lvgl/docs/others/msg.md

Messaging

Messaging (lv_msg) is a classic publisher subscriber implementation for LVGL.

IDs

Both the publishers and the subscribers needs to know the message identifiers. In lv_msg these are simple integers. For example:

#define MSG_DOOR_OPENED             1
#define MSG_DOOR_CLOSED             2
#define MSG_USER_NAME_CHANGED       100
#define MSG_USER_AVATAR_CHANGED     101

You can organize the message IDs as you wish.

Both parties also need to know about the format of the payload. E.g. in the above example MSG_DOOR_OPENED and MSG_DOOR_CLOSED might have no payload but MSG_USER_NAME_CHANGED can have a const char * payload containing the user name, and MSG_USER_AVATAR_CHANGED a const void * image source with the new avatar image.

To be more precise the message ID's type is declared like this:

typedef lv_uintptr_t lv_msg_id_t;

This way, if a value in stored in a global variable (e.g. the current temperature) then the address of that variable can be used as message ID too by simply casting it to lv_msg_id_t. It saves the creation of message IDs manually as the variable itself serves as message ID too.

Subscribe to a message

lv_msg_subscribe(msg_id, callback, user_data) can be used to subscribe to message.

Don't forget that msg_id can be a constant or a variable address too:

lv_msg_subscribe(45, my_callback_1, NULL);

int v;
lv_msg_subscribe((lv_msg_id_t)&v, my_callback_2, NULL);

The callback should look like this:

static void user_name_subscriber_cb(lv_msg_t * m)
{
    /*m: a message object with the msg_id, payload, and user_data (set during subscription)*/

    ...do something...
}

From lv_msg_t the followings can be used to get some data:

• lv_msg_get_id(m)
• lv_msg_get_payload(m)
• lv_msg_get_user_data(m)

Subscribe with an lv_obj

It's quite typical that an LVGL widget is interested in some messages. To make it simpler lv_msg_subsribe_obj(msg_id, obj, user_data) can be used. If a new message is published with msg_id an LV_EVENT_MSG_RECEIVED event will be sent to the object.

For example:

lv_obj_add_event_cb(user_name_label, user_name_label_event_cb, LV_EVENT_MSG_RECEIVED, NULL);
lv_msg_subsribe_obj(MSG_USER_NAME_CHANGED, user_name_label, NULL);

...

void user_name_label_event_cb(lv_event_t * e)
{
    lv_obj_t * label = lv_event_get_target(e);
    lv_msg_t * m = lv_event_get_msg(e);
    lv_label_set_text(label, lv_msg_get_payload(m));
}

Here msg_id also can be a variable's address:

char name[64];
lv_msg_subsribe_obj(name, user_name_label, NULL);

Unsubscribe

lv_msg_subscribe returns a pointer which can be used to unsubscribe:

void * s1;
s1 = lv_msg_subscribe(MSG_USER_DOOR_OPENED, some_callback, NULL);

...

lv_msg_unsubscribe(s1);

Send message

Messages can be sent with lv_msg_send(msg_id, payload). E.g.

lv_msg_send(MSG_USER_DOOR_OPENED, NULL);
lv_msg_send(MSG_USER_NAME_CHANGED, "John Smith");

If have subscribed to a variable with lv_msg_subscribe((lv_msg_id_t)&v, callback, NULL) and changed the variable's value the subscribers can be notified like this:

v = 10;
lv_msg_update_value(&v); //Notify all the subscribers of `(lv_msg_id_t)&v`

It's handy way of creating API for the UI too. If the UI provides some global variables (e.g. int current_tempereature;) and anyone can read and write this variable. After writing they can notify all the elements who are interested in that value. E.g. an lv_label can subscribe to (lv_msg_id_t)&current_tempereature and update its text when it's notified about the new temperature.

Example

.. include:: ../../examples/others/msg/index.rst

API

.. doxygenfile:: lv_msg.h
  :project: lvgl