1# Messaging
2
3Messaging (`lv_msg`) is a classic []publisher subscriber](https://en.wikipedia.org/wiki/Publish%E2%80%93subscribe_pattern) implementation for LVGL.
4
5## IDs
6Both the publishers and the subscribers needs to know the message identifiers.
7In `lv_msg` these are simple `uint32_t` integers. For example:
8```c
9#define MSG_DOOR_OPENED             1
10#define MSG_DOOR_CLOSED             2
11#define MSG_USER_NAME_CHANGED       100
12#define MSG_USER_AVATAR_CHANGED     101
13```
14
15You can orgnaize the message IDs as you wish.
16
17Both parties also need to know about the format of teh payload. E.g. in the above example
18`MSG_DOOR_OPENED` and `MSG_DOOR_CLOSED` has 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.
19
20
21## Send message
22
23Messages can be sent with `lv_msg_send(msg_id, payload)`. E.g.
24```c
25lv_msg_send(MSG_USER_DOOR_OPENED, NULL);
26lv_msg_send(MSG_USER_NAME_CHANGED, "John Smith");
27```
28
29## Subscribe to a message
30
31`lv_msg_subscribe(msg_id, callback, user_data)` can be used to subscribe to message.
32
33The callback should look like this:
34```c
35
36static void user_name_subscriber_cb(void * s, lv_msg_t * m)
37{
38    /*s: a subscriber obeject, can be used to unscubscribe*/
39    /*m: a message object with the msg_id, payload, and user_data (set durung subscription)*/
40
41    ...do something...
42}
43```
44
45From `lv_msg_t` the followings can be used to get some data:
46- `lv_msg_get_id(m)`
47- `lv_msg_get_payload(m)`
48- `lv_msg_get_user_data(m)`
49
50## Subscribe with an lv_obj
51It's quite typical that an LVGL widget is interested in some messages.
52To make it simpler `lv_msg_subsribe_obj(msg_id, obj, user_data)` can be used.
53If a new message is published with `msg_id` an `LV_EVENT_MSG_RECEIVED` event will be sent to the object.
54
55For example:
56```c
57lv_obj_add_event_cb(user_name_label, user_name_label_event_cb, LV_EVENT_MSG_RECEIVED, NULL);
58lv_msg_subsribe_obj(MSG_USER_NAME_CHANGED, user_name_label, NULL);
59
60...
61
62void user_name_label_event_cb(lv_event_t * e)
63{
64    lv_obj_t * label = lv_event_get_target(e);
65    lv_msg_t * m = lv_event_get_msg(e);
66    lv_label_set_text(label, lv_msg_get_payload(m));
67}
68
69```
70
71### Unsubscribe
72`lv_msg_subscribe` returns a pointer which can be used to unsubscribe:
73```c
74void * s1;
75s1 = lv_msg_subscribe(MSG_USER_DOOR_OPENED, some_callback, NULL);
76
77...
78
79lv_msg_unsubscribe(s1);
80```
81
82## Example
83
84```eval_rst
85
86.. include:: ../../examples/others/msg/index.rst
87
88```
89## API
90
91
92```eval_rst
93
94.. doxygenfile:: lv_msg.h
95  :project: lvgl
96
97```
98