Implement $map variable, which returns the current composer map item …

…id where the map is being drawn, or 'canvas' if the map is being drawn in the main QGIS window. Partly implements #9288.

resources/function_help/$map

@@ -0,0 +1,16 @@
+<h3>$map function</h3>
+Returns the id of the current map item if the map is being drawn in a composition, or "canvas" if the map is being
+drawn within the main QGIS window.
+
+<h4>Syntax</h4>
+<pre>$map</pre>
+
+<h4>Arguments</h4>
+None
+
+<h4>Example</h4>
+<!-- Show example of function.-->
+<pre>$map &rarr; "overview_map" (within a composer item)<br />
+$map &rarr; "canvas" (within the main QGIS main canvas)</pre>
+
+

src/core/composer/qgscomposermap.cpp

@@ -28,6 +28,7 @@
 #include "qgsscalecalculator.h"
 #include "qgsvectorlayer.h"
 #include "qgspallabeling.h"
+#include "qgsexpression.h"
 
 #include "qgslabel.h"
 #include "qgslabelattributes.h"
@@ -226,6 +227,9 @@ void QgsComposerMap::draw( QPainter *painter, const QgsRectangle& extent, const
   bool bkLayerCaching = s.value( "/qgis/enable_render_caching", false ).toBool();
   s.setValue( "/qgis/enable_render_caching", false );
 
+  //update $map variable. Use QgsComposerItem's id since that is user-definable
+  QgsExpression::setSpecialColumn( "$map", QgsComposerItem::id() );
+
   if ( forceWidthScale ) //force wysiwyg line widths / marker sizes
   {
     theMapRenderer.render( painter, forceWidthScale );

src/gui/qgsmapcanvas.cpp

@@ -417,6 +417,9 @@ void QgsMapCanvas::refresh()
 
   mDrawing = true;
 
+  //update $map variable to canvas
+  QgsExpression::setSpecialColumn( "$map", tr( "canvas" ) );
+
   if ( mRenderFlag && !mFrozen )
   {
     clear();