Add GetUpdatedAscendingSortIndicator() helper function

It seems like this function will need to be used in every implementation
of EVT_LIST_COL_CLICK handler when using sorting, so it makes sense to
provide it in the library itself.

include/wx/listbase.h

@@ -451,6 +451,12 @@ public:
     void RemoveSortIndicator() { ShowSortIndicator(-1); }
     virtual int GetSortIndicator() const { return -1; }
     virtual bool IsAscendingSortIndicator() const { return true; }
+    bool GetUpdatedAscendingSortIndicator(int col) const
+    {
+        // If clicking on the same column by which we already sort, toggle the sort
+        // direction, otherwise use ascending sort by default.
+        return col == GetSortIndicator() ? !IsAscendingSortIndicator() : true;
+    }
 
 protected:
     // Return pointer to the corresponding m_imagesXXX.

interface/wx/listctrl.h

@@ -1460,6 +1460,31 @@ public:
     */
     int GetSortIndicator() const;
 
+    /**
+        Returns the new value to use for sort indicator after clicking a
+        column.
+
+        This helper function can be useful in the EVT_LIST_COL_CLICK handler
+        when it updates the sort indicator after the user clicked on a column.
+
+        For example:
+        @code
+            void MyListCtrl::OnColClick(wxListEvent& event)
+            {
+                int col = event.GetColumn();
+                if ( col == -1 )
+                    return; // clicked outside any column.
+
+                const bool ascending = GetUpdatedAscendingSortIndicator(col);
+                SortItems(MyCompareFunction, ascending);
+                ShowSortIndicator(col, ascending);
+            }
+        @endcode
+
+        @since 3.1.6
+    */
+    bool GetUpdatedAscendingSortIndicator(int col) const;
+
     /**
         Returns @true if the sort indicator direction is ascending,
         @false when the direction is descending.

samples/listctrl/listtest.cpp

@@ -1089,15 +1089,8 @@ void MyListCtrl::OnColClick(wxListEvent& event)
         return; // clicked outside any column.
     }
 
-    // If clicking on the same column by which we already sort, toggle the sort
-    // direction, otherwise use ascending sort by default.
-    bool ascending;
-    if ( col == GetSortIndicator() )
-        ascending = !IsAscendingSortIndicator();
-    else
-        ascending = true;
-
     // sort on item data (SetItemData)
+    const bool ascending = GetUpdatedAscendingSortIndicator(col);
     if ( SortItems(MyCompareFunction, ascending) )
     {
         ShowSortIndicator(col, ascending);