refs #730 Avoid duplicate "empty cache value" debug messages.

src/blackmisc/valuecache.cpp

@@ -567,7 +567,20 @@ namespace BlackMisc
         if (!status.isEmpty()) // intentionally kept !empty here, debug message supposed to write default value
         {
             element.m_value.uniqueWrite() = defaultValue;
-            CLogMessage::preformatted(status);
+
+            if (status.getSeverity() == CStatusMessage::SeverityDebug)
+            {
+                QMutexLocker lock(&m_cache->m_warnedKeysMutex);
+                if (! m_cache->m_warnedKeys.contains(key))
+                {
+                    m_cache->m_warnedKeys.insert(key);
+                    CLogMessage::preformatted(status);
+                }
+            }
+            else
+            {
+                CLogMessage::preformatted(status);
+            }
         }
 
         return element;

src/blackmisc/valuecache.h

@@ -314,6 +314,10 @@ namespace BlackMisc
 
         virtual void connectPage(Private::CValuePage *page);
 
+        // only used by CValuePage::createElement
+        QSet<QString> m_warnedKeys;
+        QMutex m_warnedKeysMutex;
+
     signals:
         //! \private
         void valuesChanged(const BlackMisc::CValueCachePacket &values, QObject *changedBy);