add methods for listbox visible range

include/nana/gui/widgets/listbox.hpp

@@ -1555,6 +1555,27 @@ the nana::detail::basic_window member pointer scheme
 		 * @return the reference of *this.
 		 */
 		listbox& category_icon(const paint::image& img_expanded, const paint::image&& img_collapsed);
+
+		/// Returns first visible element
+		/**
+		 * It may return an item or a category item.
+		 * @return the index of first visible element.
+		 */
+		index_pair first_visible() const;
+
+		/// Returns last visible element
+		/**
+		 * It may return an item or a category item.
+		 * @return the index of last visible element.
+		 */
+		index_pair last_visible() const;
+
+		/// Returns all visible items
+		/**
+		 * It returns all visible items that are displayed in listbox window.
+		 * @return index_pairs containing all visible items.
+		 */
+		index_pairs visibles() const;
 	private:
 		drawerbase::listbox::essence & _m_ess() const;
 		nana::any* _m_anyobj(size_type cat, size_type index, bool allocate_if_empty) const override;

source/gui/widgets/listbox.cpp

@@ -5989,6 +5989,54 @@ namespace nana
 			return *this;
 		}
 
+		auto listbox::first_visible() const ->index_pair
+		{
+			return _m_ess().first_display();
+		}
+
+		auto listbox::last_visible() const -> index_pair
+		{
+			return _m_ess().lister.advance(_m_ess().first_display(), _m_ess().count_of_exposed(true));
+		}
+
+		auto listbox::visibles() const -> index_pairs
+		{
+			index_pairs indexes;
+
+			auto idx = _m_ess().first_display();
+
+			auto n = _m_ess().count_of_exposed(true);
+			while (n > 0)
+			{
+				if (idx.empty())
+					break;
+
+				if (idx.is_category())
+				{
+					indexes.push_back(idx);
+					--n;
+				}
+				else
+				{
+					auto const count = (std::min)(_m_ess().lister.size_item(idx.cat) - idx.item, n);
+					for (std::size_t i = 0; i < count; ++i)
+					{
+						indexes.push_back(idx);
+						++idx.item;
+					}
+					if (count)
+					{
+						n -= count;
+						--idx.item;
+					}
+				}
+
+				idx = _m_ess().lister.advance(idx, 1);
+			}
+
+			return indexes;
+		}
+
 		drawerbase::listbox::essence & listbox::_m_ess() const
 		{
 			return get_drawer_trigger().ess();