Allow event's func to be async (#1461)

Fixes #1344.

telethon/events/callbackquery.py

@@ -118,8 +118,10 @@ class CallbackQuery(EventBuilder):
             elif event.query.data != self.match:
                 return
 
-        if not self.func or self.func(event):
-            return event
+        if self.func:
+            # Return the result of func directly as it may need to be awaited
+            return self.func(event)
+        return True
 
     class Event(EventCommon, SenderGetter):
         """

telethon/events/common.py

@@ -55,7 +55,7 @@ class EventBuilder(abc.ABC):
             which will be ignored if ``blacklist_chats=True``.
 
         func (`callable`, optional):
-            A callable function that should accept the event as input
+            A callable (async or not) function that should accept the event as input
             parameter, and return a value indicating whether the event
             should be dispatched or not (any truthy value will do, it
             does not need to be a `bool`). It works like a custom filter:
@@ -105,13 +105,13 @@ class EventBuilder(abc.ABC):
 
     def filter(self, event):
         """
-        If the ID of ``event._chat_peer`` isn't in the chats set (or it is
-        but the set is a blacklist) returns `None`, otherwise the event.
+        Returns a truthy value if the event passed the filter and should be
+        used, or falsy otherwise. The return value may need to be awaited.
 
         The events must have been resolved before this can be called.
         """
         if not self.resolved:
-            return None
+            return
 
         if self.chats is not None:
             # Note: the `event.chat_id` property checks if it's `None` for us
@@ -119,10 +119,13 @@ class EventBuilder(abc.ABC):
             if inside == self.blacklist_chats:
                 # If this chat matches but it's a blacklist ignore.
                 # If it doesn't match but it's a whitelist ignore.
-                return None
+                return
 
-        if not self.func or self.func(event):
-            return event
+        if not self.func:
+            return True
+
+        # Return the result of func directly as it may need to be awaited
+        return self.func(event)
 
 
 class EventCommon(ChatGetter, abc.ABC):

telethon/events/raw.py

@@ -46,6 +46,8 @@ class Raw(EventBuilder):
         return update
 
     def filter(self, event):
-        if ((not self.types or isinstance(event, self.types))
-                and (not self.func or self.func(event))):
+        if not self.types or isinstance(event, self.types):
+            if self.func:
+                # Return the result of func directly as it may need to be awaited
+                return self.func(event)
             return event