# Messaging

Messaging (`lv_msg`) is a classic [publisher subscriber](https://en.wikipedia.org/wiki/Publish%E2%80%93subscribe_pattern) 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:
```c
#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:
```c
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:
```c
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:
```c

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:
```c
lv_obj_add_event(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:
```c
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:
```c
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.
```c
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:
```c
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

```eval_rst

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

```
## API


```eval_rst

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

```
