Add ability to set expression context variables for map canvas

python/gui/qgsmapcanvas.sip

@@ -338,6 +338,24 @@ class QgsMapCanvas : QGraphicsView
      */
     void setSnappingUtils( QgsSnappingUtils* utils );
 
+    /** Sets an expression context scope for the map canvas. This scope is injected 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. This method will overwrite the existing expression
+     * context scope for the canvas.
+     * @param scope new expression context scope
+     * @note added in QGIS 2.12
+     * @see expressionContextScope()
+     */
+    void setExpressionContextScope( const QgsExpressionContextScope& scope );
+
+    /** Returns a reference to the expression context scope for the map canvas. This scope is injected
+     * 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.
+     * @note added in QGIS 2.12
+     * @see setExpressionContextScope()
+     */
+    QgsExpressionContextScope& expressionContextScope();
+
   public slots:
 
     /** Repaints the canvas map*/

src/gui/qgsmapcanvas.cpp

@@ -660,11 +660,14 @@ void QgsMapCanvas::refreshMap()
 
   //update $map variable to canvas
   QgsExpression::setSpecialColumn( "$map", tr( "canvas" ) );
+  mExpressionContextScope.setVariable( "map_id", "canvas" );
 
   //build the expression context
   QgsExpressionContext expressionContext;
   expressionContext << QgsExpressionContextUtils::globalScope()
-  << QgsExpressionContextUtils::projectScope();
+  << QgsExpressionContextUtils::projectScope()
+  << new QgsExpressionContextScope( mExpressionContextScope );
+
   mSettings.setExpressionContext( expressionContext );
 
   // create the renderer job

src/gui/qgsmapcanvas.h

@@ -24,6 +24,7 @@
 #include <memory>
 #include <deque>
 
+#include "qgsexpressioncontext.h"
 #include "qgsrectangle.h"
 #include "qgspoint.h"
 #include "qgis.h"
@@ -409,6 +410,24 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView
      */
     void setSnappingUtils( QgsSnappingUtils* utils );
 
+    /** Sets an expression context scope for the map canvas. This scope is injected 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. This method will overwrite the existing expression
+     * context scope for the canvas.
+     * @param scope new expression context scope
+     * @note added in QGIS 2.12
+     * @see expressionContextScope()
+     */
+    void setExpressionContextScope( const QgsExpressionContextScope& scope ) { mExpressionContextScope = scope; }
+
+    /** Returns a reference to the expression context scope for the map canvas. This scope is injected
+     * 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.
+     * @note added in QGIS 2.12
+     * @see setExpressionContextScope()
+     */
+    QgsExpressionContextScope& expressionContextScope() { return mExpressionContextScope; }
+
   public slots:
 
     /** Repaints the canvas map*/
@@ -698,6 +717,8 @@ class GUI_EXPORT QgsMapCanvas : public QGraphicsView
 
     QgsSnappingUtils* mSnappingUtils;
 
+    QgsExpressionContextScope mExpressionContextScope;
+
 }; // class QgsMapCanvas
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