Avoid to create a new thread when changing state. Make the UI more reactive, especially when Qt threadpool is already full (for example when fingerprinting several files in parallel for MusicBrainz autotagging)

src/engines/gstengine.cpp

@@ -396,24 +396,10 @@ bool GstEngine::Play(quint64 offset_nanosec) {
   if (!current_pipeline_)
     return false;
 
-  QFuture<GstStateChangeReturn> future = current_pipeline_->SetState(GST_STATE_PLAYING);
-  PlayFutureWatcher* watcher = new PlayFutureWatcher(
-        PlayFutureWatcherArg(offset_nanosec, current_pipeline_->id()), this);
-  watcher->setFuture(future);
-  connect(watcher, SIGNAL(finished()), SLOT(PlayDone()));
+  GstStateChangeReturn ret = current_pipeline_->SetState(GST_STATE_PLAYING);
 
-  return true;
-}
-
-void GstEngine::PlayDone() {
-  PlayFutureWatcher* watcher = static_cast<PlayFutureWatcher*>(sender());
-  watcher->deleteLater();
-
-  GstStateChangeReturn ret = watcher->result();
-  quint64 offset_nanosec = watcher->data().first;
-
-  if (!current_pipeline_ || watcher->data().second != current_pipeline_->id()) {
-    return;
+  if (!current_pipeline_) {
+    return false;
   }
 
   if (ret == GST_STATE_CHANGE_FAILURE) {
@@ -423,13 +409,13 @@ void GstEngine::PlayDone() {
       qLog(Info) << "Redirecting to" << redirect_url;
       current_pipeline_ = CreatePipeline(redirect_url, end_nanosec_);
       Play(offset_nanosec);
-      return;
+      return false;
     }
 
     // Failure - give up
     qLog(Warning) << "Could not set thread to PLAYING.";
     current_pipeline_.reset();
-    return;
+    return false;
   }
 
   StartTimers();
@@ -444,6 +430,7 @@ void GstEngine::PlayDone() {
   emit StateChanged(Engine::Playing);
   // we've successfully started playing a media stream with this url
   emit ValidSongRequested(url_);
+  return true;
 }
 
 
@@ -773,27 +760,13 @@ int GstEngine::AddBackgroundStream(shared_ptr<GstEnginePipeline> pipeline) {
   const int stream_id = next_background_stream_id_++;
   background_streams_[stream_id] = pipeline;
 
-  QFuture<GstStateChangeReturn> future = pipeline->SetState(GST_STATE_PLAYING);
-  BoundFutureWatcher<GstStateChangeReturn, int>* watcher =
-      new BoundFutureWatcher<GstStateChangeReturn, int>(stream_id, this);
-  watcher->setFuture(future);
-  connect(watcher, SIGNAL(finished()), SLOT(BackgroundStreamPlayDone()));
-
-  return stream_id;
-}
-
-void GstEngine::BackgroundStreamPlayDone() {
-  BoundFutureWatcher<GstStateChangeReturn, int>* watcher =
-      static_cast<BoundFutureWatcher<GstStateChangeReturn, int>*>(sender());
-  watcher->deleteLater();
-
-  const int stream_id = watcher->data();
-  GstStateChangeReturn ret = watcher->result();
+  GstStateChangeReturn ret = pipeline->SetState(GST_STATE_PLAYING);
 
   if (ret == GST_STATE_CHANGE_FAILURE) {
     qLog(Warning) << "Could not set thread to PLAYING.";
     background_streams_.remove(stream_id);
   }
+  return stream_id;
 }
 
 int GstEngine::AddBackgroundStream(const QUrl& url) {

src/engines/gstengine.h

@@ -121,8 +121,6 @@ class GstEngine : public Engine::Base, public BufferConsumer {
   void FadeoutFinished();
   void SeekNow();
   void BackgroundStreamFinished();
-  void BackgroundStreamPlayDone();
-  void PlayDone();
 
  private:
   typedef QPair<quint64, int> PlayFutureWatcherArg;

src/engines/gstenginepipeline.cpp

@@ -616,8 +616,8 @@ GstState GstEnginePipeline::state() const {
   return s;
 }
 
-QFuture<GstStateChangeReturn> GstEnginePipeline::SetState(GstState state) {
-  return QtConcurrent::run(&gst_element_set_state, pipeline_, state);
+GstStateChangeReturn GstEnginePipeline::SetState(GstState state) {
+  return gst_element_set_state(pipeline_, state);
 }
 
 bool GstEnginePipeline::Seek(qint64 nanosec) {

src/engines/gstenginepipeline.h

@@ -61,7 +61,7 @@ class GstEnginePipeline : public QObject {
   void RemoveAllBufferConsumers();
 
   // Control the music playback
-  QFuture<GstStateChangeReturn> SetState(GstState state);
+  GstStateChangeReturn SetState(GstState state);
   Q_INVOKABLE bool Seek(qint64 nanosec);
   void SetEqualizerEnabled(bool enabled);
   void SetEqualizerParams(int preamp, const QList<int>& band_gains);