Make Viewer::advanceToNextFrame() more customizable

To prevent applications from reimplementing this method, which can lead to
major problems, it is made more customizable. It is now possible to change
the time source by overwriting `Viewer::advanceToNextTimePoint()` and to
add special stuff with `Viewer::advanceToNextFrameHook()`.

See vsg-dev#1132.

include/vsg/app/Viewer.h

@@ -96,6 +96,14 @@ namespace vsg
         /// hint for setting the FrameStamp::simulationTime to time since start_point()
         static constexpr double UseTimeSinceStartPoint = std::numeric_limits<double>::max();
 
+        /// get next time point in Viewer::advanceToNextFrame()
+        /// The standard implemnentation advances to the actual time by using vsg::clock::now()
+        virtual clock::time_point advanceToNextTimePoint();
+
+        /// hook for Viewer::advanceToNextFrame() for advancing to next tasks, layers etc.
+        /// The default implementation advances to the next tasks
+        virtual void advanceToNextFrameHook();
+
         /// Convenience method for advancing to the next frame.
         /// Check active status, return false if viewer no longer active.
         /// If still active, poll for pending events and place them in the Events list and advance to the next frame, generate updated FrameStamp to signify the advancement to a new frame and return true.

src/vsg/app/Viewer.cpp

@@ -150,6 +150,19 @@ bool Viewer::pollEvents(bool discardPreviousEvents)
     return result;
 }
 
+clock::time_point Viewer::advanceToNextTimePoint()
+{
+    return vsg::clock::now();
+}
+
+void Viewer::advanceToNextFrameHook()
+{
+    for (auto& task : recordAndSubmitTasks)
+    {
+        task->advance();
+    }
+}
+
 bool Viewer::advanceToNextFrame(double simulationTime)
 {
     static constexpr SourceLocation s_frame_source_location{"Viewer advanceToNextFrame", VsgFunctionName, __FILE__, __LINE__, COLOR_VIEWER, 1};
@@ -168,7 +181,7 @@ bool Viewer::advanceToNextFrame(double simulationTime)
     if (!acquireNextFrame()) return false;
 
     // create FrameStamp for frame
-    auto time = vsg::clock::now();
+    auto time = advanceToNextTimePoint();
     if (!_frameStamp)
     {
         _start_point = time;
@@ -192,10 +205,7 @@ bool Viewer::advanceToNextFrame(double simulationTime)
     // signal to instrumentation the start of frame
     if (instrumentation) instrumentation->enterFrame(&s_frame_source_location, frameReference, *_frameStamp);
 
-    for (auto& task : recordAndSubmitTasks)
-    {
-        task->advance();
-    }
+    advanceToNextFrameHook();
 
     // create an event for the new frame.
     _events.emplace_back(new FrameEvent(_frameStamp));