Merge map layer style undo commands that are created in short time (P…

…R 3277)

This makes the undo stack grow slower (especially when typing or repeatedly
changing a value) and preserves memory.

src/app/qgslayerstylingwidget.cpp

@@ -449,19 +449,18 @@ void QgsLayerStylingWidget::pushUndoItem( const QString &name )
   QDomElement rootNode = doc.createElement( "qgis" );
   doc.appendChild( rootNode );
   mCurrentLayer->writeStyle( rootNode, doc, errorMsg );
-  mCurrentLayer->undoStackStyles()->beginMacro( name );
-  mCurrentLayer->undoStackStyles()->push( new QgsMapLayerStyleCommand( mCurrentLayer, rootNode, mLastStyleXml ) );
-  mCurrentLayer->undoStackStyles()->endMacro();
+  mCurrentLayer->undoStackStyles()->push( new QgsMapLayerStyleCommand( mCurrentLayer, name, rootNode, mLastStyleXml ) );
   // Override the last style on the stack
   mLastStyleXml = rootNode.cloneNode();
 }
 
 
-QgsMapLayerStyleCommand::QgsMapLayerStyleCommand( QgsMapLayer *layer, const QDomNode &current, const QDomNode &last )
-    : QUndoCommand()
+QgsMapLayerStyleCommand::QgsMapLayerStyleCommand( QgsMapLayer *layer, const QString& text, const QDomNode &current, const QDomNode &last )
+    : QUndoCommand( text )
     , mLayer( layer )
     , mXml( current )
     , mLastState( last )
+    , mTime( QTime::currentTime() )
 {
 }
 
@@ -479,6 +478,25 @@ void QgsMapLayerStyleCommand::redo()
   mLayer->triggerRepaint();
 }
 
+bool QgsMapLayerStyleCommand::mergeWith( const QUndoCommand* other )
+{
+  if ( other->id() != id() ) // make sure other is also an QgsMapLayerStyleCommand
+    return false;
+
+  const QgsMapLayerStyleCommand* otherCmd = static_cast<const QgsMapLayerStyleCommand*>( other );
+  if ( otherCmd->mLayer != mLayer )
+    return false;  // should never happen though...
+
+  // only merge commands if they are created shortly after each other
+  // (e.g. user keeps modifying one property)
+  if ( mTime.msecsTo( otherCmd->mTime ) > 3000 )
+    return false;
+
+  mXml = otherCmd->mXml;
+  mTime = otherCmd->mTime;
+  return true;
+}
+
 QIcon QgsLayerStyleManagerWidgetFactory::icon() const
 {
   return  QgsApplication::getThemeIcon( "propertyicons/stylepreset.svg" );

src/app/qgslayerstylingwidget.h

@@ -52,15 +52,24 @@ class APP_EXPORT QgsLayerStyleManagerWidgetFactory : public QgsMapLayerConfigWid
 class APP_EXPORT QgsMapLayerStyleCommand : public QUndoCommand
 {
   public:
-    QgsMapLayerStyleCommand( QgsMapLayer* layer, const QDomNode& current, const QDomNode& last );
+    QgsMapLayerStyleCommand( QgsMapLayer* layer, const QString& text, const QDomNode& current, const QDomNode& last );
+
+    /** Return unique ID for this kind of undo command.
+     * Currently we do not have a central registry of undo command IDs, so it is a random magic number.
+     */
+    virtual int id() const override { return 0xbeef; }
 
     virtual void undo() override;
     virtual void redo() override;
 
+    /** Try to merge with other commands of this type when they are created in small time interval */
+    virtual bool mergeWith( const QUndoCommand* other ) override;
+
   private:
     QgsMapLayer* mLayer;
     QDomNode mXml;
     QDomNode mLastState;
+    QTime mTime;
 };
 
 class APP_EXPORT QgsLayerStylingWidget : public QWidget, private Ui::QgsLayerStylingWidgetBase