diff --git a/src/App/Expression.h b/src/App/Expression.h
index d80c38fad..5f3f7fb44 100644
--- a/src/App/Expression.h
+++ b/src/App/Expression.h
@@ -46,6 +46,25 @@ public:
     virtual void visit(Expression * e) = 0;
 };
 
+template<class P> class AppExport ExpressionModifier : public ExpressionVisitor {
+public:
+    ExpressionModifier(P & _prop)
+        : prop(_prop) { }
+
+    virtual ~ExpressionModifier() { }
+
+    void setExpressionChanged() {
+        if (!signaller)
+            signaller = boost::shared_ptr<typename AtomicPropertyChangeInterface<P>::AtomicPropertyChange>(AtomicPropertyChangeInterface<P>::getAtomicPropertyChange(prop));
+    }
+
+    bool getChanged() const { return signaller != 0; }
+
+protected:
+    P & prop;
+    boost::shared_ptr<typename AtomicPropertyChangeInterface<P>::AtomicPropertyChange> signaller;
+};
+
 /**
   * Base class for expressions.
   *
diff --git a/src/App/Property.h b/src/App/Property.h
index cecfb44fd..b0855f3a7 100644
--- a/src/App/Property.h
+++ b/src/App/Property.h
@@ -212,6 +212,7 @@ template<class P> class AtomicPropertyChangeInterface {
 protected:
     AtomicPropertyChangeInterface() : signalCounter(0) { }
 
+public:
     class AtomicPropertyChange {
     public:
         AtomicPropertyChange(P & prop) : mProp(prop) {
@@ -234,6 +235,8 @@ protected:
         P & mProp; /**< Referenced to property we work on */
     };
 
+    static AtomicPropertyChange * getAtomicPropertyChange(P & prop) { return new AtomicPropertyChange(prop); }
+
 private:
 
     int signalCounter; /**< Counter for invoking transaction start/stop */