[needs-docs] Allow dragging project layers from legend over map layer comboboxes to select

Greatly assists with usability of the combobox with large projects,
especially those with multiple copies of layers with the same name...

python/gui/auto_generated/qgsmaplayercombobox.sip.in

@@ -154,6 +154,17 @@ setLayer set the current layer selected in the combo
 Emitted whenever the currently selected layer changes.
 %End
 
+  protected:
+
+    virtual void dragEnterEvent( QDragEnterEvent *event );
+
+    virtual void dragLeaveEvent( QDragLeaveEvent *event );
+
+    virtual void dropEvent( QDropEvent *event );
+
+    virtual void paintEvent( QPaintEvent *e );
+
+
   protected slots:
     void indexChanged( int i );
     void rowsChanged();

src/gui/qgsmaplayercombobox.cpp

@@ -15,7 +15,9 @@
 
 #include "qgsmaplayercombobox.h"
 #include "qgsmaplayermodel.h"
-
+#include "qgsmimedatautils.h"
+#include <QDragEnterEvent>
+#include <QPainter>
 
 QgsMapLayerComboBox::QgsMapLayerComboBox( QWidget *parent )
   : QComboBox( parent )
@@ -26,6 +28,8 @@ QgsMapLayerComboBox::QgsMapLayerComboBox( QWidget *parent )
   connect( this, static_cast < void ( QComboBox::* )( int ) > ( &QComboBox::activated ), this, &QgsMapLayerComboBox::indexChanged );
   connect( mProxyModel, &QAbstractItemModel::rowsInserted, this, &QgsMapLayerComboBox::rowsChanged );
   connect( mProxyModel, &QAbstractItemModel::rowsRemoved, this, &QgsMapLayerComboBox::rowsChanged );
+
+  setAcceptDrops( true );
 }
 
 void QgsMapLayerComboBox::setExcludedProviders( const QStringList &providers )
@@ -138,3 +142,74 @@ void QgsMapLayerComboBox::rowsChanged()
   }
 }
 
+QgsMapLayer *QgsMapLayerComboBox::compatibleMapLayerFromMimeData( const QMimeData *data ) const
+{
+  const QgsMimeDataUtils::UriList uriList = QgsMimeDataUtils::decodeUriList( data );
+  for ( const QgsMimeDataUtils::Uri &u : uriList )
+  {
+    // is this uri from the current project?
+    if ( QgsMapLayer *layer = u.mapLayer() )
+    {
+      if ( mProxyModel->acceptsLayer( layer ) )
+        return layer;
+    }
+  }
+  return nullptr;
+}
+
+void QgsMapLayerComboBox::dragEnterEvent( QDragEnterEvent *event )
+{
+  if ( !( event->possibleActions() & Qt::CopyAction ) )
+    return;
+
+  if ( compatibleMapLayerFromMimeData( event->mimeData() ) )
+  {
+    // dragged an acceptable layer, phew
+    event->setDropAction( Qt::CopyAction );
+    event->accept();
+    mDragActive = true;
+    update();
+  }
+}
+
+void QgsMapLayerComboBox::dragLeaveEvent( QDragLeaveEvent *event )
+{
+  QComboBox::dragLeaveEvent( event );
+  if ( mDragActive )
+  {
+    event->accept();
+    mDragActive = false;
+    update();
+  }
+}
+
+void QgsMapLayerComboBox::dropEvent( QDropEvent *event )
+{
+  if ( !( event->possibleActions() & Qt::CopyAction ) )
+    return;
+
+  if ( QgsMapLayer *layer = compatibleMapLayerFromMimeData( event->mimeData() ) )
+  {
+    // dropped an acceptable layer, phew
+    setFocus( Qt::MouseFocusReason );
+    event->setDropAction( Qt::CopyAction );
+    event->accept();
+
+    setLayer( layer );
+  }
+  mDragActive = false;
+  update();
+}
+
+void QgsMapLayerComboBox::paintEvent( QPaintEvent *e )
+{
+  QComboBox::paintEvent( e );
+  if ( mDragActive )
+  {
+    QPainter p( this );
+    int width = 2;  // width of highlight rectangle inside frame
+    p.setPen( QPen( palette().highlight(), width ) );
+    QRect r = rect().adjusted( width, width, -width, -width );
+    p.drawRect( r );
+  }
+}

src/gui/qgsmaplayercombobox.h

@@ -139,12 +139,26 @@ class GUI_EXPORT QgsMapLayerComboBox : public QComboBox
     //! Emitted whenever the currently selected layer changes.
     void layerChanged( QgsMapLayer *layer );
 
+  protected:
+
+    void dragEnterEvent( QDragEnterEvent *event ) override;
+    void dragLeaveEvent( QDragLeaveEvent *event ) override;
+    void dropEvent( QDropEvent *event ) override;
+    void paintEvent( QPaintEvent *e ) override;
+
   protected slots:
     void indexChanged( int i );
     void rowsChanged();
 
   private:
     QgsMapLayerProxyModel *mProxyModel = nullptr;
+    bool mDragActive = false;
+
+    /**
+     * Returns a map layer, compatible with the filters set for the combo box, from
+     * the specified mime \a data (if possible!).
+     */
+    QgsMapLayer *compatibleMapLayerFromMimeData( const QMimeData *data ) const;
 };
 
 #endif // QGSMAPLAYERCOMBOBOX_H