[composer] Move code responsible for updating labels from atlas to Qg…

…sComposerLabel, fix potential crash when removing atlas coverage layer.

src/core/composer/qgsatlascomposition.cpp

@@ -413,15 +413,7 @@ void QgsAtlasComposition::prepareForFeature( int featureI )
 
   emit featureChanged( &mCurrentFeature );
 
-  // TODO - move these updates to label/shape/page item
-
-  // evaluate label expressions
-  QList<QgsComposerLabel*> labels;
-  mComposition->composerItems( labels );
-  for ( QList<QgsComposerLabel*>::iterator lit = labels.begin(); lit != labels.end(); ++lit )
-  {
-    ( *lit )->setExpressionContext( &mCurrentFeature, mCoverageLayer );
-  }
+  // TODO - move these updates to shape/page item
 
   // update shapes (in case they use data defined symbology with atlas properties)
   QList<QgsComposerShape*> shapes;

src/core/composer/qgscomposerlabel.cpp

@@ -53,6 +53,13 @@ QgsComposerLabel::QgsComposerLabel( QgsComposition *composition ):
     //otherwise fields in the label aren't correctly evaluated until atlas preview feature changes (#9457)
     setExpressionContext( mComposition->atlasComposition().currentFeature(), mComposition->atlasComposition().coverageLayer() );
   }
+
+  //connect to atlas toggling on/off and coverage layer and feature changes
+  //to update the expression context
+  connect( &mComposition->atlasComposition(), SIGNAL( toggled( bool ) ), this, SLOT( refreshExpressionContext() ) );
+  connect( &mComposition->atlasComposition(), SIGNAL( coverageLayerChanged( QgsVectorLayer* ) ), this, SLOT( refreshExpressionContext() ) );
+  connect( &mComposition->atlasComposition(), SIGNAL( featureChanged( QgsFeature* ) ), this, SLOT( refreshExpressionContext() ) );
+
 }
 
 QgsComposerLabel::~QgsComposerLabel()
@@ -187,6 +194,20 @@ void QgsComposerLabel::setExpressionContext( QgsFeature* feature, QgsVectorLayer
   update();
 }
 
+void QgsComposerLabel::refreshExpressionContext()
+{
+  QgsVectorLayer * vl = 0;
+  QgsFeature* feature = 0;
+
+  if ( mComposition->atlasComposition().enabled() )
+  {
+    vl = mComposition->atlasComposition().coverageLayer();
+    feature = mComposition->atlasComposition().currentFeature();
+  }
+
+  setExpressionContext( feature, vl );
+}
+
 QString QgsComposerLabel::displayText() const
 {
   QString displayText = mText;

src/core/composer/qgscomposerlabel.h

@@ -99,6 +99,10 @@ class CORE_EXPORT QgsComposerLabel: public QgsComposerItem
        */
     bool readXML( const QDomElement& itemElem, const QDomDocument& doc );
 
+  public slots:
+    void refreshExpressionContext();
+
+
   private slots:
     void loadingHtmlFinished( bool );