Refine paletted raster renderer classification

Change behaviour from clearing all existing values on classification
to instead keeping any existing values which do exist in the raster,
maintaining their current color and label.

src/gui/raster/qgspalettedrendererwidget.cpp

@@ -406,7 +406,7 @@ void QgsPalettedRendererWidget::classify()
       return;
     }
 
-    mGatherer = new QgsPalettedRendererClassGatherer( mRasterLayer,  mBandComboBox->currentData().toInt(), btnColorRamp->colorRamp() );
+    mGatherer = new QgsPalettedRendererClassGatherer( mRasterLayer,  mBandComboBox->currentData().toInt(), mModel->classData(), btnColorRamp->colorRamp() );
 
     connect( mGatherer, &QgsPalettedRendererClassGatherer::progressChanged, mCalculatingProgressBar, &QProgressBar::setValue );
     mCalculatingProgressBar->show();
@@ -450,7 +450,7 @@ void QgsPalettedRendererWidget::gathererThreadFinished()
 {
   mGatherer->deleteLater();
   mGatherer = nullptr;
-  mClassifyButton->setText( tr( "Add Unique Values" ) );
+  mClassifyButton->setText( tr( "Populate Values" ) );
   mClassifyButton->setEnabled( true );
   mCalculatingProgressBar->hide();
   mCancelButton->hide();

src/gui/raster/qgspalettedrendererwidget.h

@@ -38,10 +38,11 @@ class QgsPalettedRendererClassGatherer: public QThread
     Q_OBJECT
 
   public:
-    QgsPalettedRendererClassGatherer( QgsRasterLayer *layer, int bandNumber, QgsColorRamp *ramp = nullptr )
+    QgsPalettedRendererClassGatherer( QgsRasterLayer *layer, int bandNumber, const QgsPalettedRasterRenderer::ClassData &existingClasses, QgsColorRamp *ramp = nullptr )
       : mLayer( layer )
       , mBandNumber( bandNumber )
       , mRamp( ramp )
+      , mClasses( existingClasses )
       , mFeedback( nullptr )
       , mWasCanceled( false )
     {}
@@ -54,7 +55,25 @@ class QgsPalettedRendererClassGatherer: public QThread
       mFeedback = new QgsRasterBlockFeedback();
       connect( mFeedback, &QgsRasterBlockFeedback::progressChanged, this, &QgsPalettedRendererClassGatherer::progressChanged );
 
-      mClasses = QgsPalettedRasterRenderer::classDataFromRaster( mLayer->dataProvider(), mBandNumber, mRamp.get(), mFeedback );
+      QgsPalettedRasterRenderer::ClassData newClasses = QgsPalettedRasterRenderer::classDataFromRaster( mLayer->dataProvider(), mBandNumber, mRamp.get(), mFeedback );
+
+      // combine existing classes with new classes
+      QgsPalettedRasterRenderer::ClassData::iterator classIt = newClasses.begin();
+      for ( ; classIt != newClasses.end(); ++classIt )
+      {
+        // check if existing classes contains this same class
+        Q_FOREACH ( const QgsPalettedRasterRenderer::Class &existingClass, mClasses )
+        {
+          if ( existingClass.value == classIt->value )
+          {
+            classIt->color = existingClass.color;
+            classIt->label = existingClass.label;
+            break;
+          }
+        }
+      }
+      mClasses = newClasses;
+
       // be overly cautious - it's *possible* stop() might be called between deleting mFeedback and nulling it
       mFeedbackMutex.lock();
       delete mFeedback;

src/ui/qgspalettedrendererwidgetbase.ui

@@ -43,8 +43,11 @@
      </property>
      <item>
       <widget class="QPushButton" name="mClassifyButton">
+       <property name="toolTip">
+        <string>Adds all missing unique values from the raster</string>
+       </property>
        <property name="text">
-        <string>Add Unique Values</string>
+        <string>Populate Values</string>
        </property>
       </widget>
      </item>