Refactor behaviour of "event" mixin:

- Trigger a new background task for each plugin
- Call plugin.process_event
- Plugin class can then decide what to do with the particular event

InvenTree/plugin/builtin/integration/mixins.py

@@ -185,63 +185,19 @@ class EventMixin:
     """
     Mixin that provides support for responding to triggered events.
 
-    Implementing classes must provide a list of tuples,
-    which provide pairs of 'event':'function'
-
-    Notes:
-
-    Events are called by name, and based on the django signal nomenclature,
-    e.g. 'part.pre_save'
-
-    Receiving functions must be prototyped to match the 'event' they receive.
-
-    Example:
-
-    EVENTS = [
-        ('part.pre_save', 'myplugin.functions.do_stuff'),
-        ('build.complete', 'myplugin.functions.process_completed_build'),
-    ]
+    Implementing classes must provide a "process_event" function:
     """
 
-    # Override this in subclass model
-    EVENTS = []
-
+    def process_event(self, event, *args, **kwargs):
+        # Default implementation does not do anything
+        raise NotImplementedError
+    
     class MixinMeta:
         MIXIN_NAME = 'Events'
 
     def __init__(self):
         super().__init__()
-        self.add_mixin('events', 'has_events', __class__)
-        self.events = getattr(self, 'EVENTS', [])
-
-        self.validate_events()
-
-    @property
-    def has_events(self):
-        return bool(self.events) and len(self.events) > 0
-
-    def validate_events(self):
-        """
-        Check that the provided event callbacks are valid
-        """
-
-        if not self.has_events:
-            raise ValueError('EVENTS not defined')
-
-        for pair in self.events:
-            valid = True
-
-            if len(pair) == 2:
-                event = pair[0].strip()
-                func = pair[1].strip()
-
-                if len(event) == 0 or len(func) == 0:
-                    valid = False
-            else:
-                valid = False
-
-            if not valid:
-                raise ValueError("Invalid event callback: " + str(pair))
+        self.add_mixin('events', True, __class__)
 
 
 class UrlsMixin: