Layers can now be added to elevation plots via drag and drop from

layer tree.

Unfortunately due to qt api limitations, we can't just enable direct
drag and drop from the layer tree -- users have to explicitly hold
the "Ctrl" key while dragging in order to force the copy action.

src/app/elevation/qgselevationprofilewidget.cpp

@@ -157,6 +157,7 @@ QgsElevationProfileWidget::QgsElevationProfileWidget( const QString &name )
   mPanTool = new QgsPlotToolPan( mCanvas );
 
   mLayerTreeView = new QgsAppElevationProfileLayerTreeView( mLayerTree.get() );
+  connect( mLayerTreeView, &QgsAppElevationProfileLayerTreeView::addLayers, this, &QgsElevationProfileWidget::addLayersInternal );
 
   connect( mLayerTreeView, &QAbstractItemView::doubleClicked, this, [ = ]( const QModelIndex & index )
   {
@@ -482,28 +483,32 @@ void QgsElevationProfileWidget::addLayers()
 
   if ( addDialog.exec() == QDialog::Accepted )
   {
-    const QList<QgsMapLayer *> layers = addDialog.selectedLayers();
-    QList< QgsMapLayer * > updatedLayers;
-    if ( !layers.empty() )
+    addLayersInternal( addDialog.selectedLayers() );
+  }
+}
+
+void QgsElevationProfileWidget::addLayersInternal( const QList<QgsMapLayer *> &layers )
+{
+  QList< QgsMapLayer * > updatedLayers;
+  if ( !layers.empty() )
+  {
+    for ( QgsMapLayer *layer : layers )
     {
-      for ( QgsMapLayer *layer : layers )
-      {
-        if ( QgsElevationUtils::enableElevationForLayer( layer ) )
-          updatedLayers << layer;
-      }
+      if ( QgsElevationUtils::enableElevationForLayer( layer ) )
+        updatedLayers << layer;
+    }
 
-      mLayerTreeView->proxyModel()->invalidate();
-      for ( QgsMapLayer *layer : std::as_const( updatedLayers ) )
+    mLayerTreeView->proxyModel()->invalidate();
+    for ( QgsMapLayer *layer : std::as_const( updatedLayers ) )
+    {
+      if ( QgsLayerTreeLayer *node = mLayerTree->findLayer( layer ) )
       {
-        if ( QgsLayerTreeLayer *node = mLayerTree->findLayer( layer ) )
-        {
-          node->setItemVisibilityChecked( true );
-        }
+        node->setItemVisibilityChecked( true );
       }
-
-      updateCanvasLayers();
-      scheduleUpdate();
     }
+
+    updateCanvasLayers();
+    scheduleUpdate();
   }
 }
 

src/app/elevation/qgselevationprofilewidget.h

@@ -116,6 +116,7 @@ class QgsElevationProfileWidget : public QWidget
 
   private slots:
     void addLayers();
+    void addLayersInternal( const QList<QgsMapLayer *> &layers );
     void updateCanvasLayers();
     void onTotalPendingJobsCountChanged( int count );
     void setProfileCurve( const QgsGeometry &curve, bool resetView );

src/gui/elevation/qgselevationprofilelayertreeview.cpp

@@ -255,6 +255,27 @@ bool QgsElevationProfileLayerTreeModel::setData( const QModelIndex &index, const
   return QgsLayerTreeModel::setData( index, value, role );
 }
 
+bool QgsElevationProfileLayerTreeModel::canDropMimeData( const QMimeData *data, Qt::DropAction action, int row, int column, const QModelIndex &parent ) const
+{
+  if ( action == Qt::IgnoreAction )
+    return true;
+
+  if ( !data->hasFormat( QStringLiteral( "application/qgis.layertreemodeldata" ) ) )
+    return false;
+
+  // don't accept moves from other layer trees -- only allow internal drag
+  if ( action == Qt::MoveAction )
+  {
+    const QString source = data->data( QStringLiteral( "application/qgis.layertree.source" ) );
+    if ( source.isEmpty() || source != QStringLiteral( ":0x%1" ).arg( reinterpret_cast<quintptr>( this ), 2 * QT_POINTER_SIZE, 16, QLatin1Char( '0' ) ) )
+    {
+      return false;
+    }
+  }
+
+  return QgsLayerTreeModel::canDropMimeData( data, action, row, column, parent );
+}
+
 bool QgsElevationProfileLayerTreeModel::dropMimeData( const QMimeData *data, Qt::DropAction action, int row, int column, const QModelIndex &parent )
 {
   if ( action == Qt::IgnoreAction )
@@ -263,10 +284,48 @@ bool QgsElevationProfileLayerTreeModel::dropMimeData( const QMimeData *data, Qt:
   if ( !data->hasFormat( QStringLiteral( "application/qgis.layertreemodeldata" ) ) )
     return false;
 
-  // don't accept drags from other layer trees -- only allow internal drag
+  // don't accept moves from other layer trees -- only allow internal drag
   const QString source = data->data( QStringLiteral( "application/qgis.layertree.source" ) );
   if ( source.isEmpty() || source != QStringLiteral( ":0x%1" ).arg( reinterpret_cast<quintptr>( this ), 2 * QT_POINTER_SIZE, 16, QLatin1Char( '0' ) ) )
-    return false;
+  {
+    if ( action == Qt::CopyAction )
+    {
+      QByteArray encodedLayerTreeData = data->data( QStringLiteral( "application/qgis.layertreemodeldata" ) );
+
+      QDomDocument layerTreeDoc;
+      if ( !layerTreeDoc.setContent( QString::fromUtf8( encodedLayerTreeData ) ) )
+        return false;
+
+      QDomElement rootLayerTreeElem = layerTreeDoc.documentElement();
+      if ( rootLayerTreeElem.tagName() != QLatin1String( "layer_tree_model_data" ) )
+        return false;
+
+      QList<QgsMapLayer *> layersToAdd;
+
+      QDomElement elem = rootLayerTreeElem.firstChildElement();
+      while ( !elem.isNull() )
+      {
+        std::unique_ptr< QgsLayerTreeNode > node( QgsLayerTreeNode::readXml( elem, QgsProject::instance() ) );
+        if ( node && QgsLayerTree::isLayer( node.get() ) )
+        {
+          if ( QgsMapLayer *layer = qobject_cast< QgsLayerTreeLayer * >( node.get() )->layer() )
+          {
+            layersToAdd << layer;
+          }
+        }
+        elem = elem.nextSiblingElement();
+      }
+
+      if ( !layersToAdd.empty() )
+        emit addLayers( layersToAdd );
+
+      return true;
+    }
+    else
+    {
+      return false;
+    }
+  }
 
   return QgsLayerTreeModel::dropMimeData( data, action, row, column, parent );
 }
@@ -356,6 +415,7 @@ QgsElevationProfileLayerTreeView::QgsElevationProfileLayerTreeView( QgsLayerTree
   , mLayerTree( rootNode )
 {
   mModel = new QgsElevationProfileLayerTreeModel( rootNode, this );
+  connect( mModel, &QgsElevationProfileLayerTreeModel::addLayers, this, &QgsElevationProfileLayerTreeView::addLayers );
   mProxyModel = new QgsElevationProfileLayerTreeProxyModel( mModel, this );
 
   setHeaderHidden( true );

src/gui/elevation/qgselevationprofilelayertreeview.h

@@ -58,9 +58,20 @@ class GUI_EXPORT QgsElevationProfileLayerTreeModel : public QgsLayerTreeModel
     QVariant data( const QModelIndex &index, int role = Qt::DisplayRole ) const override;
     bool setData( const QModelIndex &index, const QVariant &value, int role = Qt::EditRole ) override;
 
+    bool canDropMimeData( const QMimeData *data, Qt::DropAction action,
+                          int row, int column, const QModelIndex &parent ) const override;
     bool dropMimeData( const QMimeData *data, Qt::DropAction action, int row, int column, const QModelIndex &parent ) override;
     QMimeData *mimeData( const QModelIndexList &indexes ) const override;
 
+  signals:
+
+    /**
+     * Emitted when layers should be added to the profile, e.g. via a drag and drop action.
+     *
+     * \since QGIS 3.32
+     */
+    void addLayers( const QList< QgsMapLayer * > &layers );
+
   private:
 
 #ifdef SIP_RUN
@@ -136,6 +147,15 @@ class GUI_EXPORT QgsElevationProfileLayerTreeView : public QTreeView
      */
     QgsElevationProfileLayerTreeProxyModel *proxyModel();
 
+  signals:
+
+    /**
+     * Emitted when layers should be added to the profile, e.g. via a drag and drop action.
+     *
+     * \since QGIS 3.32
+     */
+    void addLayers( const QList< QgsMapLayer * > &layers );
+
   protected:
 
     void resizeEvent( QResizeEvent *event ) override;