Ensure correct crs is available for QgsMapCanvas for slots connected

to extentsChanged when canvas CRS is changed

Fixes #51833

python/gui/auto_generated/qgsmapcanvas.sip.in

@@ -1347,6 +1347,13 @@ called when panning is in action, reset indicates end of panning
     virtual void showEvent( QShowEvent *event );
 
 
+    void emitExtentsChanged();
+%Docstring
+Emits the extentsChanged signal when appropriate.
+
+.. versionadded:: 3.30
+%End
+
 
 
 

src/gui/qgsmapcanvas.cpp

@@ -513,6 +513,12 @@ void QgsMapCanvas::setDestinationCrs( const QgsCoordinateReferenceSystem &crs )
     }
   }
 
+  // defer extent and scale changed signals until we've correctly
+  // set the destination crs, otherwise slots which connect to these signals
+  // may retrieve an outdated CRS for the map canvas
+  mBlockExtentChangedSignal++;
+  mBlockScaleChangedSignal++;
+
   if ( !rect.isEmpty() )
   {
     // we will be manually calling updateCanvasItemPositions() later, AFTER setting the updating the mSettings destination CRS, and we don't
@@ -522,8 +528,13 @@ void QgsMapCanvas::setDestinationCrs( const QgsCoordinateReferenceSystem &crs )
     mBlockItemPositionUpdates--;
   }
 
+  mBlockExtentChangedSignal--;
+  mBlockScaleChangedSignal--;
+
   mSettings.setDestinationCrs( crs );
   updateScale();
+  emitExtentsChanged();
+
   updateCanvasItemPositions();
 
   QgsDebugMsgLevel( QStringLiteral( "refreshing after destination CRS changed" ), 2 );
@@ -1426,6 +1437,7 @@ void QgsMapCanvas::setExtent( const QgsRectangle &r, bool magnified )
 
     // ### QGIS 3: do not allow empty extent - require users to call setCenter() explicitly
     QgsDebugMsgLevel( QStringLiteral( "Empty extent - keeping old scale with new center!" ), 2 );
+
     setCenter( r.center() );
   }
   else
@@ -1448,7 +1460,7 @@ void QgsMapCanvas::setExtent( const QgsRectangle &r, bool magnified )
       mSettings.setExtent( r, magnified );
     }
   }
-  emit extentsChanged();
+  emitExtentsChanged();
   updateScale();
 
   //clear all extent items after current index
@@ -1524,7 +1536,7 @@ QgsPointXY QgsMapCanvas::cursorPoint() const
 double QgsMapCanvas::rotation() const
 {
   return mapSettings().rotation();
-} // rotation
+}
 
 void QgsMapCanvas::setRotation( double degrees )
 {
@@ -1535,13 +1547,13 @@ void QgsMapCanvas::setRotation( double degrees )
 
   mSettings.setRotation( degrees );
   emit rotationChanged( degrees );
-  emit extentsChanged(); // visible extent changes with rotation
-} // setRotation
-
+  emitExtentsChanged(); // visible extent changes with rotation
+}
 
 void QgsMapCanvas::updateScale()
 {
-  emit scaleChanged( mapSettings().scale() );
+  if ( !mBlockScaleChangedSignal )
+    emit scaleChanged( mapSettings().scale() );
 }
 
 void QgsMapCanvas::zoomToFullExtent()
@@ -1577,7 +1589,7 @@ void QgsMapCanvas::zoomToPreviousExtent()
   {
     mLastExtentIndex--;
     mSettings.setExtent( mLastExtent[mLastExtentIndex] );
-    emit extentsChanged();
+    emitExtentsChanged();
     updateScale();
     refresh();
     // update controls' enabled state
@@ -1593,7 +1605,7 @@ void QgsMapCanvas::zoomToNextExtent()
   {
     mLastExtentIndex++;
     mSettings.setExtent( mLastExtent[mLastExtentIndex] );
-    emit extentsChanged();
+    emitExtentsChanged();
     updateScale();
     refresh();
     // update controls' enabled state
@@ -2524,7 +2536,7 @@ void QgsMapCanvas::resizeEvent( QResizeEvent *e )
     updateScale();
   }
 
-  emit extentsChanged();
+  emitExtentsChanged();
 }
 
 void QgsMapCanvas::paintEvent( QPaintEvent *e )
@@ -3077,6 +3089,12 @@ void QgsMapCanvas::showEvent( QShowEvent *event )
   updateDevicePixelFromScreen();
 }
 
+void QgsMapCanvas::emitExtentsChanged()
+{
+  if ( !mBlockExtentChangedSignal )
+    emit extentsChanged();
+}
+
 QPoint QgsMapCanvas::mouseLastXY()
 {
   return mCanvasProperties->mouseLastXY;

src/gui/qgsmapcanvas.h

@@ -1227,6 +1227,13 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView, public QgsExpressionContex
 
     void showEvent( QShowEvent *event ) override;
 
+    /**
+     * Emits the extentsChanged signal when appropriate.
+     *
+     * \since QGIS 3.30
+     */
+    void emitExtentsChanged();
+
     /// implementation struct
     class CanvasProperties;
 
@@ -1425,6 +1432,8 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView, public QgsExpressionContex
     QList< QgsMapCanvasInteractionBlocker * > mInteractionBlockers;
 
     int mBlockItemPositionUpdates = 0;
+    int mBlockExtentChangedSignal = 0;
+    int mBlockScaleChangedSignal = 0;
 
     std::unique_ptr< QgsTemporaryCursorOverride > mTemporaryCursorOverride;
 

tests/src/python/test_qgsmapcanvas.py

@@ -770,6 +770,33 @@ def test_temporal_animation(self):
         self.assertEqual(canvas.mapSettings().frameRate(), -1)
         self.assertEqual(canvas.mapSettings().currentFrame(), -1)
 
+    def test_crs_change_signals(self):
+        """
+        Test behavior of signals when crs is changed
+        """
+        canvas = QgsMapCanvas()
+        canvas.setDestinationCrs(QgsCoordinateReferenceSystem('EPSG:4326'))
+        canvas.setFrameStyle(0)
+        canvas.resize(600, 400)
+        self.assertEqual(canvas.width(), 600)
+        self.assertEqual(canvas.height(), 400)
+        canvas.setExtent(QgsRectangle(10, 30, 20, 35))
+
+        def on_extent_changed():
+            TestQgsMapCanvas.new_extent = canvas.extent()
+            TestQgsMapCanvas.new_crs = canvas.mapSettings().destinationCrs()
+
+        canvas.extentsChanged.connect(on_extent_changed)
+
+        TestQgsMapCanvas.new_extent = None
+        TestQgsMapCanvas.new_crs = None
+
+        canvas.setDestinationCrs(QgsCoordinateReferenceSystem('EPSG:3857'))
+
+        self.assertAlmostEqual(TestQgsMapCanvas.new_extent.xMinimum(), 1008988, places=-3)
+
+        self.assertEqual(TestQgsMapCanvas.new_crs, QgsCoordinateReferenceSystem('EPSG:3857'))
+
 
 if __name__ == '__main__':
     unittest.main()