obj, LV_OBJ_FLAG_HIDDEN);

/*Make an object non-clickable*/
lv_obj_clear_flag(obj, LV_OBJ_FLAG_CLICKABLE);

Groups

Read the Input devices overview to learn more about Groups.

Objects are added to a group with lv_group_add_obj(group, obj), and you can use lv_obj_get_group(obj) to see which group an object belongs to.

lv_obj_is_focused(obj) returns if the object is currently focused on its group or not. If the object is not added to a group, false will be returned.

Extended click area

By default, the objects can be clicked only within their bounding area. However, this can be extended with lv_obj_set_ext_click_area(obj, size).

Events

  • LV_EVENT_VALUE_CHANGED when the LV_OBJ_FLAG_CHECKABLE flag is enabled and the object clicked (on transition to/from the checked state)

  • LV_EVENT_DRAW_PART_BEGIN and LV_EVENT_DRAW_PART_END is sent for the following types:

    • LV_OBJ_DRAW_PART_RECTANGLE The main rectangle

      • part: LV_PART_MAIN

      • rect_dsc

      • draw_area: the area of the rectangle

    • LV_OBJ_DRAW_PART_BORDER_POST The border if the border_post style property is true

      • part: LV_PART_MAIN

      • rect_dsc

      • draw_area: the area of the rectangle

    • LV_OBJ_DRAW_PART_SCROLLBAR the scrollbars

      • part: LV_PART_SCROLLBAR

      • rect_dsc

      • draw_area: the area of the rectangle

Learn more about Events.

Keys

If LV_OBJ_FLAG_CHECKABLE is enabled, LV_KEY_RIGHT and LV_KEY_UP make the object checked, and LV_KEY_LEFT and LV_KEY_DOWN make it unchecked.

If LV_OBJ_FLAG_SCROLLABLE is enabled, but the object is not editable (as declared by the widget class), the arrow keys (LV_KEY_UP, LV_KEY_DOWN, LV_KEY_LEFT, LV_KEY_RIGHT) scroll the object. If the object can only scroll vertically, LV_KEY_LEFT and LV_KEY_RIGHT will scroll up/down instead, making it compatible with an encoder input device. See Input devices overview for more on encoder behaviors and the edit mode.

Learn more about Keys.

Example

Base objects with custom styles

C code  

 GitHub
#include "../../lv_examples.h"
#if LV_BUILD_EXAMPLES

void lv_example_obj_1(void)
{
    lv_obj_t * obj1;
    obj1 = lv_obj_create(lv_scr_act());
    lv_obj_set_size(obj1, 100, 50);
    lv_obj_align(obj1, LV_ALIGN_CENTER, -60, -30);

    static lv_style_t style_shadow;
    lv_style_init(&style_shadow);
    lv_style_set_shadow_width(&style_shadow, 10);
    lv_style_set_shadow_spread(&style_shadow, 5);
    lv_style_set_shadow_color(&style_shadow, lv_palette_main(LV_PALETTE_BLUE));

    lv_obj_t * obj2;
    obj2 = lv_obj_create(lv_scr_act());
    lv_obj_add_style(obj2, &style_shadow, 0);
    lv_obj_align(obj2, LV_ALIGN_CENTER, 60, 30);
}
#endif

MicroPython code  

 GitHub Simulator
obj1 = lv.obj(lv.scr_act())
obj1.set_size(100, 50)
obj1.align(lv.ALIGN.CENTER, -60, -30)

style_shadow = lv.style_t()
style_shadow.init()
style_shadow.set_shadow_width(10)
style_shadow.set_shadow_spread(5)
style_shadow.set_shadow_color(lv.palette_main(lv.PALETTE.BLUE))

obj2 = lv.obj(lv.scr_act())
obj2.add_style(style_shadow, 0)
obj2.align(lv.ALIGN.CENTER, 60, 30)


Make an object draggable

C code  

 GitHub
#include "../../lv_examples.h"
#if LV_BUILD_EXAMPLES

static void drag_event_handler(lv_event_t * e)
{
    lv_obj_t * obj = lv_event_get_target(e);

    lv_indev_t * indev = lv_indev_get_act();
    if(indev == NULL)  return;

    lv_point_t vect;
    lv_indev_get_vect(indev, &vect);

    lv_coord_t x = lv_obj_get_x(obj) + vect.x;
    lv_coord_t y = lv_obj_get_y(obj) + vect.y;
    lv_obj_set_pos(obj, x, y);
}


/**
 * Make an object dragable.
 */
void lv_example_obj_2(void)
{
    lv_obj_t * obj;
    obj = lv_obj_create(lv_scr_act());
    lv_obj_set_size(obj, 150, 100);
    lv_obj_add_event_cb(obj, drag_event_handler, LV_EVENT_PRESSING, NULL);

    lv_obj_t * label = lv_label_create(obj);
    lv_label_set_text(label, "Drag me");
    lv_obj_center(label);

}
#endif

MicroPython code  

 GitHub Simulator
def drag_event_handler(e):

    obj = e.get_target()

    indev = lv.indev_get_act()

    vect = lv.point_t()
    indev.get_vect(vect)
    x = obj.get_x() + vect.x
    y = obj.get_y() + vect.y
    obj.set_pos(x, y)


#
# Make an object dragable.
#

obj = lv.obj(lv.scr_act())
obj.set_size(150, 100)
obj.add_event_cb(drag_event_handler, lv.EVENT.PRESSING, None)

label = lv.label(obj)
label.set_text("Drag me")
label.center()


API

Typedefs

typedef uint16_t lv_state_t
typedef uint32_t lv_part_t
typedef uint32_t lv_obj_flag_t
typedef struct _lv_obj_t lv_obj_t

Enums

enum [anonymous]

Possible states of a widget. OR-ed values are possible

Values:

enumerator LV_STATE_DEFAULT
enumerator LV_STATE_CHECKED
enumerator LV_STATE_FOCUSED
enumerator LV_STATE_FOCUS_KEY
enumerator LV_STATE_EDITED
enumerator LV_STATE_HOVERED
enumerator LV_STATE_PRESSED
enumerator LV_STATE_SCROLLED
enumerator LV_STATE_DISABLED
enumerator LV_STATE_USER_1
enumerator LV_STATE_USER_2
enumerator LV_STATE_USER_3
enumerator LV_STATE_USER_4
enumerator LV_STATE_ANY

Special value can be used in some functions to target all states

enum [anonymous]

The possible parts of widgets. The parts can be considered as the internal building block of the widgets. E.g. slider = background + indicator + knob Note every part is used by every widget

Values:

enumerator LV_PART_MAIN

A background like rectangle

enumerator LV_PART_SCROLLBAR

The scrollbar(s)

enumerator LV_PART_INDICATOR

Indicator, e.g. for slider, bar, switch, or the tick box of the checkbox

enumerator LV_PART_KNOB

Like handle to grab to adjust the value

enumerator LV_PART_SELECTED

Indicate the currently selected option or section

enumerator LV_PART_ITEMS

Used if the widget has multiple similar elements (e.g. table cells)

enumerator LV_PART_TICKS

Ticks on scale e.g. for a chart or meter

enumerator LV_PART_CURSOR

Mark a specific place e.g. for text area's cursor or on a chart

enumerator LV_PART_CUSTOM_FIRST

Extension point for custom widgets

enumerator LV_PART_ANY

Special value can be used in some functions to target all parts

enum [anonymous]

On/Off features controlling the object's behavior. OR-ed values are possible

Values:

enumerator LV_OBJ_FLAG_HIDDEN

Make the object hidden. (Like it wasn't there at all)

enumerator LV_OBJ_FLAG_CLICKABLE

Make the object clickable by the input devices

enumerator LV_OBJ_FLAG_CLICK_FOCUSABLE

Add focused state to the object when clicked

enumerator LV_OBJ_FLAG_CHECKABLE

Toggle checked state when the object is clicked

enumerator LV_OBJ_FLAG_SCROLLABLE

Make the object scrollable

enumerator LV_OBJ_FLAG_SCROLL_ELASTIC

Allow scrolling inside but with slower speed

enumerator LV_OBJ_FLAG_SCROLL_MOMENTUM

Make the object scroll further when "thrown"

enumerator LV_OBJ_FLAG_SCROLL_ONE

Allow scrolling only one snappable children

enumerator LV_OBJ_FLAG_SCROLL_CHAIN

Allow propagating the scroll to a parent

enumerator LV_OBJ_FLAG_SCROLL_ON_FOCUS

Automatically scroll object to make it visible when focused

enumerator LV_OBJ_FLAG_SNAPPABLE

If scroll snap is enabled on the parent it can snap to this object

enumerator LV_OBJ_FLAG_PRESS_LOCK

Keep the object pressed even if the press slid from the object

enumerator LV_OBJ_FLAG_EVENT_BUBBLE

Propagate the events to the parent too

enumerator LV_OBJ_FLAG_GESTURE_BUBBLE

Propagate the gestures to the parent

enumerator LV_OBJ_FLAG_ADV_HITTEST

Allow performing more accurate hit (click) test. E.g. consider rounded corners.

enumerator LV_OBJ_FLAG_IGNORE_LAYOUT

Make the object position-able by the layouts

enumerator LV_OBJ_FLAG_FLOATING

Do not scroll the object when the parent scrolls and ignore layout

enumerator LV_OBJ_FLAG_LAYOUT_1

Custom flag, free to use by layouts

enumerator LV_OBJ_FLAG_LAYOUT_2

Custom flag, free to use by layouts

enumerator LV_OBJ_FLAG_WIDGET_1

Custom flag, free to use by widget

enumerator LV_OBJ_FLAG_WIDGET_2

Custom flag, free to use by widget

enumerator LV_OBJ_FLAG_USER_1

Custom flag, free to use by user

enumerator LV_OBJ_FLAG_USER_2

Custom flag, free to use by user

enumerator LV_OBJ_FLAG_USER_3

Custom flag, free to use by user

enumerator LV_OBJ_FLAG_USER_4

Custom flag, free to use by user

enum lv_obj_draw_part_type_t

type field in lv_obj_draw_part_dsc_t if class_p = lv_obj_class Used in LV_EVENT_DRAW_PART_BEGIN and LV_EVENT_DRAW_PART_END

Values:

enumerator LV_OBJ_DRAW_PART_RECTANGLE

The main rectangle

enumerator LV_OBJ_DRAW_PART_BORDER_POST

The border if style_border_post = true

enumerator LV_OBJ_DRAW_PART_SCROLLBAR

The scrollbar

Functions

void lv_init(void)

Initialize LVGL library. Should be called before any other LVGL related function.

void lv_deinit(void)

Deinit the 'lv' library Currently only implemented when not using custom allocators, or GC is enabled.

bool lv_is_initialized(void)

Returns whether the 'lv' library is currently initialized

lv_obj_t *lv_obj_create(lv_obj_t *parent)

Create a base object (a rectangle)

Parameters

parent -- pointer to a parent object. If NULL then a screen will be created.

Returns

pointer to the new object

void lv_obj_add_flag(lv_obj_t *obj, lv_obj_flag_t f)

Set one or more flags

Parameters
  • obj -- pointer to an object

  • f -- R-ed values from lv_obj_flag_t to set.

void lv_obj_clear_flag(lv_obj_t *obj, lv_obj_flag_t f)

Clear one or more flags

Parameters
  • obj -- pointer to an object

  • f -- OR-ed values from lv_obj_flag_t to set.

void lv_obj_add_state(lv_obj_t *obj, lv_state_t state)

Add one or more states to the object. The other state bits will remain unchanged. If specified in the styles, transition animation will be started from the previous state to the current.

Parameters
  • obj -- pointer to an object

  • state -- the states to add. E.g LV_STATE_PRESSED | LV_STATE_FOCUSED

void lv_obj_clear_state(lv_obj_t *obj, lv_state_t state)

Remove one or more states to the object. The other state bits will remain unchanged. If specified in the styles, transition animation will be started from the previous state to the current.

Parameters
  • obj -- pointer to an object

  • state -- the states to add. E.g LV_STATE_PRESSED | LV_STATE_FOCUSED

static inline void lv_obj_set_user_data(lv_obj_t *obj, void *user_data)

Set the user_data field of the object

Parameters
  • obj -- pointer to an object

  • user_data -- pointer to the new user_data.

bool lv_obj_has_flag(const lv_obj_t *obj, lv_obj_flag_t f)

Check if a given flag or all the given flags are set on an object.

Parameters
  • obj -- pointer to an object

  • f -- the flag(s) to check (OR-ed values can be used)

Returns

true: all flags are set; false: not all flags are set

bool lv_obj_has_flag_any(const lv_obj_t *obj, lv_obj_flag_t f)

Check if a given flag or any of the flags are set on an object.

Parameters
  • obj -- pointer to an object

  • f -- the flag(s) to check (OR-ed values can be used)

Returns

true: at lest one flag flag is set; false: none of the flags are set

lv_state_t lv_obj_get_state(const lv_obj_t *obj)

Get the state of an object

Parameters

obj -- pointer to an object

Returns

the state (OR-ed values from lv_state_t)

bool lv_obj_has_state(const lv_obj_t *obj, lv_state_t state)

Check if the object is in a given state or not.

Parameters
  • obj -- pointer to an object

  • state -- a state or combination of states to check

Returns

true: obj is in state; false: obj is not in state

void *lv_obj_get_group(const lv_obj_t *obj)

Get the group of the object

Parameters

obj -- pointer to an object

Returns

the pointer to group of the object

static inline void *lv_obj_get_user_data(lv_obj_t *obj)

Get the user_data field of the object

Parameters

obj -- pointer to an object

Returns

the pointer to the user_data of the object

void lv_obj_allocate_spec_attr(lv_obj_t *obj)

Allocate special data for an object if not allocated yet.

Parameters

obj -- pointer to an object

bool lv_obj_check_type(const lv_obj_t *obj, const lv_obj_class_t *class_p)

Check the type of obj.

Parameters
  • obj -- pointer to an object

  • class_p -- a class to check (e.g. lv_slider_class)

Returns

true: class_p is the obj class.

bool lv_obj_has_class(const lv_obj_t *obj, const lv_obj_class_t *class_p)

Check if any object has a given class (type). It checks the ancestor classes too.

Parameters
  • obj -- pointer to an object

  • class_p -- a class to check (e.g. lv_slider_class)

Returns

true: obj has the given class

const lv_obj_class_t *lv_obj_get_class(const lv_obj_t *obj)

Get the class (type) of the object

Parameters

obj -- pointer to an object

Returns

the class (type) of the object

bool lv_obj_is_valid(const lv_obj_t *obj)

Check if any object is still "alive".

Parameters

obj -- pointer to an object

Returns

true: valid

static inline lv_coord_t lv_obj_dpx(const lv_obj_t *obj, lv_coord_t n)

Scale the given number of pixels (a distance or size) relative to a 160 DPI display considering the DPI of the obj's display. It ensures that e.g. lv_dpx(100) will have the same physical size regardless to the DPI of the display.

Parameters
  • obj -- an object whose display's dpi should be considered

  • n -- the number of pixels to scale

Returns

n x current_dpi/160

Variables

const lv_obj_class_t lv_obj_class

Make the base object's class publicly available.

struct _lv_obj_spec_attr_t
#include <lv_obj.h>

Special, rarely used attributes. They are allocated automatically if any elements is set.

Public Members

struct _lv_obj_t **children

Store the pointer of the children in an array.

uint32_t child_cnt

Number of children

lv_group_t *group_p
struct _lv_event_dsc_t *event_dsc

Dynamically allocated event callback and user data array

lv_point_t scroll

The current X/Y scroll offset

lv_coord_t ext_click_pad

Extra click padding in all direction

lv_coord_t ext_draw_size

EXTend the size in every direction for drawing.

lv_scrollbar_mode_t scrollbar_mode

How to display scrollbars

lv_scroll_snap_t scroll_snap_x

Where to align the snappable children horizontally

lv_scroll_snap_t scroll_snap_y

Where to align the snappable children vertically

lv_dir_t scroll_dir

The allowed scroll direction(s)

uint8_t event_dsc_cnt

Number of event callbacks stored in event_dsc array

struct _lv_obj_t

Public Members

const lv_obj_class_t *class_p
struct _lv_obj_t *parent
_lv_obj_spec_attr_t *spec_attr
_lv_obj_style_t *styles
void *user_data
lv_area_t coords
lv_obj_flag_t flags
lv_state_t state
uint16_t layout_inv
uint16_t scr_layout_inv
uint16_t skip_trans
uint16_t style_cnt
uint16_t h_layout
uint16_t w_layout