Make paletted raster widget behave the same as categorized widget

when changing current band

Now the widget asks users if they want to delete existing categories
when they switch a band, just like how the categorized renderer
widget asks when the categorized field is changed.

python/core/raster/qgspalettedrasterrenderer.sip

@@ -91,6 +91,13 @@ Returns number of colors
 .. versionadded:: 2.1
 %End
 
+    int band() const;
+%Docstring
+ Returns the raster band used for rendering the raster.
+.. versionadded:: 3.0
+ :rtype: int
+%End
+
     virtual void writeXml( QDomDocument &doc, QDomElement &parentElem ) const;
 
 

src/core/raster/qgspalettedrasterrenderer.h

@@ -88,6 +88,12 @@ class CORE_EXPORT QgsPalettedRasterRenderer: public QgsRasterRenderer
      *  \since QGIS 2.1 */
     void setLabel( int idx, const QString &label );
 
+    /**
+     * Returns the raster band used for rendering the raster.
+     * \since QGIS 3.0
+     */
+    int band() const { return mBand; }
+
     void writeXml( QDomDocument &doc, QDomElement &parentElem ) const override;
 
     void legendSymbologyItems( QList< QPair< QString, QColor > > &symbolItems SIP_OUT ) const override;

src/gui/raster/qgspalettedrendererwidget.cpp

@@ -113,7 +113,7 @@ QgsPalettedRendererWidget::QgsPalettedRendererWidget( QgsRasterLayer *layer, con
   }
 
   connect( QgsProject::instance(), static_cast < void ( QgsProject::* )( QgsMapLayer * ) >( &QgsProject::layerWillBeRemoved ), this, &QgsPalettedRendererWidget::layerWillBeRemoved );
-  connect( mBandComboBox, &QgsRasterBandComboBox::bandChanged, this, &QgsPalettedRendererWidget::loadFromLayer );
+  connect( mBandComboBox, &QgsRasterBandComboBox::bandChanged, this, &QgsPalettedRendererWidget::bandChanged );
 }
 
 QgsPalettedRendererWidget::~QgsPalettedRendererWidget()
@@ -143,6 +143,9 @@ void QgsPalettedRendererWidget::setFromRenderer( const QgsRasterRenderer *r )
   const QgsPalettedRasterRenderer *pr = dynamic_cast<const QgsPalettedRasterRenderer *>( r );
   if ( pr )
   {
+    mBand = pr->band();
+    whileBlocking( mBandComboBox )->setBand( mBand );
+
     //read values and colors and fill into tree widget
     mModel->setClassData( pr->classes() );
 
@@ -440,6 +443,32 @@ void QgsPalettedRendererWidget::loadFromLayer()
   }
 }
 
+void QgsPalettedRendererWidget::bandChanged( int band )
+{
+  if ( band == mBand )
+    return;
+
+  bool deleteExisting = false;
+  if ( !mModel->classData().isEmpty() )
+  {
+    int res = QMessageBox::question( this,
+                                     tr( "Confirm Delete" ),
+                                     tr( "The classification band was changed from %1 to %2.\n"
+                                         "Should the existing classes be deleted?" ).arg( mBand ).arg( band ),
+                                     QMessageBox::Yes | QMessageBox::No );
+
+    deleteExisting = ( res == QMessageBox::Yes );
+  }
+
+  mBand = band;
+  mModel->blockSignals( true );
+  if ( deleteExisting )
+    mModel->deleteAll();
+
+  mModel->blockSignals( false );
+  emit widgetChanged();
+}
+
 void QgsPalettedRendererWidget::gatheredClasses()
 {
   if ( !mGatherer || mGatherer->wasCanceled() )

src/gui/raster/qgspalettedrendererwidget.h

@@ -207,6 +207,8 @@ class GUI_EXPORT QgsPalettedRendererWidget: public QgsRasterRendererWidget, priv
     //! Background class gatherer thread
     QgsPalettedRendererClassGatherer *mGatherer = nullptr;
 
+    int mBand = -1;
+
     void setSelectionColor( const QItemSelection &selection, const QColor &color );
 
   private slots:
@@ -221,6 +223,7 @@ class GUI_EXPORT QgsPalettedRendererWidget: public QgsRasterRendererWidget, priv
     void saveColorTable();
     void classify();
     void loadFromLayer();
+    void bandChanged( int band );
 
     void gatheredClasses();
     void gathererThreadFinished();