Merge pull request #642 from antlarr/album-cover-art

PICARD-1000: Show album/cluster cover art

picard/album.py

@@ -58,6 +58,7 @@ class Album(DataObject, Item):
     def __init__(self, id, discid=None):
         DataObject.__init__(self, id)
         self.metadata = Metadata()
+        self.orig_metadata = Metadata()
         self.tracks = []
         self.loaded = False
         self.load_task = None
@@ -71,6 +72,7 @@ class Album(DataObject, Item):
         self.errors = []
         self.status = None
         self._album_artists = []
+        self.update_metadata_images_enabled = True
 
     def __repr__(self):
         return '<Album %s %r>' % (self.id, self.metadata[u"album"])
@@ -83,6 +85,11 @@ class Album(DataObject, Item):
             for file in self.unmatched_files.iterfiles():
                 yield file
 
+    def enable_update_metadata_images(self, enabled):
+        self.update_metadata_images_enabled = enabled
+        if enabled:
+            self.update_metadata_images()
+
     def append_album_artist(self, id):
         """Append artist id to the list of album artists
         and return an AlbumArtist instance"""
@@ -254,6 +261,7 @@ class Album(DataObject, Item):
             self._tracks_loaded = True
 
         if not self._requests:
+            self.enable_update_metadata_images(False)
             # Prepare parser for user's script
             if config.setting["enable_tagger_scripts"]:
                 for s_pos, s_name, s_enabled, s_text in config.setting["list_of_scripts"]:
@@ -275,6 +283,7 @@ class Album(DataObject, Item):
                         self._new_metadata.strip_whitespace()
 
             for track in self.tracks:
+                track.metadata_images_changed.connect(self.update_metadata_images)
                 for file in list(track.linked_files):
                     file.move(self.unmatched_files)
             self.metadata = self._new_metadata
@@ -285,6 +294,7 @@ class Album(DataObject, Item):
             self.status = None
             self.match_files(self.unmatched_files.files)
             self.update()
+            self.enable_update_metadata_images(True)
             self.tagger.window.set_statusbar_message(
                 N_('Album %(id)s loaded: %(artist)s - %(album)s'),
                 {
@@ -378,14 +388,19 @@ class Album(DataObject, Item):
     def update(self, update_tracks=True):
         if self.item:
             self.item.update(update_tracks)
+        self.update_metadata_images()
 
     def _add_file(self, track, file):
         self._files += 1
         self.update(update_tracks=False)
+        file.metadata_images_changed.connect(self.update_metadata_images)
+        self.update_metadata_images()
 
     def _remove_file(self, track, file):
         self._files -= 1
         self.update(update_tracks=False)
+        file.metadata_images_changed.disconnect(self.update_metadata_images)
+        self.update_metadata_images()
 
     def match_files(self, files, use_recordingid=True):
         """Match files to tracks on this album, based on metadata similarity or recordingid."""
@@ -552,6 +567,52 @@ class Album(DataObject, Item):
             self.tagger.albums[mbid] = self
             self.load(priority=True, refresh=True)
 
+    def update_metadata_images(self):
+        if not self.update_metadata_images_enabled:
+            return
+
+        class State:
+            new_images = []
+            orig_images = []
+            has_common_new_images = True
+            has_common_orig_images = True
+            first_new_obj = True
+            first_orig_obj = True
+
+        state = State()
+
+        def process_images(state, obj):
+            # Check new images
+            if state.first_new_obj:
+                state.new_images = obj.metadata.images[:]
+                state.first_new_obj = False
+            else:
+                if state.new_images != obj.metadata.images:
+                    state.has_common_new_images = False
+                    state.new_images.extend([image for image in obj.metadata.images if image not in state.new_images])
+            if isinstance(obj, Track):
+                return
+            # Check orig images, but not for Tracks (which don't have orig_metadata)
+            if state.first_orig_obj:
+                state.orig_images = obj.orig_metadata.images[:]
+                state.first_orig_obj = False
+            else:
+                if state.orig_images != obj.orig_metadata.images:
+                    state.has_common_orig_images = False
+                    state.orig_images.extend([image for image in obj.orig_metadata.images if image not in state.orig_images])
+
+        for track in self.tracks:
+            process_images(state, track)
+            for file in list(track.linked_files):
+                process_images(state, file)
+        for file in list(self.unmatched_files.files):
+            process_images(state, file)
+
+        self.metadata.images = state.new_images
+        self.metadata.has_common_images = state.has_common_new_images
+        self.orig_metadata.images = state.orig_images
+        self.orig_metadata.has_common_images = state.has_common_orig_images
+
 
 class NatAlbum(Album):
 

picard/cluster.py

@@ -69,6 +69,9 @@ class Cluster(QtCore.QObject, Item):
             self.metadata.length += file.metadata.length
             file._move(self)
             file.update(signal=False)
+            cover = file.metadata.get_single_front_image()
+            if cover and cover[0] not in self.metadata.images:
+                self.metadata.append_image(cover[0])
         self.files.extend(files)
         self.metadata['totaltracks'] = len(self.files)
         self.item.add_files(files)
@@ -79,6 +82,9 @@ class Cluster(QtCore.QObject, Item):
         self.metadata['totaltracks'] = len(self.files)
         file._move(self)
         file.update(signal=False)
+        cover = file.metadata.get_single_front_image()
+        if cover and cover[0] not in self.metadata.images:
+            self.metadata.append_image(cover[0])
         self.item.add_file(file)
 
     def remove_file(self, file):

picard/const/__init__.py

@@ -113,3 +113,6 @@ PLUGINS_API = {
 
 # Default query limit
 QUERY_LIMIT = 25
+
+# Maximum number of covers to draw in a stack in CoverArtThumbnail
+MAX_COVERS_TO_STACK = 4

picard/coverart/image.py

@@ -70,6 +70,9 @@ class DataHash:
     def __eq__(self, other):
         return self._hash == other._hash
 
+    def hash(self):
+        return self._hash
+
     def delete_file(self):
         if self._filename:
             try:
@@ -211,6 +214,11 @@ class CoverArtImage:
         else:
             return False
 
+    def __hash__(self):
+        if self.datahash is None:
+            return 0
+        return hash(self.datahash.hash())
+
     def set_data(self, data):
         """Store image data in a file, if data already exists in such file
            it will be re-used and no file write occurs

picard/file.py

@@ -54,6 +54,8 @@ from picard.const import QUERY_LIMIT
 
 class File(QtCore.QObject, Item):
 
+    metadata_images_changed = QtCore.pyqtSignal()
+
     UNDEFINED = -1
     PENDING = 0
     NORMAL = 1
@@ -156,6 +158,7 @@ class File(QtCore.QObject, Item):
 
         if acoustid:
             self.metadata["acoustid_id"] = acoustid
+        self.metadata_images_changed.emit()
 
     def has_error(self):
         return self.state == File.ERROR

picard/metadata.py

@@ -55,6 +55,11 @@ class Metadata(dict):
     def append_image(self, coverartimage):
         self.images.append(coverartimage)
 
+    def set_front_image(self, coverartimage):
+        # First remove all front images
+        self.images[:] = [img for img in self.images if not img.is_front_image()]
+        self.images.append(coverartimage)
+
     @property
     def images_to_be_saved_to_tags(self):
         if not config.setting["save_images_to_tags"]:

picard/track.py

@@ -19,6 +19,7 @@
 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 
 from functools import partial
+from PyQt4 import QtCore
 from picard import config, log
 from picard.metadata import Metadata, run_track_metadata_processors
 from picard.dataobj import DataObject
@@ -44,6 +45,8 @@ class TrackArtist(DataObject):
 
 class Track(DataObject, Item):
 
+    metadata_images_changed = QtCore.pyqtSignal()
+
     def __init__(self, id, album=None):
         DataObject.__init__(self, id)
         self.album = album
@@ -110,7 +113,7 @@ class Track(DataObject, Item):
         return True
 
     def can_view_info(self):
-        return self.num_linked_files == 1
+        return self.num_linked_files == 1 or self.metadata.images
 
     def column(self, column):
         m = self.metadata

picard/ui/coverartbox.py

@@ -27,6 +27,8 @@ from picard.coverart.image import CoverArtImage, CoverArtImageError
 from picard.track import Track
 from picard.file import File
 from picard.util import encode_filename, imageinfo
+from picard.util.lrucache import LRUCache
+from picard.const import MAX_COVERS_TO_STACK
 
 if sys.platform == 'darwin':
     try:
@@ -38,7 +40,6 @@ if sys.platform == 'darwin':
 
 
 class ActiveLabel(QtGui.QLabel):
-
     """Clickable QLabel."""
 
     clicked = QtCore.pyqtSignal()
@@ -83,7 +84,7 @@ class ActiveLabel(QtGui.QLabel):
 
 class CoverArtThumbnail(ActiveLabel):
 
-    def __init__(self, active=False, drops=False, *args, **kwargs):
+    def __init__(self, active=False, drops=False, pixmap_cache=None, *args, **kwargs):
         super(CoverArtThumbnail, self).__init__(active, drops, *args, **kwargs)
         self.data = None
         self.shadow = QtGui.QPixmap(":/images/CoverArtShadow.png")
@@ -93,6 +94,7 @@ class CoverArtThumbnail(ActiveLabel):
         self.clicked.connect(self.open_release_page)
         self.image_dropped.connect(self.fetch_remote_image)
         self.related_images = list()
+        self._pixmap_cache = pixmap_cache
 
     def __eq__(self, other):
         if len(self.related_images) or len(other.related_images):
@@ -103,7 +105,20 @@ class CoverArtThumbnail(ActiveLabel):
     def show(self):
         self.set_data(self.data, True)
 
-    def set_data(self, data, force=False, pixmap=None):
+    def decorate_cover(self, pixmap):
+        offx, offy, w, h = (1, 1, 121, 121)
+        cover = QtGui.QPixmap(self.shadow)
+        pixmap = pixmap.scaled(w, h, QtCore.Qt.KeepAspectRatio, QtCore.Qt.SmoothTransformation)
+        painter = QtGui.QPainter(cover)
+        bgcolor = QtGui.QColor.fromRgb(0, 0, 0, 128)
+        painter.fillRect(QtCore.QRectF(offx, offy, w, h), bgcolor)
+        x = offx + (w - pixmap.width()) / 2
+        y = offy + (h - pixmap.height()) / 2
+        painter.drawPixmap(x, y, pixmap)
+        painter.end()
+        return cover
+
+    def set_data(self, data, force=False, has_common_images=True):
         if not force and self.data == data:
             return
 
@@ -111,43 +126,98 @@ class CoverArtThumbnail(ActiveLabel):
         if not force and self.parent().isHidden():
             return
 
-        cover = self.shadow
-        if self.data:
-            if pixmap is None:
+        if not self.data:
+            self.setPixmap(self.shadow)
+            return
+
+        w, h, displacements = (128, 128, 20)
+        key = hash(tuple(sorted(self.data)) + (has_common_images,))
+        try:
+            pixmap = self._pixmap_cache[key]
+        except KeyError:
+            if len(self.data) == 1:
                 pixmap = QtGui.QPixmap()
-                pixmap.loadFromData(self.data.data)
-            if not pixmap.isNull():
-                offx, offy, w, h = (1, 1, 121, 121)
-                cover = QtGui.QPixmap(self.shadow)
-                pixmap = pixmap.scaled(w, h, QtCore.Qt.KeepAspectRatio, QtCore.Qt.SmoothTransformation)
-                painter = QtGui.QPainter(cover)
-                bgcolor = QtGui.QColor.fromRgb(0, 0, 0, 128)
-                painter.fillRect(QtCore.QRectF(offx, offy, w, h), bgcolor)
-                x = offx + (w - pixmap.width()) / 2
-                y = offy + (h - pixmap.height()) / 2
-                painter.drawPixmap(x, y, pixmap)
+                pixmap.loadFromData(self.data[0].data)
+                pixmap = self.decorate_cover(pixmap)
+            else:
+                limited = len(self.data) > MAX_COVERS_TO_STACK
+                if limited:
+                    data_to_paint = data[:MAX_COVERS_TO_STACK - 1]
+                    offset = displacements * len(data_to_paint)
+                else:
+                    data_to_paint = data
+                    offset = displacements * (len(data_to_paint) - 1)
+                stack_width, stack_height = (w + offset, h + offset)
+                pixmap = QtGui.QPixmap(stack_width, stack_height)
+                bgcolor = self.palette().color(QtGui.QPalette.Window)
+                painter = QtGui.QPainter(pixmap)
+                painter.fillRect(QtCore.QRectF(0, 0, stack_width, stack_height), bgcolor)
+                cx = stack_width - w / 2
+                cy = h / 2
+                if limited:
+                    x, y = (cx - self.shadow.width() / 2, cy - self.shadow.height() / 2)
+                    for i in range(3):
+                        painter.drawPixmap(x, y, self.shadow)
+                        x -= displacements / 3
+                        y += displacements / 3
+                    cx -= displacements
+                    cy += displacements
+                else:
+                    cx = stack_width - w / 2
+                    cy = h / 2
+                for image in reversed(data_to_paint):
+                    if isinstance(image, QtGui.QPixmap):
+                        thumb = image
+                    else:
+                        thumb = QtGui.QPixmap()
+                        thumb.loadFromData(image.data)
+                    thumb = self.decorate_cover(thumb)
+                    x, y = (cx - thumb.width() / 2, cy - thumb.height() / 2)
+                    painter.drawPixmap(x, y, thumb)
+                    cx -= displacements
+                    cy += displacements
+                if not has_common_images:
+                    color = QtGui.QColor("darkgoldenrod")
+                    border_length = 10
+                    for k in range(border_length):
+                        color.setAlpha(255 - k * 255 / border_length)
+                        painter.setPen(color)
+                        painter.drawLine(x, y - k - 1, x + 121 + k + 1, y - k - 1)
+                        painter.drawLine(x + 121 + k + 2, y - 1 - k, x + 121 + k + 2, y + 121 + 4)
+                    for k in range(5):
+                        bgcolor.setAlpha(80 + k * 255 / 7)
+                        painter.setPen(bgcolor)
+                        painter.drawLine(x + 121 + 2, y + 121 + 2 + k, x + 121 + border_length + 2, y + 121 + 2 + k)
                 painter.end()
-        self.setPixmap(cover)
+                pixmap = pixmap.scaled(w, h, QtCore.Qt.KeepAspectRatio, QtCore.Qt.SmoothTransformation)
+            self._pixmap_cache[key] = pixmap
+
+        self.setPixmap(pixmap)
 
     def set_metadata(self, metadata):
         data = None
         self.related_images = []
         if metadata and metadata.images:
             self.related_images = metadata.images
-            for image in metadata.images:
-                if image.is_front_image():
-                    data = image
-                    break
-            else:
+            data = [image for image in metadata.images if image.is_front_image()]
+            if not data:
                 # There's no front image, choose the first one available
-                data = metadata.images[0]
-        self.set_data(data)
+                data = [metadata.images[0]]
+        has_common_images = getattr(metadata, 'has_common_images', True)
+        self.set_data(data, has_common_images=has_common_images)
         release = None
         if metadata:
             release = metadata.get("musicbrainz_albumid", None)
         if release:
             self.setActive(True)
-            self.setToolTip(_(u"View release on MusicBrainz"))
+            text = _(u"View release on MusicBrainz")
+            if hasattr(metadata, 'has_common_images'):
+                if has_common_images:
+                    note = _(u'Common images on all tracks')
+                else:
+                    note = _(u'Tracks contain different images')
+                text += '<br /><i>%s</i>' % note
+            self.setToolTip(text)
         else:
             self.setActive(False)
             self.setToolTip("")
@@ -172,12 +242,13 @@ class CoverArtBox(QtGui.QGroupBox):
         self.setStyleSheet('''QGroupBox{background-color:none;border:1px;}''')
         self.setFlat(True)
         self.item = None
+        self.pixmap_cache = LRUCache(40)
         self.cover_art_label = QtGui.QLabel('')
         self.cover_art_label.setAlignment(QtCore.Qt.AlignTop | QtCore.Qt.AlignHCenter)
-        self.cover_art = CoverArtThumbnail(False, True, parent)
+        self.cover_art = CoverArtThumbnail(False, True, self.pixmap_cache, parent)
         spacerItem = QtGui.QSpacerItem(40, 20, QtGui.QSizePolicy.Minimum, QtGui.QSizePolicy.Expanding)
         self.orig_cover_art_label = QtGui.QLabel('')
-        self.orig_cover_art = CoverArtThumbnail(False, False, parent)
+        self.orig_cover_art = CoverArtThumbnail(False, False, self.pixmap_cache, parent)
         self.orig_cover_art_label.setAlignment(QtCore.Qt.AlignTop | QtCore.Qt.AlignHCenter)
         self.show_details_button = QtGui.QPushButton(_(u'Show more details'), self)
         self.layout.addWidget(self.cover_art_label)
@@ -209,7 +280,7 @@ class CoverArtBox(QtGui.QGroupBox):
         # We want to show the 2 coverarts only if they are different
         # and orig_cover_art data is set and not the default cd shadow
         if self.orig_cover_art.data is None or self.cover_art == self.orig_cover_art:
-            self.show_details_button.setHidden(len(self.cover_art.related_images) <= 1)
+            self.show_details_button.setHidden(len(self.cover_art.related_images) == 0)
             self.orig_cover_art.setHidden(True)
             self.cover_art_label.setText('')
             self.orig_cover_art_label.setText('')
@@ -290,6 +361,7 @@ class CoverArtBox(QtGui.QGroupBox):
         try:
             coverartimage = CoverArtImage(
                 url=url.toString(),
+                types=[u'front'],
                 data=data
             )
         except CoverArtImageError as e:
@@ -297,21 +369,29 @@ class CoverArtBox(QtGui.QGroupBox):
             return
         if isinstance(self.item, Album):
             album = self.item
-            album.metadata.append_image(coverartimage)
+            album.enable_update_metadata_images(False)
             for track in album.tracks:
-                track.metadata.append_image(coverartimage)
+                track.metadata.set_front_image(coverartimage)
+                track.metadata_images_changed.emit()
             for file in album.iterfiles():
-                file.metadata.append_image(coverartimage)
+                file.metadata.set_front_image(coverartimage)
+                file.metadata_images_changed.emit()
                 file.update()
+            album.enable_update_metadata_images(True)
         elif isinstance(self.item, Track):
             track = self.item
-            track.metadata.append_image(coverartimage)
+            track.album.enable_update_metadata_images(False)
+            track.metadata.set_front_image(coverartimage)
+            track.metadata_images_changed.emit()
             for file in track.iterfiles():
-                file.metadata.append_image(coverartimage)
+                file.metadata.set_front_image(coverartimage)
+                file.metadata_images_changed.emit()
                 file.update()
+            track.album.enable_update_metadata_images(True)
         elif isinstance(self.item, File):
             file = self.item
-            file.metadata.append_image(coverartimage)
+            file.metadata.set_front_image(coverartimage)
+            file.metadata_images_changed.emit()
             file.update()
         self.cover_art.set_metadata(self.item.metadata)
         self.show()

picard/ui/infodialog.py

@@ -24,6 +24,7 @@ from PyQt4 import QtGui, QtCore
 from picard import log
 from picard.file import File
 from picard.track import Track
+from picard.album import Album
 from picard.coverart.image import CoverArtImageIOError
 from picard.util import format_time, encode_filename, bytes2human, webbrowser2, union_sorted_lists
 from picard.ui import PicardDialog
@@ -100,10 +101,14 @@ class InfoDialog(PicardDialog):
         self.obj = obj
         self.ui = Ui_InfoDialog()
         self.display_existing_artwork = False
-        if isinstance(obj, File) and isinstance(obj.parent, Track) or \
-                isinstance(obj, Track):
+        if (isinstance(obj, File)
+            and isinstance(obj.parent, Track)
+            or isinstance(obj, Track)):
             # Display existing artwork only if selected object is track object
             # or linked to a track object
+            if getattr(obj, 'orig_metadata', None) is not None:
+                self.display_existing_artwork = True
+        elif isinstance(obj, Album) and obj.get_num_total_files() > 0:
             self.display_existing_artwork = True
 
         self.ui.setupUi(self)
@@ -236,8 +241,8 @@ class FileInfoDialog(InfoDialog):
         InfoDialog.__init__(self, file, parent)
         self.setWindowTitle(_("Info") + " - " + file.base_filename)
 
-    def _display_info_tab(self):
-        file = self.obj
+    @staticmethod
+    def format_file_info(file):
         info = []
         info.append((_('Filename:'), file.filename))
         if '~format' in file.orig_metadata:
@@ -265,9 +270,13 @@ class FileInfoDialog(InfoDialog):
             else:
                 ch = str(ch)
             info.append((_('Channels:'), ch))
-        text = '<br/>'.join(map(lambda i: '<b>%s</b><br/>%s' %
+        return '<br/>'.join(map(lambda i: '<b>%s</b><br/>%s' %
                                 (cgi.escape(i[0]),
                                  cgi.escape(i[1])), info))
+
+    def _display_info_tab(self):
+        file = self.obj
+        text = FileInfoDialog.format_file_info(file)
         self.ui.info.setText(text)
 
 
@@ -296,6 +305,29 @@ class AlbumInfoDialog(InfoDialog):
             tabWidget.setTabText(tab_index, _("&Info"))
             self.tab_hide(tab)
 
+class TrackInfoDialog(FileInfoDialog):
+
+    def __init__(self, track, parent=None):
+        InfoDialog.__init__(self, track, parent)
+        self.setWindowTitle(_("Track Info"))
+
+    def _display_info_tab(self):
+        track = self.obj
+        tab = self.ui.info_tab
+        tabWidget = self.ui.tabWidget
+        tab_index = tabWidget.indexOf(tab)
+        if track.num_linked_files == 0:
+            tabWidget.setTabText(tab_index, _("&Info"))
+            self.tab_hide(tab)
+            return
+
+        tabWidget.setTabText(tab_index, _("&Info"))
+        text = ungettext("%i file in this track", "%i files in this track",
+                         track.num_linked_files) % track.num_linked_files
+        info_files = [FileInfoDialog.format_file_info(file) for file in track.linked_files]
+        text += '<hr />' + '<hr />'.join(info_files)
+        self.ui.info.setText(text)
+
 
 class ClusterInfoDialog(InfoDialog):
 

picard/ui/mainwindow.py

@@ -33,7 +33,7 @@ from picard.ui.metadatabox import MetadataBox
 from picard.ui.filebrowser import FileBrowser
 from picard.ui.tagsfromfilenames import TagsFromFileNamesDialog
 from picard.ui.options.dialog import OptionsDialog
-from picard.ui.infodialog import FileInfoDialog, AlbumInfoDialog, ClusterInfoDialog
+from picard.ui.infodialog import FileInfoDialog, AlbumInfoDialog, TrackInfoDialog, ClusterInfoDialog
 from picard.ui.infostatus import InfoStatus
 from picard.ui.passworddialog import PasswordDialog, ProxyDialog
 from picard.ui.logview import LogView, HistoryView
@@ -844,6 +844,9 @@ class MainWindow(QtGui.QMainWindow):
         elif isinstance(self.selected_objects[0], Cluster):
             cluster = self.selected_objects[0]
             dialog = ClusterInfoDialog(cluster, self)
+        elif isinstance(self.selected_objects[0], Track):
+            track = self.selected_objects[0]
+            dialog = TrackInfoDialog(track, self)
         else:
             file = self.tagger.get_files_from_objects(self.selected_objects)[0]
             dialog = FileInfoDialog(file, self)
@@ -955,6 +958,9 @@ class MainWindow(QtGui.QMainWindow):
                         }
                     self.set_statusbar_message(msg, mparms, echo=None,
                                                history=None)
+            elif isinstance(obj, Album):
+                metadata = obj.metadata
+                orig_metadata = obj.orig_metadata
             elif obj.can_edit_tags():
                 metadata = obj.metadata
 

picard/util/lrucache.py

@@ -0,0 +1,79 @@
+# -*- coding: utf-8 -*-
+#
+# Picard, the next-generation MusicBrainz tagger
+# Copyright (C) 2017 Antonio Larrosa <alarrosa@suse.com>
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+
+
+class LRUCache(dict):
+    """
+    Helper class to cache items using a Least Recently Used policy.
+
+    It's originally used to cache generated pixmaps in the CoverArtBox object
+    but it's generic enough to be used for other purposes if necessary.
+    The cache will never hold more than max_size items and the item least
+    recently used will be discarded.
+
+    >>> cache = LRUCache(3)
+    >>> cache['item1'] = 'some value'
+    >>> cache['item2'] = 'some other value'
+    >>> cache['item3'] = 'yet another value'
+    >>> cache['item1']
+    'some value'
+    >>> cache['item4'] = 'This will push item 2 out of the cache'
+    >>> cache['item2']
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in <module>
+      File "lrucache.py", line 48, in __getitem__
+        return super(LRUCache, self).__getitem__(key)
+    KeyError: 'item2'
+    >>> cache['item5'] = 'This will push item3 out of the cache'
+    >>> cache['item3']
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in <module>
+      File "lrucache.py", line 48, in __getitem__
+        return super(LRUCache, self).__getitem__(key)
+    KeyError: 'item3'
+    >>> cache['item1']
+    'some value'
+    """
+
+    def __init__(self, max_size):
+        self._ordered_keys = []
+        self._max_size = max_size
+
+    def __getitem__(self, key):
+        if key in self:
+            self._ordered_keys.remove(key)
+            self._ordered_keys.insert(0, key)
+        return super(LRUCache, self).__getitem__(key)
+
+    def __setitem__(self, key, value):
+        if key in self:
+            self._ordered_keys.remove(key)
+        self._ordered_keys.insert(0, key)
+
+        r = super(LRUCache, self).__setitem__(key, value)
+
+        if len(self) > self._max_size:
+            item = self._ordered_keys.pop()
+            super(LRUCache, self).__delitem__(item)
+
+        return r
+
+    def __delitem__(self, key):
+        self._ordered_keys.remove(key)
+        super(LRUCache, self).__delitem__(key)