[FEATURE] map canvas @canvas_cursor_point variable (#7225)

Author: nirvn

python/gui/auto_generated/qgsmapcanvas.sip.in

@@ -611,6 +611,8 @@ context scope for the canvas.
 
 .. seealso:: :py:func:`expressionContextScope`
 
+.. seealso:: :py:func:`defaultExpressionContextScope`
+
 .. versionadded:: 2.12
 %End
 
@@ -622,10 +624,23 @@ overrides for expression evaluation for the map canvas render.
 
 .. seealso:: :py:func:`setExpressionContextScope`
 
+.. seealso:: :py:func:`defaultExpressionContextScope`
+
 .. versionadded:: 2.12
 %End
 
 
+    QgsExpressionContextScope *defaultExpressionContextScope() /Factory/;
+%Docstring
+Creates a new scope which contains default variables and functions relating to the map canvas.
+
+.. seealso:: :py:func:`expressionContextScope`
+
+.. seealso:: :py:func:`setExpressionContextScope`
+
+.. versionadded:: 3.4
+%End
+
     void setSegmentationTolerance( double tolerance );
 %Docstring
 Sets the segmentation tolerance applied when rendering curved geometries

src/core/expression/qgsexpression.cpp

@@ -748,6 +748,9 @@ void QgsExpression::initVariableHelp()
   sVariableHelpTexts.insert( QStringLiteral( "grid_number" ), QCoreApplication::translate( "variable_help", "Current grid annotation value." ) );
   sVariableHelpTexts.insert( QStringLiteral( "grid_axis" ), QCoreApplication::translate( "variable_help", "Current grid annotation axis (e.g., 'x' for longitude, 'y' for latitude)." ) );
 
+  // map canvas item variables
+  sVariableHelpTexts.insert( QStringLiteral( "canvas_cursor_point" ), QCoreApplication::translate( "variable_help", "Last cursor position on the canvas in the project's geographical coordinates." ) );
+
   // map tool capture variables
   sVariableHelpTexts.insert( QStringLiteral( "snapping_results" ), QCoreApplication::translate( "variable_help",
                              "<p>An array with an item for each snapped point.</p>"

src/gui/qgsmapcanvas.cpp

@@ -466,6 +466,13 @@ QgsMapLayer *QgsMapCanvas::currentLayer()
   return mCurrentLayer;
 }
 
+QgsExpressionContextScope *QgsMapCanvas::defaultExpressionContextScope()
+{
+  QgsExpressionContextScope *s = new QgsExpressionContextScope( QObject::tr( "Map Canvas" ) );
+  s->setVariable( QStringLiteral( "canvas_cursor_point" ), QgsGeometry::fromPointXY( cursorPoint() ), true );
+
+  return s;
+}
 
 void QgsMapCanvas::refresh()
 {
@@ -510,6 +517,7 @@ void QgsMapCanvas::refreshMap()
                     << QgsExpressionContextUtils::projectScope( QgsProject::instance() )
                     << QgsExpressionContextUtils::atlasScope( nullptr )
                     << QgsExpressionContextUtils::mapSettingsScope( mSettings )
+                    << defaultExpressionContextScope()
                     << new QgsExpressionContextScope( mExpressionContextScope );
 
   mSettings.setExpressionContext( expressionContext );
@@ -862,6 +870,10 @@ QgsPointXY QgsMapCanvas::center() const
   return r.center();
 }
 
+QgsPointXY QgsMapCanvas::cursorPoint() const
+{
+  return mCursorPoint;
+}
 
 double QgsMapCanvas::rotation() const
 {
@@ -1622,9 +1634,8 @@ void QgsMapCanvas::mouseMoveEvent( QMouseEvent *e )
   }
 
   // show x y on status bar
-  QPoint xy = e->pos();
-  QgsPointXY coord = getCoordinateTransform()->toMapCoordinates( xy );
-  emit xyCoordinates( coord );
+  mCursorPoint = getCoordinateTransform()->toMapCoordinates( mCanvasProperties->mouseLastXY );
+  emit xyCoordinates( mCursorPoint );
 }
 
 void QgsMapCanvas::setMapTool( QgsMapTool *tool, bool clean )

src/gui/qgsmapcanvas.h

@@ -544,6 +544,7 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView
      * context scope for the canvas.
      * \param scope new expression context scope
      * \see expressionContextScope()
+     * \see defaultExpressionContextScope()
      * \since QGIS 2.12
      */
     void setExpressionContextScope( const QgsExpressionContextScope &scope ) { mExpressionContextScope = scope; }
@@ -553,18 +554,28 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView
      * into the expression context used for rendering the map, and can be used to apply specific variable
      * overrides for expression evaluation for the map canvas render.
      * \see setExpressionContextScope()
+     * \see defaultExpressionContextScope()
      * \since QGIS 2.12
      */
     QgsExpressionContextScope &expressionContextScope() { return mExpressionContextScope; }
 
     /**
      * Returns a const reference to the expression context scope for the map canvas.
      * \see setExpressionContextScope()
+     * \see defaultExpressionContextScope()
      * \note not available in Python bindings
      * \since QGIS 2.12
      */
     const QgsExpressionContextScope &expressionContextScope() const { return mExpressionContextScope; } SIP_SKIP
 
+    /**
+     * Creates a new scope which contains default variables and functions relating to the map canvas.
+     * \see expressionContextScope()
+     * \see setExpressionContextScope()
+     * \since QGIS 3.4
+     */
+    QgsExpressionContextScope *defaultExpressionContextScope() SIP_FACTORY;
+
     /**
      * Sets the segmentation tolerance applied when rendering curved geometries
     \param tolerance the segmentation tolerance*/
@@ -990,12 +1001,20 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView
 
     QString mTheme;
 
+    QgsPointXY mCursorPoint;
+
     bool mAnnotationsVisible = true;
 
     bool mUsePreviewJobs = false;
 
     QHash< QString, int > mLastLayerRenderTime;
 
+    /**
+     * Returns the last cursor position on the canvas in geographical coordinates
+     * \since QGIS 3.4
+     */
+    QgsPointXY cursorPoint() const;
+
     /**
      * Force a resize of the map canvas item
      * \since QGIS 2.16

src/gui/symbology/qgssymbolwidgetcontext.cpp

@@ -83,6 +83,7 @@ QList<QgsExpressionContextScope *> QgsSymbolWidgetContext::globalProjectAtlasMap
   if ( mMapCanvas )
   {
     scopes << QgsExpressionContextUtils::mapSettingsScope( mMapCanvas->mapSettings() )
+           << mMapCanvas->defaultExpressionContextScope()
            << new QgsExpressionContextScope( mMapCanvas->expressionContextScope() );
   }
   else