Feature: Dropdown options with callback (#826)

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

include/ftxui/component/component.hpp

@@ -75,6 +75,8 @@ Component Radiobox(ConstStringListRef entries,
                    RadioboxOption options = {});
 
 Component Dropdown(ConstStringListRef entries, int* selected);
+Component Dropdown(DropdownOption options);
+
 Component Toggle(ConstStringListRef entries, int* selected);
 
 // General slider constructor:

include/ftxui/component/component_options.hpp

@@ -263,6 +263,21 @@ struct WindowOptions {
   std::function<Element(const WindowRenderState&)> render;
 };
 
+/// @brief Option for the Dropdown component.
+/// @ingroup component
+/// A dropdown menu is a checkbox opening/closing a radiobox.
+struct DropdownOption {
+  /// Whether the dropdown is open or closed:
+  Ref<bool> open = false;
+  // The options for the checkbox:
+  CheckboxOption checkbox;
+  // The options for the radiobox:
+  RadioboxOption radiobox;
+  // The transformation function:
+  std::function<Element(bool open, Element checkbox, Element radiobox)>
+      transform;
+};
+
 }  // namespace ftxui
 
 #endif /* end of include guard: FTXUI_COMPONENT_COMPONENT_OPTIONS_HPP */