Events

Events are triggered in LVGL when something happens which might be interesting to the user, e.g. when an object:

  • is clicked

  • is scrolled

  • has its value changed

  • is redrawn, etc.

Besides widgets, events can registered from displays and input devices too.

Add events to a widget

The user can assign callback functions to an object to see its events. In practice, it looks like this:

lv_obj_t * btn = lv_button_create(lv_screen_active());
lv_obj_add_event_cb(btn, my_event_cb, LV_EVENT_CLICKED, NULL);   /*Assign an event callback*/

...

static void my_event_cb(lv_event_t * event)
{
    printf("Clicked\n");
}

In the example LV_EVENT_CLICKED means that only the click event will call my_event_cb. See the list of event codes for all the options. LV_EVENT_ALL can be used to receive all events.

The last parameter of lv_obj_add_event() is a pointer to any custom data that will be available in the event. It will be described later in more detail.

More events can be added to an object, like this:

lv_obj_add_event_cb(obj, my_event_cb_1, LV_EVENT_CLICKED, NULL);
lv_obj_add_event_cb(obj, my_event_cb_2, LV_EVENT_PRESSED, NULL);
lv_obj_add_event_cb(obj, my_event_cb_3, LV_EVENT_ALL, NULL);       /*No filtering, receive all events*/

Even the same event callback can be used on an object with different user_data. For example:

lv_obj_add_event_cb(obj, increment_on_click, LV_EVENT_CLICKED, &num1);
lv_obj_add_event_cb(obj, increment_on_click, LV_EVENT_CLICKED, &num2);

The events will be called in the order as they were added.

Other objects can use the same event callback.

In the very same way events can attached to the input devices and displays like this

lv_display_add_event_cb(disp, event_cb, LV_EVENT_RESOLUTION_CHANGED, NULL);
lv_indev_add_event_cb(indev, event_cb, LV_EVENT_CLICKED, NULL);

Remove event(s) from widgets

uint32_t i;
uint32_t event_cnt = lv_obj_get_event_count(obj);
for(i = 0; i < event_cnt; i++) {
    lv_event_dsc_t * event_dsc = lv_obj_get_event_dsc(obj, i);
    if(lv_event_dsc_get_cb(event_dsc) == some_event_cb) {
        lv_obj_remove_event(obj, i);
        break;
    }
}

Event codes

The event codes can be grouped into these categories: - Input device events - Drawing events - Other events - Special events - Custom events

All objects (such as Buttons/Labels/Sliders etc.) regardless their type receive the Input device, Drawing and Other events.

However, the Special events are specific to a particular widget type. See the widgets' documentation to learn when they are sent,

Custom events are added by the user and are never sent by LVGL.

The following event codes exist:

Input device events

Drawing events

Special events

Other events

Display events

Custom events

Any custom event codes can be registered by uint32_t MY_EVENT_1 = lv_event_register_id()

They can be sent to any object with lv_event_send(obj, MY_EVENT_1, &some_data)

Sending events

To manually send events to an object, use lv_obj_send_event(obj, <EVENT_CODE>, &some_data).

For example, this can be used to manually close a message box by simulating a button press (although there are simpler ways to do this):

/*Simulate the press of the first button (indexes start from zero)*/
uint32_t btn_id = 0;
lv_event_send(mbox, LV_EVENT_VALUE_CHANGED, &btn_id);

The same works for display and input devices with lv_display_send_event(obj, <EVENT_CODE>, &some_data) and lv_indev_send_event(obj, <EVENT_CODE>, &some_data).

Refresh event

LV_EVENT_REFRESH is a special event because it's designed to let the user notify an object to refresh itself. Some examples:

  • notify a label to refresh its text according to one or more variables (e.g. current time)

  • refresh a label when the language changes

  • enable a button if some conditions are met (e.g. the correct PIN is entered)

  • add/remove styles to/from an object if a limit is exceeded, etc

Fields of lv_event_t

lv_event_t is the only parameter passed to the event callback and it contains all data about the event. The following values can be gotten from it:

Event bubbling

If lv_obj_add_flag(obj, LV_OBJ_FLAG_EVENT_BUBBLE) is enabled all events will be sent to an object's parent too. If the parent also has LV_OBJ_FLAG_EVENT_BUBBLE enabled the event will be sent to its parent and so on.

The target parameter of the event is always the current target object, not the original object. To get the original target call lv_event_get_target_obj(e) in the event handler.

Examples

Button click event

#include "../lv_examples.h"
#if LV_BUILD_EXAMPLES && LV_USE_SWITCH

static void event_cb(lv_event_t * e)
{
    LV_LOG_USER("Clicked");

    static uint32_t cnt = 1;
    lv_obj_t * btn = lv_event_get_target(e);
    lv_obj_t * label = lv_obj_get_child(btn, 0);
    lv_label_set_text_fmt(label, "%"LV_PRIu32, cnt);
    cnt++;
}

/**
 * Add click event to a button
 */
void lv_example_event_1(void)
{
    lv_obj_t * btn = lv_button_create(lv_screen_active());
    lv_obj_set_size(btn, 100, 50);
    lv_obj_center(btn);
    lv_obj_add_event_cb(btn, event_cb, LV_EVENT_CLICKED, NULL);

    lv_obj_t * label = lv_label_create(btn);
    lv_label_set_text(label, "Click me!");
    lv_obj_center(label);
}

#endif

Handle multiple events

#include "../lv_examples.h"
#if LV_BUILD_EXAMPLES && LV_USE_SWITCH

static void event_cb(lv_event_t * e)
{
    lv_event_code_t code = lv_event_get_code(e);
    lv_obj_t * label = lv_event_get_user_data(e);

    switch(code) {
        case LV_EVENT_PRESSED:
            lv_label_set_text(label, "The last button event:\nLV_EVENT_PRESSED");
            break;
        case LV_EVENT_CLICKED:
            lv_label_set_text(label, "The last button event:\nLV_EVENT_CLICKED");
            break;
        case LV_EVENT_LONG_PRESSED:
            lv_label_set_text(label, "The last button event:\nLV_EVENT_LONG_PRESSED");
            break;
        case LV_EVENT_LONG_PRESSED_REPEAT:
            lv_label_set_text(label, "The last button event:\nLV_EVENT_LONG_PRESSED_REPEAT");
            break;
        default:
            break;
    }
}

/**
 * Handle multiple events
 */
void lv_example_event_2(void)
{
    lv_obj_t * btn = lv_button_create(lv_screen_active());
    lv_obj_set_size(btn, 100, 50);
    lv_obj_center(btn);

    lv_obj_t * btn_label = lv_label_create(btn);
    lv_label_set_text(btn_label, "Click me!");
    lv_obj_center(btn_label);

    lv_obj_t * info_label = lv_label_create(lv_screen_active());
    lv_label_set_text(info_label, "The last button event:\nNone");

    lv_obj_add_event_cb(btn, event_cb, LV_EVENT_ALL, info_label);
}

#endif

Event bubbling

#include "../lv_examples.h"
#if LV_BUILD_EXAMPLES && LV_USE_FLEX

static void event_cb(lv_event_t * e)
{
    /*The original target of the event. Can be the buttons or the container*/
    lv_obj_t * target = lv_event_get_target(e);

    /*The current target is always the container as the event is added to it*/
    lv_obj_t * cont = lv_event_get_current_target(e);

    /*If container was clicked do nothing*/
    if(target == cont) return;

    /*Make the clicked buttons red*/
    lv_obj_set_style_bg_color(target, lv_palette_main(LV_PALETTE_RED), 0);
}

/**
 * Demonstrate event bubbling
 */
void lv_example_event_3(void)
{

    lv_obj_t * cont = lv_obj_create(lv_screen_active());
    lv_obj_set_size(cont, 290, 200);
    lv_obj_center(cont);
    lv_obj_set_flex_flow(cont, LV_FLEX_FLOW_ROW_WRAP);

    uint32_t i;
    for(i = 0; i < 30; i++) {
        lv_obj_t * btn = lv_button_create(cont);
        lv_obj_set_size(btn, 70, 50);
        lv_obj_add_flag(btn, LV_OBJ_FLAG_EVENT_BUBBLE);

        lv_obj_t * label = lv_label_create(btn);
        lv_label_set_text_fmt(label, "%"LV_PRIu32, i);
        lv_obj_center(label);
    }

    lv_obj_add_event_cb(cont, event_cb, LV_EVENT_CLICKED, NULL);
}

#endif

Draw event

#include "../lv_examples.h"

#if LV_BUILD_EXAMPLES

static uint32_t size = 0;
static bool size_dec = false;

static void timer_cb(lv_timer_t * timer)
{
    lv_obj_invalidate(timer->user_data);
    if(size_dec) size--;
    else size++;

    if(size == 50) size_dec = true;
    else if(size == 0) size_dec = false;
}

static void event_cb(lv_event_t * e)
{
    lv_obj_t * obj = lv_event_get_target(e);
    lv_draw_task_t * draw_task = lv_event_get_draw_task(e);
    lv_draw_dsc_base_t * base_dsc = draw_task->draw_dsc;
    if(base_dsc->part == LV_PART_MAIN) {
        lv_draw_rect_dsc_t draw_dsc;
        lv_draw_rect_dsc_init(&draw_dsc);
        draw_dsc.bg_color = lv_color_hex(0xffaaaa);
        draw_dsc.radius = LV_RADIUS_CIRCLE;
        draw_dsc.border_color = lv_color_hex(0xff5555);
        draw_dsc.border_width = 2;
        draw_dsc.outline_color = lv_color_hex(0xff0000);
        draw_dsc.outline_pad = 3;
        draw_dsc.outline_width = 2;

        lv_area_t a;
        a.x1 = 0;
        a.y1 = 0;
        a.x2 = size;
        a.y2 = size;
        lv_area_align(&obj->coords, &a, LV_ALIGN_CENTER, 0, 0);

        lv_draw_rect(base_dsc->layer, &draw_dsc, &a);
    }
}

/**
 * Demonstrate the usage of draw event
 */
void lv_example_event_4(void)
{
    lv_obj_t * cont = lv_obj_create(lv_screen_active());
    lv_obj_set_size(cont, 200, 200);
    lv_obj_center(cont);
    lv_obj_add_event_cb(cont, event_cb, LV_EVENT_DRAW_TASK_ADDED, NULL);
    lv_obj_add_flag(cont, LV_OBJ_FLAG_SEND_DRAW_TASK_EVENTS);
    lv_timer_create(timer_cb, 30, cont);
}

#endif

API

lv_obj_event.h

lv_types.h

lv_event.h