[feature] When interactively moving a callout line, holding shift

will cause the callouti line angle to snap to 15 degree increments

This allows for easier creation of nicely parallel callout lines

src/app/labeling/qgsmaptoolmovelabel.cpp

@@ -25,6 +25,7 @@
 #include "qgsadvanceddigitizingdockwidget.h"
 #include "qgsvectorlayerlabeling.h"
 #include "qgscallout.h"
+#include "qgsstatusbar.h"
 
 QgsMapToolMoveLabel::QgsMapToolMoveLabel( QgsMapCanvas *canvas, QgsAdvancedDigitizingDockWidget *cadDock )
   : QgsMapToolLabel( canvas, cadDock )
@@ -67,7 +68,15 @@ void QgsMapToolMoveLabel::cadCanvasMoveEvent( QgsMapMouseEvent *e )
   else if ( mCalloutMoveRubberBand )
   {
     const int index = mCurrentCalloutMoveOrigin ? 0 : 1;
-    mCalloutMoveRubberBand->movePoint( index, e->mapPoint() );
+
+    QgsPointXY mapPoint = e->mapPoint();
+    if ( e->modifiers() & Qt::ShiftModifier )
+    {
+      // shift modifier = snap to common angles
+      mapPoint = snapCalloutPointToCommonAngle( mapPoint, true );
+    }
+
+    mCalloutMoveRubberBand->movePoint( index, mapPoint );
     mCalloutMoveRubberBand->update();
   }
   else
@@ -231,17 +240,23 @@ void QgsMapToolMoveLabel::cadCanvasPressEvent( QgsMapMouseEvent *e )
       case Qt::LeftButton:
       {
         // second click drops label/callout
+        const bool isCalloutMove = !mCurrentCallout.layerID.isEmpty();
+        QgsPointXY releaseCoords = e->mapPoint();
+        if ( isCalloutMove && e->modifiers() & Qt::ShiftModifier )
+        {
+          // shift modifier = snap to common angles
+          releaseCoords = snapCalloutPointToCommonAngle( releaseCoords, false );
+        }
+
         deleteRubberBands();
 
-        const bool isCalloutMove = !mCurrentCallout.layerID.isEmpty();
         QgsVectorLayer *vlayer = !isCalloutMove ? mCurrentLabel.layer : QgsProject::instance()->mapLayer<QgsVectorLayer *>( mCurrentCallout.layerID );
         if ( !vlayer )
         {
           return;
         }
         const QgsFeatureId featureId = !isCalloutMove ? mCurrentLabel.pos.featureId : mCurrentCallout.featureId;
 
-        QgsPointXY releaseCoords = e->mapPoint();
         double xdiff = releaseCoords.x() - mStartPointMapCoords.x();
         double ydiff = releaseCoords.y() - mStartPointMapCoords.y();
 
@@ -549,5 +564,41 @@ bool QgsMapToolMoveLabel::currentCalloutDataDefinedPosition( double &x, bool &xS
   return true;
 }
 
+QgsPointXY QgsMapToolMoveLabel::snapCalloutPointToCommonAngle( const QgsPointXY &mapPoint, bool showStatusMessage ) const
+{
+  const int index = mCurrentCalloutMoveOrigin ? 0 : 1;
+
+  QgsPointXY start = *mCalloutMoveRubberBand->getPoint( 0, index == 0 ? 1 : 0 );
+  const double cursorDistance = start.distance( mapPoint );
+
+  // snap to common angles (15 degree increments)
+  double closestDist = std::numeric_limits< double >::max();
+  double closestX = 0;
+  double closestY = 0;
+  int bestAngle = 0;
+
+  const double angleOffset = -canvas()->rotation();
+
+  for ( int angle = 0; angle < 360; angle += 15 )
+  {
+    const QgsPointXY end = start.project( cursorDistance * 2, angle + angleOffset );
+    double minDistX = 0;
+    double minDistY = 0;
+    const double angleDist = QgsGeometryUtils::sqrDistToLine( mapPoint.x(), mapPoint.y(), start.x(), start.y(), end.x(), end.y(), minDistX, minDistY, 4 * std::numeric_limits<double>::epsilon() );
+    if ( angleDist < closestDist )
+    {
+      closestDist = angleDist;
+      closestX = minDistX;
+      closestY = minDistY;
+      bestAngle = angle;
+    }
+  }
+
+  if ( showStatusMessage )
+    QgisApp::instance()->statusBarIface()->showMessage( tr( "Callout angle: %1°" ).arg( bestAngle ), 2000 );
+
+  return QgsPointXY( closestX, closestY );
+}
+
 
 

src/app/labeling/qgsmaptoolmovelabel.h

@@ -57,6 +57,8 @@ class APP_EXPORT QgsMapToolMoveLabel: public QgsMapToolLabel
   private:
     bool currentCalloutDataDefinedPosition( double &x, bool &xSuccess, double &y, bool &ySuccess, int &xCol, int &yCol );
 
+    QgsPointXY snapCalloutPointToCommonAngle( const QgsPointXY &mapPoint, bool showStatusMessage ) const;
+
 };
 
 #endif // QGSMAPTOOLMOVELABEL_H