add message interface

README.md

@@ -78,7 +78,9 @@ If both `menu.dll` and `menu.lua` exists in scripts folder, one of it may be nam
 
 - `max_title_length=80`: Limits the title length in dynamic submenu, set to 0 to disable.
 
-## Updating menu from script
+## Scripting
+
+### Properties
 
 The menu data is stored in `user-data/menu/items` property with the following structure:
 
@@ -96,6 +98,34 @@ Updating this property will trigger an update of the menu UI.
 
 To reduce update frequency, it's recommended to update this property in [mp.register_idle(fn)](https://mpv.io/manual/master/#lua-scripting-mp-register-idle(fn)).
 
+### Messages
+
+#### `menu-ready`
+
+Broadcasted when `dyn_menu.lua` has initilized itself.
+
+#### `get <keyword> <src>`
+
+Get the menu item structure of `keyword`, and send a json reply to `src`.
+
+The reply is sent via `srcript-message-to <src> menu-get-reply <json>`, data structure:
+
+```json
+{
+  "keyword": "chapters"
+  "item": {
+    "title": "Chapters",
+    "type": "submenu",
+    "submenu": {}
+  }
+}
+```
+If `keyword` not found, it will contain an additional `error` field, and no `item` field.
+
+#### `update <keyword> <json>`
+
+Update the menu item structure of `keyword` with `json`.
+
 ## Credits
 
 This project contains code copied from [mpv](https://github.com/mpv-player/mpv).