[needs-docs] Refine snapping logic for layouts

Previously grids would always take precedence when both a grid
and guide were within tolerance of a point.

Now, guides will always take precedence - since they have been
manually set by users we make the assumption that they have
been explicitly placed at highly desirable snapping locations,
and should be selected over the general grid.

Additionally, grid snapping was previously only done if BOTH
x and y could be snapped to the grid. We now snap to the nearest
grid line for x/y separately. This means if a point is close
to a vertical grid line but not a horizontal one it will still
snap to that nearby vertical grid line.

python/core/layout/qgslayoutsnapper.sip

@@ -77,10 +77,11 @@ class QgsLayoutSnapper
  :rtype: QPointF
 %End
 
-    QPointF snapPointToGrid( QPointF point, double scaleFactor, bool &snapped /Out/ ) const;
+    QPointF snapPointToGrid( QPointF point, double scaleFactor, bool &snappedX /Out/, bool &snappedY /Out/ ) const;
 %Docstring
  Snaps a layout coordinate ``point`` to the grid. If ``point``
- was snapped, ``snapped`` will be set to true.
+ was snapped horizontally, ``snappedX`` will be set to true. If ``point``
+ was snapped vertically, ``snappedY`` will be set to true.
 
  The ``scaleFactor`` argument should be set to the transformation from
  scalar transform from layout coordinates to pixels, i.e. the

src/core/layout/qgslayoutsnapper.cpp

@@ -26,36 +26,43 @@ QPointF QgsLayoutSnapper::snapPoint( QPointF point, double scaleFactor, bool &sn
 {
   snapped = false;
 
-  // highest priority - grid
-  bool snappedToGrid = false;
-  QPointF res = snapPointToGrid( point, scaleFactor, snappedToGrid );
-  if ( snappedToGrid )
+  // highest priority - guides
+  bool snappedXToGuides = false;
+  double newX = snapPointToGuides( point.x(), QgsLayoutGuide::Vertical, scaleFactor, snappedXToGuides );
+  if ( snappedXToGuides )
   {
     snapped = true;
-    return res;
+    point.setX( newX );
+  }
+  bool snappedYToGuides = false;
+  double newY = snapPointToGuides( point.y(), QgsLayoutGuide::Horizontal, scaleFactor, snappedYToGuides );
+  if ( snappedYToGuides )
+  {
+    snapped = true;
+    point.setY( newY );
   }
 
-  bool snappedToHozGuides = false;
-  double newX = snapPointToGuides( point.x(), QgsLayoutGuide::Vertical, scaleFactor, snappedToHozGuides );
-  if ( snappedToHozGuides )
+  bool snappedXToGrid = false;
+  bool snappedYToGrid = false;
+  QPointF res = snapPointToGrid( point, scaleFactor, snappedXToGrid, snappedYToGrid );
+  if ( snappedXToGrid && !snappedXToGuides )
   {
     snapped = true;
-    point.setX( newX );
+    point.setX( res.x() );
   }
-  bool snappedToVertGuides = false;
-  double newY = snapPointToGuides( point.y(), QgsLayoutGuide::Horizontal, scaleFactor, snappedToVertGuides );
-  if ( snappedToVertGuides )
+  if ( snappedYToGrid && !snappedYToGuides )
   {
     snapped = true;
-    point.setY( newY );
+    point.setY( res.y() );
   }
 
   return point;
 }
 
-QPointF QgsLayoutSnapper::snapPointToGrid( QPointF point, double scaleFactor, bool &snapped ) const
+QPointF QgsLayoutSnapper::snapPointToGrid( QPointF point, double scaleFactor, bool &snappedX, bool &snappedY ) const
 {
-  snapped = false;
+  snappedX = false;
+  snappedY = false;
   if ( !mLayout || !mSnapToGrid )
   {
     return point;
@@ -89,7 +96,7 @@ QPointF QgsLayoutSnapper::snapPointToGrid( QPointF point, double scaleFactor, bo
   }
   else
   {
-    snapped = true;
+    snappedX = true;
   }
   if ( fabs( ySnapped - point.y() ) > alignThreshold )
   {
@@ -98,7 +105,7 @@ QPointF QgsLayoutSnapper::snapPointToGrid( QPointF point, double scaleFactor, bo
   }
   else
   {
-    snapped = true;
+    snappedY = true;
   }
 
   return QPointF( xSnapped, ySnapped );

src/core/layout/qgslayoutsnapper.h

@@ -90,7 +90,8 @@ class CORE_EXPORT QgsLayoutSnapper
 
     /**
      * Snaps a layout coordinate \a point to the grid. If \a point
-     * was snapped, \a snapped will be set to true.
+     * was snapped horizontally, \a snappedX will be set to true. If \a point
+     * was snapped vertically, \a snappedY will be set to true.
      *
      * The \a scaleFactor argument should be set to the transformation from
      * scalar transform from layout coordinates to pixels, i.e. the
@@ -99,7 +100,7 @@ class CORE_EXPORT QgsLayoutSnapper
      * If snapToGrid() is disabled, this method will return the point
      * unchanged.
      */
-    QPointF snapPointToGrid( QPointF point, double scaleFactor, bool &snapped SIP_OUT ) const;
+    QPointF snapPointToGrid( QPointF point, double scaleFactor, bool &snappedX SIP_OUT, bool &snappedY SIP_OUT ) const;
 
     /**
      * Snaps a layout coordinate \a point to the grid. If \a point

tests/src/python/test_qgslayoutsnapper.py

@@ -21,10 +21,9 @@
                        QgsLayoutMeasurement,
                        QgsUnitTypes,
                        QgsLayoutPoint,
-                       QgsLayoutItemPage)
+                       QgsLayoutItemPage,
+                       QgsLayoutGuide)
 from qgis.PyQt.QtCore import QPointF
-from qgis.PyQt.QtGui import (QPen,
-                             QColor)
 
 from qgis.testing import start_app, unittest
 
@@ -65,58 +64,112 @@ def testSnapPointToGrid(self):
         s.setSnapToGrid(True)
         s.setSnapTolerance(1)
 
-        point, snapped = s.snapPointToGrid(QPointF(1, 1), 1)
-        self.assertTrue(snapped)
+        point, snappedX, snappedY = s.snapPointToGrid(QPointF(1, 1), 1)
+        self.assertTrue(snappedX)
+        self.assertTrue(snappedY)
         self.assertEqual(point, QPointF(0, 0))
 
-        point, snapped = s.snapPointToGrid(QPointF(9, 1), 1)
-        self.assertTrue(snapped)
+        point, snappedX, snappedY = s.snapPointToGrid(QPointF(9, 1), 1)
+        self.assertTrue(snappedX)
+        self.assertTrue(snappedY)
         self.assertEqual(point, QPointF(10, 0))
 
-        point, snapped = s.snapPointToGrid(QPointF(1, 11), 1)
-        self.assertTrue(snapped)
+        point, snappedX, snappedY = s.snapPointToGrid(QPointF(1, 11), 1)
+        self.assertTrue(snappedX)
+        self.assertTrue(snappedY)
         self.assertEqual(point, QPointF(0, 10))
 
-        point, snapped = s.snapPointToGrid(QPointF(13, 11), 1)
-        self.assertTrue(snapped)
+        point, snappedX, snappedY = s.snapPointToGrid(QPointF(13, 11), 1)
+        self.assertFalse(snappedX)
+        self.assertTrue(snappedY)
         self.assertEqual(point, QPointF(13, 10))
 
-        point, snapped = s.snapPointToGrid(QPointF(11, 13), 1)
-        self.assertTrue(snapped)
+        point, snappedX, snappedY = s.snapPointToGrid(QPointF(11, 13), 1)
+        self.assertTrue(snappedX)
+        self.assertFalse(snappedY)
         self.assertEqual(point, QPointF(10, 13))
 
-        point, snapped = s.snapPointToGrid(QPointF(13, 23), 1)
-        self.assertFalse(snapped)
+        point, snappedX, snappedY = s.snapPointToGrid(QPointF(13, 23), 1)
+        self.assertFalse(snappedX)
+        self.assertFalse(snappedY)
         self.assertEqual(point, QPointF(13, 23))
 
         # grid disabled
         s.setSnapToGrid(False)
-        point, snapped = s.snapPointToGrid(QPointF(1, 1), 1)
-        self.assertFalse(snapped)
+        point, nappedX, snappedY = s.snapPointToGrid(QPointF(1, 1), 1)
+        self.assertFalse(nappedX)
+        self.assertFalse(snappedY)
         self.assertEqual(point, QPointF(1, 1))
         s.setSnapToGrid(True)
 
         # with different pixel scale
-        point, snapped = s.snapPointToGrid(QPointF(0.5, 0.5), 1)
-        self.assertTrue(snapped)
+        point, snappedX, snappedY = s.snapPointToGrid(QPointF(0.5, 0.5), 1)
+        self.assertTrue(snappedX)
+        self.assertTrue(snappedY)
         self.assertEqual(point, QPointF(0, 0))
-        point, snapped = s.snapPointToGrid(QPointF(0.5, 0.5), 3)
-        self.assertFalse(snapped)
+        point, snappedX, snappedY = s.snapPointToGrid(QPointF(0.5, 0.5), 3)
+        self.assertFalse(snappedX)
+        self.assertFalse(snappedY)
         self.assertEqual(point, QPointF(0.5, 0.5))
 
         # with offset grid
         l.gridSettings().setOffset(QgsLayoutPoint(2, 0))
-        point, snapped = s.snapPointToGrid(QPointF(13, 23), 1)
-        self.assertTrue(snapped)
+        point, snappedX, snappedY = s.snapPointToGrid(QPointF(13, 23), 1)
+        self.assertTrue(snappedX)
+        self.assertFalse(snappedY)
         self.assertEqual(point, QPointF(12, 23))
 
+    def testSnapPointToGuides(self):
+        p = QgsProject()
+        l = QgsLayout(p)
+        page = QgsLayoutItemPage(l)
+        page.setPageSize('A4')
+        l.pageCollection().addPage(page)
+        s = QgsLayoutSnapper(l)
+        guides = l.guides()
+
+        s.setSnapToGuides(True)
+        s.setSnapTolerance(1)
+
+        # no guides
+        point, snapped = s.snapPointToGuides(0.5, QgsLayoutGuide.Vertical, 1)
+        self.assertFalse(snapped)
+
+        guides.addGuide(QgsLayoutGuide(QgsLayoutGuide.Vertical, QgsLayoutMeasurement(1)))
+        point, snapped = s.snapPointToGuides(0.5, QgsLayoutGuide.Vertical, 1)
+        self.assertTrue(snapped)
+        self.assertEqual(point, 1)
+
+        # outside tolerance
+        point, snapped = s.snapPointToGuides(5.5, QgsLayoutGuide.Vertical, 1)
+        self.assertFalse(snapped)
+
+        # snapping off
+        s.setSnapToGuides(False)
+        point, snapped = s.snapPointToGuides(0.5, QgsLayoutGuide.Vertical, 1)
+        self.assertFalse(snapped)
+
+        s.setSnapToGuides(True)
+        # snap to hoz
+        point, snapped = s.snapPointToGuides(0.5, QgsLayoutGuide.Horizontal, 1)
+        self.assertFalse(snapped)
+        guides.addGuide(QgsLayoutGuide(QgsLayoutGuide.Horizontal, QgsLayoutMeasurement(1)))
+        point, snapped = s.snapPointToGuides(0.5, QgsLayoutGuide.Horizontal, 1)
+        self.assertTrue(snapped)
+        self.assertEqual(point, 1)
+
+        # with different pixel scale
+        point, snapped = s.snapPointToGuides(0.5, QgsLayoutGuide.Horizontal, 3)
+        self.assertFalse(snapped)
+
     def testSnapPoint(self):
         p = QgsProject()
         l = QgsLayout(p)
         page = QgsLayoutItemPage(l)
         page.setPageSize('A4')
         l.pageCollection().addPage(page)
         s = QgsLayoutSnapper(l)
+        guides = l.guides()
 
         # first test snapping to grid
         l.gridSettings().setResolution(QgsLayoutMeasurement(5, QgsUnitTypes.LayoutMillimeters))
@@ -132,6 +185,14 @@ def testSnapPoint(self):
         self.assertFalse(snapped)
         self.assertEqual(point, QPointF(1, 1))
 
+        # test that guide takes precedence
+        s.setSnapToGrid(True)
+        s.setSnapToGuides(True)
+        guides.addGuide(QgsLayoutGuide(QgsLayoutGuide.Horizontal, QgsLayoutMeasurement(0.5)))
+        point, snapped = s.snapPoint(QPointF(1, 1), 1)
+        self.assertTrue(snapped)
+        self.assertEqual(point, QPointF(0, 0.5))
+
 
 if __name__ == '__main__':
     unittest.main()