Another try at debouncing book details.

1) The debouncing is done where the display is called: views.py and book_details.py.
2) The 250ms timeout defined in book_info.py is arbitrary. I like it but some might want it smaller.
3) This commit implements delayed evaluation of composites when using get_metadata().

src/calibre/db/cache.py

@@ -394,19 +394,16 @@ class Cache:
             default_value={}))
         mi.application_id = book_id
         mi.id = book_id
-        composites = []
         for key, meta in self.field_metadata.custom_iteritems():
             mi.set_user_metadata(key, meta)
-            if meta['datatype'] == 'composite':
-                composites.append(key)
-            else:
+            if meta['datatype'] != 'composite':
+                # composites are evaluated on demand in metadata.book.base
+                # because their value is None
                 val = self._field_for(key, book_id)
                 if isinstance(val, tuple):
                     val = list(val)
                 extra = self._field_for(key+'_index', book_id)
                 mi.set(key, val=val, extra=extra)
-        for key in composites:
-            mi.set(key, val=self._composite_for(key, book_id, mi))
 
         mi.link_maps = self._get_all_link_maps_for_book(book_id)
 

src/calibre/ebooks/metadata/book/base.py

@@ -150,9 +150,7 @@ class Metadata:
         if field in _data['user_metadata']:
             d = _data['user_metadata'][field]
             val = d['#value#']
-            if d['datatype'] != 'composite':
-                return val
-            if val is None:
+            if val is None and d['datatype'] == 'composite':
                 d['#value#'] = 'RECURSIVE_COMPOSITE FIELD (Metadata) ' + field
                 val = d['#value#'] = self.formatter.safe_format(
                                             d['display']['composite_template'],
@@ -191,8 +189,9 @@ class Metadata:
                 langs = [val]
             _data['languages'] = langs
         elif field in _data['user_metadata']:
-            _data['user_metadata'][field]['#value#'] = val
-            _data['user_metadata'][field]['#extra#'] = extra
+            d = _data['user_metadata'][field]
+            d['#value#'] = val
+            d['#extra#'] = extra
         else:
             # You are allowed to stick arbitrary attributes onto this object as
             # long as they don't conflict with global or user metadata names
@@ -205,11 +204,24 @@ class Metadata:
     def has_key(self, key):
         return key in object.__getattribute__(self, '_data')
 
+    def _evaluate_all_composites(self):
+        custom_fields = object.__getattribute__(self, '_data')['user_metadata']
+        for field in custom_fields:
+            self._evaluate_composite(field)
+
+    def _evaluate_composite(self, field):
+        f = object.__getattribute__(self, '_data')['user_metadata'].get(field, None)
+        if f is not None:
+            if f['datatype'] == 'composite' and f['#value#'] is None:
+                self.get(field)
+
     def deepcopy(self, class_generator=lambda : Metadata(None)):
         ''' Do not use this method unless you know what you are doing, if you
         want to create a simple clone of this object, use :meth:`deepcopy_metadata`
         instead. Class_generator must be a function that returns an instance
         of Metadata or a subclass of it.'''
+        # We don't need to evaluate all the composites here because we
+        # are returning a "real" Metadata instance that has __get_attribute__.
         m = class_generator()
         if not isinstance(m, Metadata):
             return None
@@ -217,6 +229,8 @@ class Metadata:
         return m
 
     def deepcopy_metadata(self):
+        # We don't need to evaluate all the composites here because we
+        # are returning a "real" Metadata instance that has __get_attribute__.
         m = Metadata(None)
         object.__setattr__(m, '_data', copy.deepcopy(object.__getattribute__(self, '_data')))
         return m
@@ -228,6 +242,8 @@ class Metadata:
             return default
 
     def get_extra(self, field, default=None):
+        # Don't need to evaluate all composites because a composite can't have
+        # an extra value
         _data = object.__getattribute__(self, '_data')
         if field in _data['user_metadata']:
             try:
@@ -247,8 +263,7 @@ class Metadata:
         needed is large. Also, we don't want any manipulations of the returned
         dict to show up in the book.
         '''
-        ans = object.__getattribute__(self,
-            '_data')['identifiers']
+        ans = object.__getattribute__(self, '_data')['identifiers']
         if not ans:
             ans = {}
         return copy.deepcopy(ans)
@@ -273,16 +288,14 @@ class Metadata:
         typ, val = self._clean_identifier(typ, val)
         if not typ:
             return
-        identifiers = object.__getattribute__(self,
-            '_data')['identifiers']
+        identifiers = object.__getattribute__(self, '_data')['identifiers']
 
         identifiers.pop(typ, None)
         if val:
             identifiers[typ] = val
 
     def has_identifier(self, typ):
-        identifiers = object.__getattribute__(self,
-            '_data')['identifiers']
+        identifiers = object.__getattribute__(self, '_data')['identifiers']
         return typ in identifiers
 
     # field-oriented interface. Intended to be the same as in LibraryDatabase
@@ -373,6 +386,9 @@ class Metadata:
         return a dict containing all the custom field metadata associated with
         the book.
         '''
+        # Must evaluate all composites because we are returning a dict, not a
+        # Metadata instance
+        self._evaluate_all_composites()
         _data = object.__getattribute__(self, '_data')
         user_metadata = _data['user_metadata']
         if not make_copy:
@@ -388,9 +404,12 @@ class Metadata:
         None. field is the key name, not the label. Return a copy if requested,
         just in case the user wants to change values in the dict.
         '''
-        _data = object.__getattribute__(self, '_data')
-        _data = _data['user_metadata']
+        _data = object.__getattribute__(self, '_data')['user_metadata']
         if field in _data:
+            # Must evaluate the field because it might be a composite. It won't
+            # be evaluated on demand because we are returning its dict, not a
+            # Metadata instance
+            self._evaluate_composite(field)
             if make_copy:
                 return copy.deepcopy(_data[field])
             return _data[field]

src/calibre/gui2/dialogs/book_info.py

@@ -4,6 +4,8 @@
 
 import textwrap
 from enum import IntEnum
+from functools import partial
+
 from qt.core import (
     QAction, QApplication, QBrush, QCheckBox, QDialog, QDialogButtonBox, QGridLayout,
     QHBoxLayout, QIcon, QKeySequence, QLabel, QListView, QModelIndex, QPalette, QPixmap,
@@ -22,6 +24,8 @@ from calibre.gui2.widgets import CoverView
 from calibre.gui2.widgets2 import Dialog, HTMLDisplay
 from calibre.startup import connect_lambda
 
+BOOK_DETAILS_DISPLAY_DELAY = 250 # 250ms is arbitrary
+
 
 class Cover(CoverView):
 
@@ -221,6 +225,7 @@ class BookInfo(QDialog):
         self.path_to_book = None
         self.current_row = None
         self.slave_connected = False
+        self.slave_debounce_timer = QTimer()
         if library_path is not None:
             self.view = None
             db = get_gui().library_broker.get_library(library_path)
@@ -319,6 +324,7 @@ class BookInfo(QDialog):
         ret = QDialog.done(self, r)
         if self.slave_connected:
             self.view.model().new_bookdisplay_data.disconnect(self.slave)
+        self.slave_debounce_timer.stop() # OK if it isn't running
         self.view = self.link_delegate = self.gui = None
         self.closed.emit(self)
         return ret
@@ -343,6 +349,13 @@ class BookInfo(QDialog):
         QTimer.singleShot(1, self.resize_cover)
 
     def slave(self, mi):
+        self.slave_debounce_timer.stop()
+        t = self.book_display_info_timer = QTimer()
+        t.setSingleShot(True)
+        t.timeout.connect(partial(self._timed_slave, mi))
+        t.start(BOOK_DETAILS_DISPLAY_DELAY)
+
+    def _timed_slave(self, mi):
         self.refresh(mi.row_number, mi)
 
     def move(self, delta=1):

src/calibre/gui2/library/views.py

@@ -13,7 +13,7 @@ from qt.core import (
     QAbstractItemView, QDialog, QDialogButtonBox, QDrag, QEvent, QFont, QFontMetrics,
     QGridLayout, QHeaderView, QIcon, QItemSelection, QItemSelectionModel, QLabel, QMenu,
     QMimeData, QModelIndex, QPoint, QPushButton, QSize, QSpinBox, QStyle,
-    QStyleOptionHeader, Qt, QTableView, QUrl, pyqtSignal,
+    QStyleOptionHeader, Qt, QTableView, QTimer, QUrl, pyqtSignal,
 )
 
 from calibre import force_unicode
@@ -1615,7 +1615,16 @@ class BooksView(QTableView):  # {{{
             self._model.search_done.connect(self.alternate_views.restore_current_book_state)
 
     def connect_to_book_display(self, bd):
-        self._model.new_bookdisplay_data.connect(bd)
+        self.connect_to_book_display_timer = QTimer()
+        self._model.new_bookdisplay_data.connect(partial(self._timed_connect_to_book_display, bd))
+
+    def _timed_connect_to_book_display(self, bd, data):
+        self.connect_to_book_display_timer.stop()
+        t = self.connect_to_book_display_timer = QTimer()
+        t.setSingleShot(True)
+        t.timeout.connect(partial(bd, data))
+        from calibre.gui2.dialogs.book_info import BOOK_DETAILS_DISPLAY_DELAY
+        t.start(BOOK_DETAILS_DISPLAY_DELAY)
 
     def search_done(self, ok):
         self._search_done(self, ok)