Dive pictures: Introduce thumbnailer class

Create a new class, which performs all thumbnailing code.
This is mostly code reshuffling. Thumbnails are extracted
either from a cache or thumbnail calculation is started in
a worker thread.

Since getHashedImage() is called from a worker thread it
makes no sense to call subfunctions in yet another worker
thread. Remove these calls.

In contrast to the previous code, on error the background
thread produces a failure image, but it is not yet shown.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/imagedownloader.h

@@ -5,6 +5,7 @@
 #include <QImage>
 #include <QFuture>
 #include <QNetworkReply>
+#include <QThreadPool>
 
 class ImageDownloader : public QObject {
 	Q_OBJECT
@@ -18,6 +19,31 @@ private:
 	QString filename;
 };
 
+class PictureEntry;
+class Thumbnailer : public QObject {
+	Q_OBJECT
+public:
+	static Thumbnailer *instance();
+
+	// Schedule a thumbnail for fetching or calculation.
+	// Returns a placehlder thumbnail. The actual thumbnail will be sent
+	// via a signal later.
+	QImage fetchThumbnail(PictureEntry &entry, int size);
+
+	// If we change dive, clear all unfinished thumbnail creations
+	void clearWorkQueue();
+signals:
+	void thumbnailChanged(QString filename, QImage thumbnail);
+private:
+	Thumbnailer();
+	void processItem(QString filename, int size);
+
+	mutable QMutex lock;
+	QThreadPool pool;
+
+	QMap<QString,QFuture<void>> workingOn;
+};
+
 QImage getHashedImage(const QString &filename);
 
 #endif // IMAGEDOWNLOADER_H