[FEATURE] Allow dropping a map layer from the layer tree onto

a projection selection widget to set the projection to match that layer

Just a little timesaving shortcut!

python/gui/auto_generated/qgsprojectionselectionwidget.sip.in

@@ -119,6 +119,15 @@ to the preset CRSes shown in the widget.
 Opens the dialog for selecting a new CRS
 %End
 
+  protected:
+
+    virtual void dragEnterEvent( QDragEnterEvent *event );
+
+    virtual void dragLeaveEvent( QDragLeaveEvent *event );
+
+    virtual void dropEvent( QDropEvent *event );
+
+
 };
 
 /************************************************************************

src/gui/CMakeLists.txt

@@ -317,6 +317,7 @@ SET(QGIS_GUI_SRCS
   qgsgui.cpp
   qgsguiutils.cpp
   qgshighlight.cpp
+  qgshighlightablecombobox.cpp
   qgshistogramwidget.cpp
   qgshelp.cpp
   qgsidentifymenu.cpp
@@ -520,6 +521,7 @@ SET(QGIS_GUI_HDRS
   qgsguiutils.h
   qgshelp.h
   qgshighlight.h
+  qgshighlightablecombobox.h
   qgshistogramwidget.h
   qgsidentifymenu.h
   qgskeyvaluewidget.h

src/gui/qgshighlightablecombobox.cpp

@@ -0,0 +1,40 @@
+/***************************************************************************
+    qgshighlightablecombobox.cpp
+     ---------------------------
+    Date                 : 20/12/2019
+    Copyright            : (C) 2019 by Nyall Dawson
+    Email                : nyall dot dawson at gmail dot com
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+
+#include "qgshighlightablecombobox.h"
+#include <QPainter>
+
+QgsHighlightableComboBox::QgsHighlightableComboBox( QWidget *parent )
+  : QComboBox( parent )
+{}
+
+void QgsHighlightableComboBox::paintEvent( QPaintEvent *e )
+{
+  QComboBox::paintEvent( e );
+  if ( mHighlight )
+  {
+    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 );
+  }
+}
+
+void QgsHighlightableComboBox::setHighlighted( bool highlighted )
+{
+  mHighlight = highlighted;
+  update();
+}

src/gui/qgshighlightablecombobox.h

@@ -0,0 +1,65 @@
+/***************************************************************************
+    qgshighlightablecombobox.h
+     -------------------------
+    Date                 : 20/12/2019
+    Copyright            : (C) 2019 by Nyall Dawson
+    Email                : nyall dot dawson at gmail dot com
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+
+#ifndef QGSHIGHLIGHTABLECOMBOBOX_H
+#define QGSHIGHLIGHTABLECOMBOBOX_H
+
+#include <QComboBox>
+
+#include "qgscoordinatereferencesystem.h"
+#include "qgis_gui.h"
+
+#define SIP_NO_FILE
+
+/**
+ * \class QgsHighlightableComboBox
+ * \ingroup gui
+ *
+ * A QComboBox subclass with the ability to "highlight" the edges of the widget.
+ *
+ * \since QGIS 3.12
+ */
+class GUI_EXPORT QgsHighlightableComboBox : public QComboBox
+{
+    Q_OBJECT
+  public:
+
+    /**
+     * Constructor for QgsHighlightableComboBox with the specified parent widget.
+     */
+    QgsHighlightableComboBox( QWidget *parent = nullptr );
+
+    /**
+     * Returns TRUE if the combo box is currently highlighted.
+     * \see setHighlighted()
+     */
+    bool isHighlighted() const { return mHighlight; }
+
+    /**
+     * Sets whether the combo box is currently \a highlighted.
+     * \see isHighlighted()
+     */
+    void setHighlighted( bool highlighted );
+
+  protected:
+    void paintEvent( QPaintEvent *e ) override;
+
+  private:
+
+    bool mHighlight = false;
+
+};
+
+#endif // QGSHIGHLIGHTABLECOMBOBOX_H

src/gui/qgsprojectionselectionwidget.cpp

@@ -20,18 +20,17 @@
 #include "qgsprojectionselectiondialog.h"
 #include "qgsproject.h"
 #include "qgssettings.h"
+#include "qgshighlightablecombobox.h"
 
 QgsProjectionSelectionWidget::QgsProjectionSelectionWidget( QWidget *parent )
   : QWidget( parent )
 {
-
-
   QHBoxLayout *layout = new QHBoxLayout();
   layout->setContentsMargins( 0, 0, 0, 0 );
   layout->setSpacing( 6 );
   setLayout( layout );
 
-  mCrsComboBox = new QComboBox( this );
+  mCrsComboBox = new QgsHighlightableComboBox( this );
   mCrsComboBox->addItem( tr( "invalid projection" ), QgsProjectionSelectionWidget::CurrentCrs );
   mCrsComboBox->setSizePolicy( QSizePolicy::Ignored, QSizePolicy::Preferred );
 
@@ -58,14 +57,15 @@ QgsProjectionSelectionWidget::QgsProjectionSelectionWidget( QWidget *parent )
 
   setFocusPolicy( Qt::StrongFocus );
   setFocusProxy( mButton );
+  setAcceptDrops( true );
 
   connect( mButton, &QToolButton::clicked, this, &QgsProjectionSelectionWidget::selectCrs );
   connect( mCrsComboBox, static_cast<void ( QComboBox::* )( int )>( &QComboBox::currentIndexChanged ), this, &QgsProjectionSelectionWidget::comboIndexChanged );
 }
 
 QgsCoordinateReferenceSystem QgsProjectionSelectionWidget::crs() const
 {
-  switch ( ( CrsOption )mCrsComboBox->currentData().toInt() )
+  switch ( static_cast< CrsOption >( mCrsComboBox->currentData().toInt() ) )
   {
     case QgsProjectionSelectionWidget::LayerCrs:
       return mLayerCrs;
@@ -194,6 +194,68 @@ void QgsProjectionSelectionWidget::selectCrs()
   }
 }
 
+void QgsProjectionSelectionWidget::dragEnterEvent( QDragEnterEvent *event )
+{
+  if ( !( event->possibleActions() & Qt::CopyAction ) )
+  {
+    event->ignore();
+    return;
+  }
+
+  if ( mapLayerFromMimeData( event->mimeData() ) )
+  {
+    // dragged an acceptable layer, phew
+    event->setDropAction( Qt::CopyAction );
+    event->accept();
+    mCrsComboBox->setHighlighted( true );
+    update();
+  }
+  else
+  {
+    event->ignore();
+  }
+}
+
+void QgsProjectionSelectionWidget::dragLeaveEvent( QDragLeaveEvent *event )
+{
+  if ( mCrsComboBox->isHighlighted() )
+  {
+    event->accept();
+    mCrsComboBox->setHighlighted( false );
+    update();
+  }
+  else
+  {
+    event->ignore();
+  }
+}
+
+void QgsProjectionSelectionWidget::dropEvent( QDropEvent *event )
+{
+  if ( !( event->possibleActions() & Qt::CopyAction ) )
+  {
+    event->ignore();
+    return;
+  }
+
+  if ( QgsMapLayer *layer = mapLayerFromMimeData( event->mimeData() ) )
+  {
+    // dropped a map layer
+    setFocus( Qt::MouseFocusReason );
+    event->setDropAction( Qt::CopyAction );
+    event->accept();
+
+    if ( layer->crs().isValid() )
+      setCrs( layer->crs() );
+  }
+  else
+  {
+    event->ignore();
+  }
+  mCrsComboBox->setHighlighted( false );
+  update();
+}
+
 void QgsProjectionSelectionWidget::addNotSetOption()
 {
   mCrsComboBox->insertItem( 0, mNotSetText, QgsProjectionSelectionWidget::CrsNotSet );
@@ -203,7 +265,7 @@ void QgsProjectionSelectionWidget::addNotSetOption()
 
 void QgsProjectionSelectionWidget::comboIndexChanged( int idx )
 {
-  switch ( ( CrsOption )mCrsComboBox->itemData( idx ).toInt() )
+  switch ( static_cast< CrsOption >( mCrsComboBox->itemData( idx ).toInt() ) )
   {
     case QgsProjectionSelectionWidget::LayerCrs:
       emit crsChanged( mLayerCrs );
@@ -349,7 +411,7 @@ int QgsProjectionSelectionWidget::firstRecentCrsIndex() const
 {
   for ( int i = 0; i < mCrsComboBox->count(); ++i )
   {
-    if ( ( CrsOption )mCrsComboBox->itemData( i ).toInt() == RecentCrs )
+    if ( static_cast< CrsOption >( mCrsComboBox->itemData( i ).toInt() ) == RecentCrs )
     {
       return i;
     }
@@ -365,3 +427,17 @@ void QgsProjectionSelectionWidget::updateTooltip()
   else
     setToolTip( QString() );
 }
+
+QgsMapLayer *QgsProjectionSelectionWidget::mapLayerFromMimeData( 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() )
+    {
+      return layer;
+    }
+  }
+  return nullptr;
+}

src/gui/qgsprojectionselectionwidget.h

@@ -27,6 +27,7 @@
 #include "qgis_gui.h"
 
 class QgsProjectionSelectionDialog;
+class QgsHighlightableComboBox;
 
 /**
  * \class QgsProjectionSelectionWidget
@@ -132,13 +133,19 @@ class GUI_EXPORT QgsProjectionSelectionWidget : public QWidget
      */
     void selectCrs();
 
+  protected:
+
+    void dragEnterEvent( QDragEnterEvent *event ) override;
+    void dragLeaveEvent( QDragLeaveEvent *event ) override;
+    void dropEvent( QDropEvent *event ) override;
+
   private:
 
     QgsCoordinateReferenceSystem mCrs;
     QgsCoordinateReferenceSystem mLayerCrs;
     QgsCoordinateReferenceSystem mProjectCrs;
     QgsCoordinateReferenceSystem mDefaultCrs;
-    QComboBox *mCrsComboBox = nullptr;
+    QgsHighlightableComboBox *mCrsComboBox = nullptr;
     QToolButton *mButton = nullptr;
     QgsProjectionSelectionDialog *mDialog = nullptr;
     QString mNotSetText;
@@ -155,6 +162,8 @@ class GUI_EXPORT QgsProjectionSelectionWidget : public QWidget
     int firstRecentCrsIndex() const;
     void updateTooltip();
 
+    QgsMapLayer *mapLayerFromMimeData( const QMimeData *data ) const;
+
   private slots:
 
     void comboIndexChanged( int idx );