viewer: implement lookahead to ensure footage is ready in advance

app/widget/viewer/viewer.cpp

@@ -60,7 +60,7 @@ QVector<ViewerWidget*> ViewerWidget::instances_;
 //       changing values. 1/4 second seems to be a good middleground.
 const rational ViewerWidget::kAudioPlaybackInterval = rational(1, 4);
 
-const rational kVideoPlaybackInterval = rational(2);
+const rational kVideoPlaybackInterval = rational(1, 2);
 
 ViewerWidget::ViewerWidget(QWidget *parent) :
   super(false, true, parent),
@@ -560,6 +560,35 @@ void ViewerWidget::ShowSubtitleProperties()
   }
 }
 
+void ViewerWidget::DryRunFinished()
+{
+  RenderTicketWatcher *w = static_cast<RenderTicketWatcher*>(sender());
+
+  if (dry_run_watchers_.contains(w)) {
+    RequestNextDryRun();
+  }
+
+  delete w;
+}
+
+void ViewerWidget::RequestNextDryRun()
+{
+  if (IsPlaying()) {
+    rational next_time = Timecode::timestamp_to_time(dry_run_next_frame_, timebase());
+    if (FrameExistsAtTime(next_time)) {
+      if (next_time > GetTime() + RenderManager::kDryRunInterval) {
+        QTimer::singleShot(timebase().toDouble() / playback_speed_, this, &ViewerWidget::RequestNextDryRun);
+      } else {
+        RenderTicketWatcher *watcher = new RenderTicketWatcher(this);
+        connect(watcher, &RenderTicketWatcher::Finished, this, &ViewerWidget::DryRunFinished);
+        watcher->SetTicket(auto_cacher_.GetSingleFrame(next_time, true));
+        dry_run_next_frame_ += playback_speed_;
+        dry_run_watchers_.append(watcher);
+      }
+    }
+  }
+}
+
 void ViewerWidget::CloseAudioProcessor()
 {
   audio_processor_.Close();
@@ -828,6 +857,9 @@ void ViewerWidget::PlayInternal(int speed, bool in_to_out_only)
       for (int i=0; i<prequeue_length_; i++) {
         RequestNextFrameForQueue();
       }
+
+      dry_run_next_frame_ = playback_queue_next_frame_;
+      RequestNextDryRun();
     }
   }
 
@@ -845,6 +877,7 @@ void ViewerWidget::PlayInternal(int speed, bool in_to_out_only)
       QueueNextAudioBuffer();
     }
   }
+
   // Force screen to stay awake
   PreventSleep(true);
 }
@@ -893,6 +926,7 @@ void ViewerWidget::PauseInternal()
 
   prequeuing_video_ = false;
   prequeuing_audio_ = 0;
+  dry_run_watchers_.clear();
 
   // Reset screen timeout timer
   PreventSleep(false);