DropdownButtonFormField<T> constructor Null safety

Implementation

DropdownButtonFormField({
  super.key,
  required List<DropdownMenuItem<T>>? items,
  DropdownButtonBuilder? selectedItemBuilder,
  T? value,
  Widget? hint,
  Widget? disabledHint,
  required this.onChanged,
  VoidCallback? onTap,
  int elevation = 8,
  TextStyle? style,
  Widget? icon,
  Color? iconDisabledColor,
  Color? iconEnabledColor,
  double iconSize = 24.0,
  bool isDense = true,
  bool isExpanded = false,
  double? itemHeight,
  Color? focusColor,
  FocusNode? focusNode,
  bool autofocus = false,
  Color? dropdownColor,
  InputDecoration? decoration,
  super.onSaved,
  super.validator,
  AutovalidateMode? autovalidateMode,
  double? menuMaxHeight,
  bool? enableFeedback,
  AlignmentGeometry alignment = AlignmentDirectional.centerStart,
  BorderRadius? borderRadius,
  // When adding new arguments, consider adding similar arguments to
  // DropdownButton.
}) : assert(items == null || items.isEmpty || value == null ||
            items.where((DropdownMenuItem<T> item) {
              return item.value == value;
            }).length == 1,
              "There should be exactly one item with [DropdownButton]'s value: "
              '$value. \n'
              'Either zero or 2 or more [DropdownMenuItem]s were detected '
              'with the same value',
            ),
     assert(elevation != null),
     assert(iconSize != null),
     assert(isDense != null),
     assert(isExpanded != null),
     assert(itemHeight == null || itemHeight >= kMinInteractiveDimension),
     assert(autofocus != null),
     decoration = decoration ?? InputDecoration(focusColor: focusColor),
     super(
       initialValue: value,
       autovalidateMode: autovalidateMode ?? AutovalidateMode.disabled,
       builder: (FormFieldState<T> field) {
         final _DropdownButtonFormFieldState<T> state = field as _DropdownButtonFormFieldState<T>;
         final InputDecoration decorationArg =  decoration ?? InputDecoration(focusColor: focusColor);
         final InputDecoration effectiveDecoration = decorationArg.applyDefaults(
           Theme.of(field.context).inputDecorationTheme,
         );

         final bool showSelectedItem = items != null && items.where((DropdownMenuItem<T> item) => item.value == state.value).isNotEmpty;
         bool isHintOrDisabledHintAvailable() {
           final bool isDropdownDisabled = onChanged == null || (items == null || items.isEmpty);
           if (isDropdownDisabled) {
             return hint != null || disabledHint != null;
           } else {
             return hint != null;
           }
         }
         final bool isEmpty = !showSelectedItem && !isHintOrDisabledHintAvailable();

         // An unfocusable Focus widget so that this widget can detect if its
         // descendants have focus or not.
         return Focus(
           canRequestFocus: false,
           skipTraversal: true,
           child: Builder(builder: (BuildContext context) {
             return DropdownButtonHideUnderline(
               child: DropdownButton<T>._formField(
                 items: items,
                 selectedItemBuilder: selectedItemBuilder,
                 value: state.value,
                 hint: hint,
                 disabledHint: disabledHint,
                 onChanged: onChanged == null ? null : state.didChange,
                 onTap: onTap,
                 elevation: elevation,
                 style: style,
                 icon: icon,
                 iconDisabledColor: iconDisabledColor,
                 iconEnabledColor: iconEnabledColor,
                 iconSize: iconSize,
                 isDense: isDense,
                 isExpanded: isExpanded,
                 itemHeight: itemHeight,
                 focusColor: focusColor,
                 focusNode: focusNode,
                 autofocus: autofocus,
                 dropdownColor: dropdownColor,
                 menuMaxHeight: menuMaxHeight,
                 enableFeedback: enableFeedback,
                 alignment: alignment,
                 borderRadius: borderRadius,
                 inputDecoration: effectiveDecoration.copyWith(errorText: field.errorText),
                 isEmpty: isEmpty,
                 isFocused: Focus.of(context).hasFocus,
               ),
             );
           }),
         );
       },
     );