preserve central value of distribution

GUI/Model/Beam/DistributionItems.h

@@ -38,6 +38,7 @@ public:
 
     virtual DoubleProperty& center() = 0;
     virtual const DoubleProperty& center() const = 0;
+    virtual void setCenter(double v) = 0;
 
     //! Serialization of contents.
     //!
@@ -63,6 +64,7 @@ public:
 
     DoubleProperty& center() override { return mean(); }
     const DoubleProperty& center() const override { return mean(); }
+    void setCenter(double v) override { m_mean.setDVal(v); }
 
     void writeTo(QXmlStreamWriter* w) const override;
     void readFrom(QXmlStreamReader* r) override;
@@ -88,6 +90,7 @@ public:
 
     DoubleProperty& center() override { return m_center; }
     const DoubleProperty& center() const override { return m_center; }
+    void setCenter(double v) override { m_center.setDVal(v); }
 
     DoubleProperty& halfwidth() { return m_halfwidth; }
     const DoubleProperty& halfwidth() const { return m_halfwidth; }
@@ -149,6 +152,7 @@ public:
 
     DoubleProperty& center() override { return median(); }
     const DoubleProperty& center() const override { return median(); }
+    void setCenter(double v) override { m_median.setDVal(v); }
 
     DoubleProperty& scaleParameter() { return m_scale_parameter; }
     const DoubleProperty& scaleParameter() const { return m_scale_parameter; }

GUI/View/Device/DistributionSelector.cpp

@@ -33,13 +33,17 @@ DistributionSelector::DistributionSelector(std::optional<MeanConfig> mean_config
     m_form_layout = new QFormLayout(this);
     m_form_layout->setContentsMargins(0, 0, 0, 0);
 
-    m_distribution_combo = GUI::Util::createComboBoxFromPolyPtr(
-        item->distributionSelection(),
-        [this](int) {
+    m_distribution_combo = GUI::Util::createGeneralComboBoxFromPolyPtr(
+        m_item->distributionSelection(),
+        [this](int index) {
+            double old_center = m_item->distributionItem()->center().dVal();
+            m_item->distributionSelection().setCertainIndex(index);
+            m_item->distributionItem()->setCenter(old_center);
             createDistributionWidgets();
             emit distributionChanged();
         },
         true);
+
     m_distribution_combo->setEnabled(allow_distr);
     m_form_layout->addRow("Distribution:", m_distribution_combo);
 

GUI/View/Numeric/ComboUtil.h

@@ -43,12 +43,9 @@ QComboBox* createComboBox(std::function<ComboProperty()> comboFunction,
 //! Furthermore, the combo box can prohibit accidental changes by the mouse wheel. Otherwise it
 //! would be dangerous if the combo is on a scrollable form - unintended and unnoticed changes would
 //! take place when just scrolling through the form.
-//!
-//! Changes in the combobox will be notified to the PolyItem already. The additional (and
-//! optional) slot can be used to be notified about an already executed change.
 template <typename B, typename C>
-QComboBox* createComboBoxFromPolyPtr(PolyPtr<B, C>& d, std::function<void(int)> slot,
-                                     bool inScrollArea)
+QComboBox* createGeneralComboBoxFromPolyPtr(PolyPtr<B, C>& d, std::function<void(int)> full_slot,
+                                            bool inScrollArea)
 {
     auto* combo = new QComboBox;
     combo->addItems(d.menuEntries());
@@ -59,15 +56,30 @@ QComboBox* createComboBoxFromPolyPtr(PolyPtr<B, C>& d, std::function<void(int)>
     if (inScrollArea)
         WheelEventEater::install(combo);
 
-    QObject::connect(combo, &QComboBox::currentIndexChanged, [&d, slot](int index) {
-        d.setCertainIndex(index);
-        if (slot)
-            slot(index);
+    QObject::connect(combo, &QComboBox::currentIndexChanged, [full_slot](int index) {
+        if (full_slot)
+            full_slot(index);
     });
 
     return combo;
 }
-
+//! Create a combo box with the information found in a selection property.
+//!
+//! Changes in the combobox will be notified to the PolyItem already. The additional (and
+//! optional) slot can be used to be notified about an already executed change.
+template <typename B, typename C>
+QComboBox* createComboBoxFromPolyPtr(PolyPtr<B, C>& d, std::function<void(int)> optional_slot,
+                                     bool inScrollArea)
+{
+    return createGeneralComboBoxFromPolyPtr(
+        d,
+        [&d, optional_slot](int index) {
+            d.setCertainIndex(index);
+            if (optional_slot)
+                optional_slot(index);
+        },
+        inScrollArea);
+}
 } // namespace GUI::Util
 
 #endif // BORNAGAIN_GUI_VIEW_NUMERIC_COMBOUTIL_H