Add AbstractPlugin.on_session_buffer_changed API (#1873)

So that plugins can do background/implicit requests when the buffer changes.

plugin/core/sessions.py

@@ -672,6 +672,12 @@ def on_open_uri_async(self, uri: DocumentUri, callback: Callable[[str, str, str]
         """
         return False
 
+    def on_session_buffer_changed_async(self, session_buffer: SessionBufferProtocol) -> None:
+        """
+        Called when the context of the session buffer has changed or a new buffer was opened.
+        """
+        pass
+
     def on_session_end_async(self) -> None:
         """
         Notifies about the session ending (also if the session has crashed). Provides an opportunity to clean up
@@ -1253,6 +1259,10 @@ def open_location_async(self, location: Union[Location, LocationLink], flags: in
         uri, r = get_uri_and_range_from_location(location)
         return self.open_uri_async(uri, r, flags, group)
 
+    def notify_plugin_on_session_buffer_change(self, session_buffer: SessionBufferProtocol) -> None:
+        if self._plugin:
+            self._plugin.on_session_buffer_changed_async(session_buffer)
+
     def _maybe_resolve_code_action(self, code_action: CodeAction) -> Promise[Union[CodeAction, Error]]:
         if "edit" not in code_action and self.has_capability("codeActionProvider.resolveProvider"):
             # TODO: Should we accept a SessionBuffer? What if this capability is registered with a documentSelector?

plugin/session_buffer.py

@@ -105,6 +105,7 @@ def _check_did_open(self, view: sublime.View) -> None:
                 return
             self.session.send_notification(did_open(view, language_id))
             self.opened = True
+            self.session.notify_plugin_on_session_buffer_change(self)
 
     def _check_did_close(self) -> None:
         if self.opened and self.should_notify_did_close():
@@ -238,6 +239,7 @@ def purge_changes_async(self, view: sublime.View) -> None:
             except MissingUriError:
                 pass  # we're closing
             self.pending_changes = None
+            self.session.notify_plugin_on_session_buffer_change(self)
 
     def on_pre_save_async(self, view: sublime.View) -> None:
         if self.should_notify_will_save():