Add const/references to all signal parameters

src/collection/collection.h

@@ -81,7 +81,7 @@ class SCollection : public QObject {
   void SongsRatingChanged(const SongList &songs, const bool save_tags = false);
 
  signals:
-  void Error(QString);
+  void Error(const QString &error);
   void ExitFinished();
 
  private:

src/collection/collectionbackend.h

@@ -254,23 +254,23 @@ class CollectionBackend : public CollectionBackendInterface {
   void ExpireSongs(const int directory_id, const int expire_unavailable_songs_days);
 
  signals:
-  void DirectoryDiscovered(CollectionDirectory, CollectionSubdirectoryList);
-  void DirectoryDeleted(CollectionDirectory);
+  void DirectoryDiscovered(const CollectionDirectory &dir, const CollectionSubdirectoryList &subdir);
+  void DirectoryDeleted(const CollectionDirectory &dir);
 
-  void SongsDiscovered(SongList);
-  void SongsDeleted(SongList);
-  void SongsStatisticsChanged(SongList, bool = false);
+  void SongsDiscovered(const SongList &songs);
+  void SongsDeleted(const SongList &songs);
+  void SongsStatisticsChanged(const SongList &songs, const bool save_tags = false);
 
   void DatabaseReset();
 
-  void TotalSongCountUpdated(int);
-  void TotalArtistCountUpdated(int);
-  void TotalAlbumCountUpdated(int);
-  void SongsRatingChanged(SongList, bool);
+  void TotalSongCountUpdated(const int count);
+  void TotalArtistCountUpdated(const int count);
+  void TotalAlbumCountUpdated(const int count);
+  void SongsRatingChanged(const SongList &songs, const bool save_tags);
 
   void ExitFinished();
 
-  void Error(QString);
+  void Error(const QString &error);
 
  private:
   struct CompilationInfo {

src/collection/collectionfilterwidget.h

@@ -96,7 +96,7 @@ class CollectionFilterWidget : public QWidget {
   void UpPressed();
   void DownPressed();
   void ReturnPressed();
-  void Filter(QString text);
+  void Filter(const QString &text);
 
  protected:
   void keyReleaseEvent(QKeyEvent *e) override;

src/collection/collectionmodel.h

@@ -199,10 +199,10 @@ class CollectionModel : public SimpleTreeModel<CollectionItem> {
   static QString ContainerKey(const GroupBy group_by, const bool separate_albums_by_grouping, const Song &song);
 
  signals:
-  void TotalSongCountUpdated(int count);
-  void TotalArtistCountUpdated(int count);
-  void TotalAlbumCountUpdated(int count);
-  void GroupingChanged(CollectionModel::Grouping g, bool separate_albums_by_grouping);
+  void TotalSongCountUpdated(const int count);
+  void TotalArtistCountUpdated(const int count);
+  void TotalAlbumCountUpdated(const int count);
+  void GroupingChanged(const CollectionModel::Grouping g, const bool separate_albums_by_grouping);
 
  public slots:
   void SetFilterMode(CollectionFilterOptions::FilterMode filter_mode);

src/collection/collectionview.h

@@ -89,7 +89,7 @@ class CollectionView : public AutoExpandingTreeView {
   void TotalSongCountUpdated_();
   void TotalArtistCountUpdated_();
   void TotalAlbumCountUpdated_();
-  void Error(QString);
+  void Error(const QString &error);
 
  protected:
   // QWidget

src/collection/collectionwatcher.h

@@ -70,18 +70,18 @@ class CollectionWatcher : public QObject {
   void RescanSongsAsync(const SongList &songs);
 
  signals:
-  void NewOrUpdatedSongs(SongList);
-  void SongsMTimeUpdated(SongList);
-  void SongsDeleted(SongList);
-  void SongsUnavailable(SongList songs, bool unavailable = true);
-  void SongsReadded(SongList songs, bool unavailable = false);
-  void SubdirsDiscovered(CollectionSubdirectoryList subdirs);
-  void SubdirsMTimeUpdated(CollectionSubdirectoryList subdirs);
+  void NewOrUpdatedSongs(const SongList &songs);
+  void SongsMTimeUpdated(const SongList &songs);
+  void SongsDeleted(const SongList &songs);
+  void SongsUnavailable(const SongList &songs, const bool unavailable = true);
+  void SongsReadded(const SongList &songs, const bool unavailable = false);
+  void SubdirsDiscovered(const CollectionSubdirectoryList &subdirs);
+  void SubdirsMTimeUpdated(const CollectionSubdirectoryList &subdirs);
   void CompilationsNeedUpdating();
-  void UpdateLastSeen(int directory_id, int expire_unavailable_songs_days);
+  void UpdateLastSeen(const int directory_id, const int expire_unavailable_songs_days);
   void ExitFinished();
 
-  void ScanStarted(int task_id);
+  void ScanStarted(const int task_id);
 
  public slots:
   void AddDirectory(const CollectionDirectory &dir, const CollectionSubdirectoryList &subdirs);

src/collection/groupbydialog.h

@@ -49,7 +49,7 @@ class GroupByDialog : public QDialog {
   void accept() override;
 
  signals:
-  void Accepted(CollectionModel::Grouping g, bool separate_albums_by_grouping);
+  void Accepted(const CollectionModel::Grouping g, const bool separate_albums_by_grouping);
 
  private slots:
   void Reset();