docs(py): add definition of end_program_callback

content/python/callbacks.md

@@ -12,9 +12,11 @@ Library supported callback functions.
 ## Scene callbacks
 
 ### on_msg_callback
-This is called whenever there is a new message sent to the client. Use this whenever you want to sniff out __all__ incoming messages.
+
+This is called whenever there is a new message sent to the client. Use this whenever you want to sniff out **all** incoming messages.
 
 #### Usage:
+
 ```python
 # [scene] is the Scene that called the callback
 # [obj] will be an Object instance
@@ -31,11 +33,13 @@ scene.on_msg_callback = on_msg_callback
 ```
 
 ### new_obj_callback
+
 This is called whenever there is a new object that has been created in the scene,
 one that the user does not have a reference to. Use this to make references to any
 new objects that may appear during a programs lifetime. Also a good way to find camera ID's.
 
 #### Usage:
+
 ```python
 # [scene] is the Scene that called the callback
 # [obj] will be an Object instance
@@ -52,12 +56,14 @@ scene.new_obj_callback = new_obj_callback
 ```
 
 ### delete_obj_callback
+
 This is called whenever there is an object has been deleted in the scene.
 arena-py will look for all "action" = "delete" messages and call this callback.
 Use this to delete references and to be notified when an object is removed by
 another user or program.
 
 #### Usage:
+
 ```python
 # [scene] is the Scene that called the callback
 # [obj] will be an Object instance
@@ -73,13 +79,35 @@ def delete_obj_callback(scene, obj, msg):
 scene.delete_obj_callback = delete_obj_callback
 ```
 
+### end_program_callback
+
+This is called whenever the program is ending from the SIGINT, Crtl-C, or other kill process.
+Use this to cleanup resources you don't want in the scene when the program ends.
+This is especially useful for persisted objects use created that you want to be removed.
+
+#### Usage:
+
+```python
+# [scene] is the Scene that called the callback
+def end_program_callback(scene):
+    ## do stuff with scene root objects here
+    scene.delete_object(appParentObject)
+    scene.delete_object(otherAppParentObject)
+    print("App Terminated.")
+    # etc.
+
+scene.end_program_callback = end_program_callback
+```
+
 ## User callbacks
 
 ### user_join_callback
+
 This is called whenever the library detects/finds a new user that it hasn't seen before in a scene.
 Note: this is not neccesarily called when a user "joins" a scene, rather, it is called when the library first sees a `Camera` object/receives an "update" message from a user.
 
 #### Usage:
+
 ```python
 def user_join_callback(camera):
     ## Get access to user state
@@ -94,9 +122,11 @@ scene.user_join_callback = user_join_callback
 ```
 
 ### user_left_callback
+
 This is called whenever a user leaves a scene/sends a delete message.
 
 #### Usage:
+
 ```python
 def user_left_callback(camera):
     ## Get access to user state
@@ -111,7 +141,9 @@ scene.user_left_callback = user_left_callback
 ```
 
 ### Adding callbacks when you instantiate the class
+
 You can also add callbacks like so:
+
 ```python
 from arena import *
 
@@ -128,7 +160,9 @@ scene = Scene(..., on_msg_callback=on_msg_callback, new_obj_callback=new_obj_cal
 ```
 
 ## Custom Message Callbacks
+
 If you need to use an MQTT client, the `Scene` object exposes a way to use its MQTT client to subscribe to custom topics.
+
 ```python
 def led_toggle(client, userdata, msg):
     # do stuff here