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98 lines
2.5 KiB
Markdown
98 lines
2.5 KiB
Markdown
# Messaging
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Messaging (`lv_msg`) is a classic []publisher subscriber](https://en.wikipedia.org/wiki/Publish%E2%80%93subscribe_pattern) implementation for LVGL.
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## IDs
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Both the publishers and the subscribers needs to know the message identifiers.
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In `lv_msg` these are simple `uint32_t` integers. For example:
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```c
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#define MSG_DOOR_OPENED 1
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#define MSG_DOOR_CLOSED 2
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#define MSG_USER_NAME_CHANGED 100
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#define MSG_USER_AVATAR_CHANGED 101
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```
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You can orgnaize the message IDs as you wish.
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Both parties also need to know about the format of teh payload. E.g. in the above example
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`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.
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## Send message
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Messages can be sent with `lv_msg_send(msg_id, payload)`. E.g.
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```c
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lv_msg_send(MSG_USER_DOOR_OPENED, NULL);
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lv_msg_send(MSG_USER_NAME_CHANGED, "John Smith");
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```
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## Subscribe to a message
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`lv_msg_subscribe(msg_id, callback, user_data)` can be used to subscribe to message.
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The callback should look like this:
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```c
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static void user_name_subscriber_cb(void * s, lv_msg_t * m)
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{
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/*s: a subscriber obeject, can be used to unscubscribe*/
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/*m: a message object with the msg_id, payload, and user_data (set durung subscription)*/
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...do something...
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}
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```
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From `lv_msg_t` the followings can be used to get some data:
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- `lv_msg_get_id(m)`
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- `lv_msg_get_payload(m)`
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- `lv_msg_get_user_data(m)`
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## Subscribe with an lv_obj
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It's quite typical that an LVGL widget is interested in some messages.
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To make it simpler `lv_msg_subsribe_obj(msg_id, obj, user_data)` can be used.
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If a new message is published with `msg_id` an `LV_EVENT_MSG_RECEIVED` event will be sent to the object.
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For example:
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```c
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lv_obj_add_event_cb(user_name_label, user_name_label_event_cb, LV_EVENT_MSG_RECEIVED, NULL);
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lv_msg_subsribe_obj(MSG_USER_NAME_CHANGED, user_name_label, NULL);
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...
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void user_name_label_event_cb(lv_event_t * e)
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{
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lv_obj_t * label = lv_event_get_target(e);
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lv_msg_t * m = lv_event_get_msg(e);
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lv_label_set_text(label, lv_msg_get_payload(m));
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}
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```
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### Unsubscribe
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`lv_msg_subscribe` returns a pointer which can be used to unsubscribe:
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```c
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void * s1;
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s1 = lv_msg_subscribe(MSG_USER_DOOR_OPENED, some_callback, NULL);
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...
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lv_msg_unsubscribe(s1);
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```
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## Example
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```eval_rst
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.. include:: ../../examples/others/msg/index.rst
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```
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## API
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```eval_rst
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.. doxygenfile:: lv_msg.h
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:project: lvgl
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```
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