[layouts] Fix data defined atlas margin isn't evaluated

Fixes #19896

Author: nyalldawson

src/core/layout/qgslayoutitemmap.cpp

@@ -540,7 +540,7 @@ bool QgsLayoutItemMap::writePropertiesToElement( QDomElement &mapElem, QDomDocum
   }
 
   // follow map theme
-  mapElem.setAttribute( QStringLiteral( "followPreset" ), mFollowVisibilityPreset ? "true" : "false" );
+  mapElem.setAttribute( QStringLiteral( "followPreset" ), mFollowVisibilityPreset ? QStringLiteral( "true" ) : QStringLiteral( "false" ) );
   mapElem.setAttribute( QStringLiteral( "followPresetName" ), mFollowVisibilityPresetName );
 
   //map rotation
@@ -1960,9 +1960,10 @@ void QgsLayoutItemMap::updateAtlasFeature()
     }
     newExtent = QgsRectangle( xa1, ya1, xa2, ya2 );
 
-    if ( mAtlasMargin > 0.0 )
+    const double evaluatedAtlasMargin = atlasMargin();
+    if ( evaluatedAtlasMargin > 0.0 )
     {
-      newExtent.scale( 1 + mAtlasMargin );
+      newExtent.scale( 1 + evaluatedAtlasMargin );
     }
   }
 

tests/src/python/test_qgslayoutatlas.py

@@ -626,6 +626,51 @@ def rotation_test(self):
 
         QgsProject.instance().removeMapLayer(polygonLayer)
 
+    def test_datadefined_margin(self):
+        polygonLayer = QgsVectorLayer('Polygon?field=margin:int', 'test_polygon', 'memory')
+        poly = QgsFeature(polygonLayer.fields())
+        poly.setAttributes([0])
+        poly.setGeometry(QgsGeometry.fromWkt('Polygon((30 30, 40 30, 40 40, 30 40, 30 30))'))
+        polygonLayer.dataProvider().addFeatures([poly])
+        poly = QgsFeature(polygonLayer.fields())
+        poly.setAttributes([10])
+        poly.setGeometry(QgsGeometry.fromWkt('Polygon((10 10, 20 10, 20 20, 10 20, 10 10))'))
+        polygonLayer.dataProvider().addFeatures([poly])
+        poly = QgsFeature(polygonLayer.fields())
+        poly.setAttributes([20])
+        poly.setGeometry(QgsGeometry.fromWkt('Polygon((50 50, 60 50, 60 60, 50 60, 50 50))'))
+        polygonLayer.dataProvider().addFeatures([poly])
+        QgsProject.instance().addMapLayer(polygonLayer)
+
+        layout = QgsPrintLayout(QgsProject.instance())
+        map = QgsLayoutItemMap(layout)
+        map.setCrs(polygonLayer.crs())
+        map.attemptSetSceneRect(QRectF(20, 20, 130, 130))
+        map.setFrameEnabled(True)
+        map.setLayers([polygonLayer])
+        map.setExtent(QgsRectangle(0, 0, 100, 50))
+        layout.addLayoutItem(map)
+
+        atlas = layout.atlas()
+        atlas.setCoverageLayer(polygonLayer)
+        atlas.setEnabled(True)
+
+        map.setAtlasDriven(True)
+        map.setAtlasScalingMode(QgsLayoutItemMap.Auto)
+        map.setAtlasMargin(77.0)
+        map.dataDefinedProperties().setProperty(QgsLayoutObject.MapAtlasMargin, QgsProperty.fromExpression('margin/2'))
+
+        atlas.beginRender()
+        atlas.first()
+
+        self.assertEqual(map.extent(), QgsRectangle(25, 30, 45, 40))
+        self.assertTrue(atlas.next())
+        self.assertEqual(map.extent(), QgsRectangle(4.5, 9.75, 25.5, 20.25))
+        self.assertTrue(atlas.next())
+        self.assertEqual(map.extent(), QgsRectangle(44, 49.5, 66, 60.5))
+
+        QgsProject.instance().removeMapLayer(polygonLayer)
+
 
 if __name__ == '__main__':
     unittest.main()