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.
Add events to the object¶
The user can assign callback functions to an object to see its events. In practice, it looks like this:
lv_obj_t * btn = lv_btn_create(lv_scr_act());
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_cb
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.
Remove event(s) from an object¶
Events can be removed from an object with the lv_obj_remove_event_cb(obj, event_cb)
function or lv_obj_remove_event_dsc(obj, event_dsc)
. event_dsc
is a pointer returned by lv_obj_add_event_cb
.
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¶
LV_EVENT_PRESSED
An object has been pressedLV_EVENT_PRESSING
An object is being pressed (called continuously while pressing)LV_EVENT_PRESS_LOST
An object is still being pressed but slid cursor/finger off of the objectLV_EVENT_SHORT_CLICKED
An object was pressed for a short period of time, then released. Not called if scrolled.LV_EVENT_LONG_PRESSED
An object has been pressed for at least thelong_press_time
specified in the input device driver. Not called if scrolled.LV_EVENT_LONG_PRESSED_REPEAT
Called afterlong_press_time
in everylong_press_repeat_time
ms. Not called if scrolled.LV_EVENT_CLICKED
Called on release if an object did not scroll (regardless of long press)LV_EVENT_RELEASED
Called in every case when an object has been releasedLV_EVENT_SCROLL_BEGIN
Scrolling begins. The event parameter isNULL
or anlv_anim_t *
with a scroll animation descriptor that can be modified if required.LV_EVENT_SCROLL_END
Scrolling ends.LV_EVENT_SCROLL
An object was scrolledLV_EVENT_GESTURE
A gesture is detected. Get the gesture withlv_indev_get_gesture_dir(lv_indev_get_act());
LV_EVENT_KEY
A key is sent to an object. Get the key withlv_indev_get_key(lv_indev_get_act());
LV_EVENT_FOCUSED
An object is focusedLV_EVENT_DEFOCUSED
An object is unfocusedLV_EVENT_LEAVE
An object is unfocused but still selectedLV_EVENT_HIT_TEST
Perform advanced hit-testing. Uselv_hit_test_info_t * a = lv_event_get_hit_test_info(e)
and check ifa->point
can click the object or not. If not seta->res = false
Drawing events¶
LV_EVENT_COVER_CHECK
Check if an object fully covers an area. The event parameter islv_cover_check_info_t *
.LV_EVENT_REFR_EXT_DRAW_SIZE
Get the required extra draw area around an object (e.g. for a shadow). The event parameter islv_coord_t *
to store the size. Only overwrite it with a larger value.LV_EVENT_DRAW_MAIN_BEGIN
Starting the main drawing phase.LV_EVENT_DRAW_MAIN
Perform the main drawingLV_EVENT_DRAW_MAIN_END
Finishing the main drawing phaseLV_EVENT_DRAW_POST_BEGIN
Starting the post draw phase (when all children are drawn)LV_EVENT_DRAW_POST
Perform the post draw phase (when all children are drawn)LV_EVENT_DRAW_POST_END
Finishing the post draw phase (when all children are drawn)LV_EVENT_DRAW_PART_BEGIN
Starting to draw a part. The event parameter islv_obj_draw_dsc_t *
. Learn more here.LV_EVENT_DRAW_PART_END
Finishing to draw a part. The event parameter islv_obj_draw_dsc_t *
. Learn more here.
In LV_EVENT_DRAW_...
events it's not allowed to adjust the widgets' properties. E.g. you can not call lv_obj_set_width()
.
In other words only get
functions can be called.
Other events¶
LV_EVENT_DELETE
Object is being deletedLV_EVENT_CHILD_CHANGED
Child was removed/addedLV_EVENT_CHILD_CREATED
Child was created, always bubbles up to all parentsLV_EVENT_CHILD_DELETED
Child was deleted, always bubbles up to all parentsLV_EVENT_SIZE_CHANGED
Object coordinates/size have changedLV_EVENT_STYLE_CHANGED
Object's style has changedLV_EVENT_BASE_DIR_CHANGED
The base dir has changedLV_EVENT_GET_SELF_SIZE
Get the internal size of a widgetLV_EVENT_SCREEN_UNLOAD_START
A screen unload started, fired immediately when lv_scr_load/lv_scr_load_anim is calledLV_EVENT_SCREEN_LOAD_START
A screen load started, fired when the screen change delay is expiredLV_EVENT_SCREEN_LOADED
A screen was loaded, called when all animations are finishedLV_EVENT_SCREEN_UNLOADED
A screen was unloaded, called when all animations are finished
Special events¶
LV_EVENT_VALUE_CHANGED
The object's value has changed (i.e. slider moved)LV_EVENT_INSERT
Text is being inserted into the object. The event data ischar *
being inserted.LV_EVENT_REFRESH
Notify the object to refresh something on it (for the user)LV_EVENT_READY
A process has finishedLV_EVENT_CANCEL
A process has been canceled
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_event_send(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);
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:
lv_event_get_code(e)
get the event codelv_event_get_current_target(e)
get the object to which an event was sent. I.e. the object whose event handler is being called.lv_event_get_target(e)
get the object that originally triggered the event (different fromlv_event_get_target
if event bubbling is enabled)lv_event_get_user_data(e)
get the pointer passed as the last parameter oflv_obj_add_event_cb
.lv_event_get_param(e)
get the parameter passed as the last parameter oflv_event_send
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_original_target(e)
in the event handler.
Examples¶
Button click event¶
C code
GitHub#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_btn_create(lv_scr_act());
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
class Event_1():
def __init__(self):
self.cnt = 1
#
# Add click event to a button
#
btn = lv.btn(lv.scr_act())
btn.set_size(100, 50)
btn.center()
btn.add_event_cb(self.event_cb, lv.EVENT.CLICKED, None)
label = lv.label(btn)
label.set_text("Click me!")
label.center()
def event_cb(self,e):
print("Clicked")
btn = e.get_target()
label = btn.get_child(0)
label.set_text(str(self.cnt))
self.cnt += 1
evt1 = Event_1()
Handle multiple events¶
C code
GitHub#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_btn_create(lv_scr_act());
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_scr_act());
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
def event_cb(e,label):
code = e.get_code()
if code == lv.EVENT.PRESSED:
label.set_text("The last button event:\nLV_EVENT_PRESSED")
elif code == lv.EVENT.CLICKED:
label.set_text("The last button event:\nLV_EVENT_CLICKED")
elif code == lv.EVENT.LONG_PRESSED:
label.set_text("The last button event:\nLV_EVENT_LONG_PRESSED")
elif code == lv.EVENT.LONG_PRESSED_REPEAT:
label.set_text("The last button event:\nLV_EVENT_LONG_PRESSED_REPEAT")
btn = lv.btn(lv.scr_act())
btn.set_size(100, 50)
btn.center()
btn_label = lv.label(btn)
btn_label.set_text("Click me!")
btn_label.center()
info_label = lv.label(lv.scr_act())
info_label.set_text("The last button event:\nNone")
btn.add_event_cb(lambda e: event_cb(e,info_label), lv.EVENT.ALL, None)
Event bubbling¶
C code
GitHub#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_scr_act());
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_btn_create(cont);
lv_obj_set_size(btn, 80, 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
def event_cb(e):
# The original target of the event. Can be the buttons or the container
target = e.get_target()
# print(type(target))
# If container was clicked do nothing
if type(target) != type(lv.btn()):
return
# Make the clicked buttons red
target.set_style_bg_color(lv.palette_main(lv.PALETTE.RED), 0)
#
# Demonstrate event bubbling
#
cont = lv.obj(lv.scr_act())
cont.set_size(320, 200)
cont.center()
cont.set_flex_flow(lv.FLEX_FLOW.ROW_WRAP)
for i in range(30):
btn = lv.btn(cont)
btn.set_size(80, 50)
btn.add_flag(lv.obj.FLAG.EVENT_BUBBLE)
label = lv.label(btn)
label.set_text(str(i))
label.center()
cont.add_event_cb(event_cb, lv.EVENT.CLICKED, None)