Add a signal plugins can connect to to be notified of db events

src/calibre/gui2/ui.py

@@ -110,6 +110,7 @@ class Main(MainWindow, MainWindowMixin, DeviceMixin, EmailMixin,  # {{{
     proceed_requested = pyqtSignal(object, object)
     book_converted = pyqtSignal(object, object)
     enter_key_pressed_in_book_list = pyqtSignal(object)  # used by action chains plugin
+    event_in_db = pyqtSignal(object, object, object)
     shutting_down = False
 
     def __init__(self, opts, parent=None, gui_debug=None):
@@ -200,6 +201,13 @@ class Main(MainWindow, MainWindowMixin, DeviceMixin, EmailMixin,  # {{{
         else:
             stmap[st.name] = st
 
+    def add_db_listener(self, callback):
+        self.library_broker.start_listening_for_db_events()
+        self.event_in_db.connect(callback)
+
+    def remove_db_listener(self, callback):
+        self.event_in_db.disconnect(callback)
+
     def initialize(self, library_path, db, actions, show_gui=True):
         opts = self.opts
         self.preferences_action, self.quit_action = actions
@@ -1103,6 +1111,10 @@ class Main(MainWindow, MainWindowMixin, DeviceMixin, EmailMixin,  # {{{
         self.shutting_down = True
         self.show_shutdown_message()
         self.server_change_notification_timer.stop()
+        try:
+            self.event_in_db.disconnect()
+        except Exception:
+            pass
 
         from calibre.customize.ui import has_library_closed_plugins
         if has_library_closed_plugins():

src/calibre/srv/library_broker.py

@@ -201,12 +201,16 @@ class GuiLibraryBroker(LibraryBroker):
         from calibre.gui2 import gprefs
         self.last_used_times = defaultdict(lambda: -EXPIRED_AGE)
         self.gui_library_id = None
+        self.listening_for_db_events = False
         LibraryBroker.__init__(self, load_gui_libraries(gprefs))
         self.gui_library_changed(db)
 
     def init_library(self, library_path, is_default_library):
         library_path = self.original_path_map.get(library_path, library_path)
-        return LibraryDatabase(library_path, is_second_db=True)
+        db = LibraryDatabase(library_path, is_second_db=True)
+        if self.listening_for_db_events:
+            db.add_listener(self.on_db_event)
+        return db
 
     def get(self, library_id=None):
         try:
@@ -214,6 +218,22 @@ class GuiLibraryBroker(LibraryBroker):
         finally:
             self.last_used_times[library_id or self.default_library] = monotonic()
 
+    def start_listening_for_db_events(self):
+        with self:
+            self.listening_for_db_events = True
+            for db in self.loaded_dbs:
+                db.new_api.add_listener(self.on_db_event)
+
+    def on_db_event(self, event_type, library_id, event_data):
+        from calibre.gui2.ui import get_gui
+        gui = get_gui()
+        if gui is not None:
+            db = LibraryBroker.get(self, library_id)
+            with self:
+                db = self.loaded_dbs.get(library_id)
+            if db is not None:
+                gui.event_in_db.emit(db, event_type, event_data)
+
     def get_library(self, original_library_path):
         library_path = canonicalize_path(original_library_path)
         with self: