Cache album art in the library model between resets (to prevent flickering when filtering), and don't load art for the same album multiple times.

Fixes issue 2528

src/library/librarymodel.cpp

@@ -35,6 +35,7 @@
 #include <QFuture>
 #include <QFutureWatcher>
 #include <QMetaEnum>
+#include <QPixmapCache>
 #include <QSettings>
 #include <QStringList>
 #include <QUrl>
@@ -91,7 +92,7 @@ LibraryModel::LibraryModel(LibraryBackend* backend, Application* app,
           SIGNAL(ImageLoaded(quint64,QImage)),
           SLOT(AlbumArtLoaded(quint64,QImage)));
 
-  no_cover_icon_pretty_ = QImage(":nocover.png").scaled(
+  no_cover_icon_pretty_ = QPixmap(":nocover.png").scaled(
         kPrettyCoverSize, kPrettyCoverSize,
         Qt::KeepAspectRatio, Qt::SmoothTransformation);
 }
@@ -390,35 +391,62 @@ void LibraryModel::SongsDeleted(const SongList& songs) {
   }
 }
 
+QString LibraryModel::AlbumIconPixmapCacheKey(const QModelIndex& index) const {
+  QStringList path;
+  QModelIndex index_copy(index);
+  while (index_copy.isValid()) {
+    path.prepend(index_copy.data().toString());
+    index_copy = index_copy.parent();
+  }
+
+  return "libraryart:" + path.join("/");
+}
+
 QVariant LibraryModel::AlbumIcon(const QModelIndex& index) {
-  // Cache the art in the item's metadata field
   LibraryItem* item = IndexToItem(index);
   if (!item)
     return no_cover_icon_pretty_;
-  if (!item->metadata.image().isNull())
-    return item->metadata.image();
 
-  // No art is cached - load art for the first Song in the album.
+  // Check the cache for a pixmap we already loaded.
+  const QString cache_key = AlbumIconPixmapCacheKey(index);
+  QPixmap cached_pixmap;
+  if (QPixmapCache::find(cache_key, &cached_pixmap)) {
+    return cached_pixmap;
+  }
+
+  // Maybe we're loading a pixmap already?
+  if (pending_cache_keys_.contains(cache_key)) {
+    return no_cover_icon_pretty_;
+  }
+
+  // No art is cached and we're not loading it already.  Load art for the first
+  // Song in the album.
   SongList songs = GetChildSongs(index);
   if (!songs.isEmpty()) {
     const quint64 id = app_->album_cover_loader()->LoadImageAsync(
           cover_loader_options_, songs.first());
-    pending_art_[id] = item;
+    pending_art_[id] = ItemAndCacheKey(item, cache_key);
+    pending_cache_keys_.insert(cache_key);
   }
 
   return no_cover_icon_pretty_;
 }
 
 void LibraryModel::AlbumArtLoaded(quint64 id, const QImage& image) {
-  LibraryItem* item = pending_art_.take(id);
+  ItemAndCacheKey item_and_cache_key = pending_art_.take(id);
+  LibraryItem* item = item_and_cache_key.first;
+  const QString& cache_key = item_and_cache_key.second;
   if (!item)
     return;
 
+  pending_cache_keys_.remove(cache_key);
+
+  // Insert this image in the cache.
   if (image.isNull()) {
     // Set the no_cover image so we don't continually try to load art.
-    item->metadata.set_image(no_cover_icon_pretty_);
+    QPixmapCache::insert(cache_key, no_cover_icon_pretty_);
   } else {
-    item->metadata.set_image(image);
+    QPixmapCache::insert(cache_key, QPixmap::fromImage(image));
   }
 
   const QModelIndex index = ItemToIndex(item);

src/library/librarymodel.h

@@ -223,6 +223,7 @@ class LibraryModel : public SimpleTreeModel<LibraryItem> {
   QString DividerDisplayText(GroupBy type, const QString& key) const;
 
   // Helpers
+  QString AlbumIconPixmapCacheKey(const QModelIndex& index) const;
   QVariant AlbumIcon(const QModelIndex& index);
   QVariant data(const LibraryItem* item, int role) const;
   bool CompareItems(const LibraryItem* a, const LibraryItem* b) const;
@@ -259,7 +260,7 @@ class LibraryModel : public SimpleTreeModel<LibraryItem> {
   QIcon album_icon_;
   // used as a generic icon to show when no cover art is found,
   // fixed to the same size as the artwork (32x32)
-  QImage no_cover_icon_pretty_;
+  QPixmap no_cover_icon_pretty_;
   QIcon no_cover_icon_;
   QIcon playlists_dir_icon_;
   QIcon playlist_icon_;
@@ -270,7 +271,10 @@ class LibraryModel : public SimpleTreeModel<LibraryItem> {
   bool show_dividers_;
 
   AlbumCoverLoaderOptions cover_loader_options_;
-  QMap<quint64, LibraryItem*> pending_art_;
+
+  typedef QPair<LibraryItem*, QString> ItemAndCacheKey;
+  QMap<quint64, ItemAndCacheKey> pending_art_;
+  QSet<QString> pending_cache_keys_;
 };
 
 Q_DECLARE_METATYPE(LibraryModel::Grouping);