lvgl/docs/overview/event.md

.. include:: /header.rst 
:github_url: |github_link_base|/overview/event.md

Events

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

• is clicked
• is dragged
• its value has changed, etc.

The user can assign a callback function to an object to see these events. In practice, it looks like this:

lv_obj_t * btn = lv_btn_create(lv_scr_act(), NULL);
lv_obj_set_event_cb(btn, my_event_cb);   /*Assign an event callback*/

...

static void my_event_cb(lv_obj_t * obj, lv_event_t event)
{
    switch(event) {
        case LV_EVENT_PRESSED:
            printf("Pressed\n");
            break;

        case LV_EVENT_SHORT_CLICKED:
            printf("Short clicked\n");
            break;

        case LV_EVENT_CLICKED:
            printf("Clicked\n");
            break;

        case LV_EVENT_LONG_PRESSED:
            printf("Long press\n");
            break;

        case LV_EVENT_LONG_PRESSED_REPEAT:
            printf("Long press repeat\n");
            break;

        case LV_EVENT_RELEASED:
            printf("Released\n");
            break;
    }

       /*Etc.*/
}

More objects can use the same event callback.

Event types

The following event types exist:

Generic events

All objects (such as Buttons/Labels/Sliders etc.) receive these generic events regardless of their type.

Related to the input devices

These are sent when an object is pressed/released etc. by the user. They are used not only for Pointers but can used for Keypad, Encoder and Button input devices as well. Visit the Overview of input devices section to learn more about them.

• LV_EVENT_PRESSED The object has been pressed
• LV_EVENT_PRESSING The object is being pressed (sent continuously while pressing)
• LV_EVENT_PRESS_LOST The input device is still being pressed but is no longer on the object
• LV_EVENT_SHORT_CLICKED Released before LV_INDEV_LONG_PRESS_TIME time. Not called if dragged.
• LV_EVENT_LONG_PRESSED Pressing for LV_INDEV_LONG_PRESS_TIME time. Not called if dragged.
• LV_EVENT_LONG_PRESSED_REPEAT Called after LV_INDEV_LONG_PRESS_TIME in every LV_INDEV_LONG_PRESS_REP_TIME ms. Not called if dragged.
• LV_EVENT_CLICKED Called on release if not dragged (regardless to long press)
• LV_EVENT_RELEASED Called in every case when the object has been released even if it was dragged. Not called if slid from the object while pressing and released outside of the object. In this case, LV_EVENT_PRESS_LOST is sent.

Related to pointer

These events are sent only by pointer-like input devices (E.g. mouse or touchpad)

• LV_EVENT_DRAG_BEGIN Dragging of the object has started
• LV_EVENT_DRAG_END Dragging finished (including drag throw)
• LV_EVENT_DRAG_THROW_BEGIN Drag throw started (released after drag with "momentum")

Related to keypad and encoder

These events are sent by keypad and encoder input devices. Learn more about Groups in [overview/indev](Input devices) section.

• LV_EVENT_KEY A Key is sent to the object. Typically when it was pressed or repeated after a long press. The key can be retrived by uint32_t * key = lv_event_get_data()
• LV_EVENT_FOCUSED The object is focused in its group
• LV_EVENT_DEFOCUSED The object is defocused in its group

General events

Other general events sent by the library.

• LV_EVENT_DELETE The object is being deleted. Free the related user-allocated data.

Special events

These events are specific to a particular object type.

• LV_EVENT_VALUE_CHANGED The object value has changed (e.g. for a Slider)
• LV_EVENT_INSERT Something is inserted to the object. (Typically to a Text area)
• LV_EVENT_APPLY "Ok", "Apply" or similar specific button has clicked. (Typically from a Keyboard object)
• LV_EVENT_CANCEL "Close", "Cancel" or similar specific button has clicked. (Typically from a Keyboard object)
• LV_EVENT_REFRESH Query to refresh the object. Never sent by the library but can be sent by the user.

Visit particular Object type's documentation to understand which events are used by an object type.

Custom data

Some events might contain custom data. For example, LV_EVENT_VALUE_CHANGED in some cases tells the new value. For more information, see the particular Object type's documentation. To get the custom data in the event callback use lv_event_get_data().

The type of the custom data depends on the sending object but if it's a

• single number then it's uint32_t * or int32_t *
• text then char * or const char *

Send events manually

Arbitrary events

To manually send events to an object, use lv_event_send(obj, LV_EVENT_..., &custom_data).

For example, it can be used to manually close a message box by simulating a button press (although there are simpler ways of doing 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);

Refresh event

LV_EVENT_REFRESH is special event because it's designed to be used by the user to 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

To simplest way to handle similar cases is utilizing the following functions.

lv_event_send_refresh(obj) is just a wrapper to lv_event_send(obj, LV_EVENT_REFRESH, NULL). So it simply sends an LV_EVENT_REFRESH to an object.

lv_event_send_refresh_recursive(obj) sends LV_EVENT_REFRESH event to an object and all of its children. If NULL is passed as parameter all objects of all displays will be refreshed.