Flutter's CustomDropdownButton. Exposes some extra params for fine tunning the UI.

custom_dropdown.dart

/// new "custom" parameters: 
/// -menuRadius: menu corner radius [BorderRadius]
/// -menuDecoration: background [BoxDecoration]
/// -menuPadding: menu vertical padding [EdgeInsets]
/// -menuClip: To clip (mask) menu's content [Clip.none]
/// -itemPadding: item's horizontal padding [EdgeInsets]


// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'dart:math' as math;
import 'dart:ui' show window;

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/services.dart';

const Duration _kDropdownMenuDuration = Duration(milliseconds: 300);
const double _kMenuItemHeight = kMinInteractiveDimension;
const double _kDenseButtonHeight = 24.0;
const EdgeInsets _kMenuItemPadding = EdgeInsets.symmetric(horizontal: 16.0);
const EdgeInsetsGeometry _kAlignedButtonPadding =
    EdgeInsetsDirectional.only(start: 16.0, end: 4.0);
const EdgeInsets _kUnalignedButtonPadding = EdgeInsets.zero;
const EdgeInsets _kAlignedMenuMargin = EdgeInsets.zero;
const EdgeInsetsGeometry _kUnalignedMenuMargin =
    EdgeInsetsDirectional.only(start: 16.0, end: 24.0);

class _DropdownMenuPainter extends CustomPainter {
  _DropdownMenuPainter({
    BoxDecoration? decoration,
    this.color,
    this.elevation,
    this.selectedIndex,
    BorderRadius? radius,
    required this.resize,
    required this.getSelectedItemOffset,
  })  : decoration = BoxDecoration(
          // If you add an image here, you must provide a real
          // configuration in the paint() function and you must provide some sort
          // of onChanged callback here.
          image: decoration?.image,
          shape: decoration?.shape ?? BoxShape.rectangle,
          backgroundBlendMode: decoration?.backgroundBlendMode,
          gradient: decoration?.gradient,
          color: decoration?.color ?? color,
          border: decoration?.border,
          borderRadius:
              decoration?.borderRadius ?? radius ?? BorderRadius.circular(5.0),
          boxShadow: decoration?.boxShadow ?? kElevationToShadow[elevation],
        ),
        super(repaint: resize);
  final BoxDecoration decoration;
  final Color? color;
  final int? elevation;
  final int? selectedIndex;
  final Animation<double> resize;
  final ValueGetter<double> getSelectedItemOffset;
  late BoxPainter _painter = decoration.createBoxPainter();

  @override
  void paint(Canvas canvas, Size size) {
    final double selectedItemOffset = getSelectedItemOffset();
    final Tween<double> top = Tween<double>(
      begin: selectedItemOffset.clamp(
          0.0, math.max(size.height - _kMenuItemHeight, 0.0)),
      end: 0.0,
    );

    final Tween<double> bottom = Tween<double>(
      begin: (top.begin! + _kMenuItemHeight)
          .clamp(math.min(_kMenuItemHeight, size.height), size.height),
      end: size.height,
    );

    final Rect rect = Rect.fromLTRB(
        0.0, top.evaluate(resize), size.width, bottom.evaluate(resize));

    _painter.paint(canvas, rect.topLeft, ImageConfiguration(size: rect.size));
  }

  @override
  bool shouldRepaint(_DropdownMenuPainter oldPainter) {
    return oldPainter.color != color ||
        oldPainter.decoration != decoration ||
        oldPainter.elevation != elevation ||
        oldPainter.selectedIndex != selectedIndex ||
        oldPainter.resize != resize;
  }
}

// The widget that is the button wrapping the menu items.
class _DropdownMenuItemButton<T> extends StatefulWidget {
  const _DropdownMenuItemButton({
    Key? key,
    this.padding,
    required this.route,
    required this.buttonRect,
    required this.constraints,
    required this.itemIndex,
  }) : super(key: key);

  final _DropdownRoute<T> route;
  final EdgeInsets? padding;
  final Rect buttonRect;
  final BoxConstraints constraints;
  final int itemIndex;

  @override
  _DropdownMenuItemButtonState<T> createState() =>
      _DropdownMenuItemButtonState<T>();
}

class _DropdownMenuItemButtonState<T>
    extends State<_DropdownMenuItemButton<T>> {
  void _handleFocusChange(bool focused) {
    final bool inTraditionalMode;
    switch (FocusManager.instance.highlightMode) {
      case FocusHighlightMode.touch:
        inTraditionalMode = false;
        break;
      case FocusHighlightMode.traditional:
        inTraditionalMode = true;
        break;
    }

    if (focused && inTraditionalMode) {
      final _MenuLimits menuLimits = widget.route.getMenuLimits(
        widget.buttonRect,
        widget.constraints.maxHeight,
        widget.itemIndex,
      );
      widget.route.scrollController!.animateTo(
        menuLimits.scrollOffset,
        curve: Curves.easeInOut,
        duration: const Duration(milliseconds: 100),
      );
    }
  }

  void _handleOnTap() {
    final DropdownMenuItem<T> dropdownMenuItem =
        widget.route.items[widget.itemIndex].item!;

    dropdownMenuItem.onTap?.call();

    Navigator.pop(
      context,
      _DropdownRouteResult<T>(dropdownMenuItem.value),
    );
  }

  static final Map<LogicalKeySet, Intent> _webShortcuts =
      <LogicalKeySet, Intent>{
    // On the web, up/down don't change focus, *except* in a <select>
    // element, which is what a dropdown emulates.
    LogicalKeySet(LogicalKeyboardKey.arrowDown):
        const DirectionalFocusIntent(TraversalDirection.down),
    LogicalKeySet(LogicalKeyboardKey.arrowUp):
        const DirectionalFocusIntent(TraversalDirection.up),
  };

  @override
  Widget build(BuildContext context) {
    final CurvedAnimation opacity;
    final double unit = 0.5 / (widget.route.items.length + 1.5);
    if (widget.itemIndex == widget.route.selectedIndex) {
      opacity = CurvedAnimation(
          parent: widget.route.animation!, curve: const Threshold(0.0));
    } else {
      final double start =
          (0.5 + (widget.itemIndex + 1) * unit).clamp(0.0, 1.0);
      final double end = (start + 1.5 * unit).clamp(0.0, 1.0);
      opacity = CurvedAnimation(
          parent: widget.route.animation!, curve: Interval(start, end));
    }
    Widget child = FadeTransition(
      opacity: opacity,
      child: InkWell(
        // borderRadius: BorderRadius.circular(20),
        autofocus: widget.itemIndex == widget.route.selectedIndex,
        child: Container(
          padding: widget.padding,
          child: widget.route.items[widget.itemIndex],
        ),
        onTap: _handleOnTap,
        onFocusChange: _handleFocusChange,
      ),
    );
    if (kIsWeb) {
      child = Shortcuts(
        shortcuts: _webShortcuts,
        child: child,
      );
    }
    return child;
  }
}

class _DropdownMenu<T> extends StatefulWidget {
  const _DropdownMenu({
    Key? key,
    this.padding,
    required this.route,
    required this.buttonRect,
    required this.constraints,
    this.radius,
    this.backgroundDecoration,
    this.clipList = Clip.none,
    this.listPadding,
    this.dropdownColor,
  }) : super(key: key);

  final _DropdownRoute<T> route;
  final BorderRadius? radius;
  final EdgeInsets? padding;
  final EdgeInsets? listPadding;
  final Clip clipList;
  final Rect buttonRect;
  final BoxConstraints constraints;
  final Color? dropdownColor;
  final BoxDecoration? backgroundDecoration;

  @override
  _DropdownMenuState<T> createState() => _DropdownMenuState<T>();
}

class _DropdownMenuState<T> extends State<_DropdownMenu<T>> {
  late CurvedAnimation _fadeOpacity;
  late CurvedAnimation _resize;

  @override
  void initState() {
    super.initState();
    // We need to hold these animations as state because of their curve
    // direction. When the route's animation reverses, if we were to recreate
    // the CurvedAnimation objects in build, we'd lose
    // CurvedAnimation._curveDirection.
    _fadeOpacity = CurvedAnimation(
      parent: widget.route.animation!,
      curve: const Interval(0.0, 0.25),
      reverseCurve: const Interval(0.75, 1.0),
    );
    _resize = CurvedAnimation(
      parent: widget.route.animation!,
      curve: const Interval(0.25, 0.5),
      reverseCurve: const Threshold(0.0),
    );
  }

  @override
  Widget build(BuildContext context) {
    // The menu is shown in three stages (unit timing in brackets):
    // [0s - 0.25s] - Fade in a rect-sized menu container with the selected item.
    // [0.25s - 0.5s] - Grow the otherwise empty menu container from the center
    //   until it's big enough for as many items as we're going to show.
    // [0.5s - 1.0s] Fade in the remaining visible items from top to bottom.
    //
    // When the menu is dismissed we just fade the entire thing out
    // in the first 0.25s.
    assert(debugCheckHasMaterialLocalizations(context));
    final MaterialLocalizations localizations =
        MaterialLocalizations.of(context);
    final _DropdownRoute<T> route = widget.route;
    final List<Widget> children = <Widget>[
      for (int itemIndex = 0; itemIndex < route.items.length; ++itemIndex)
        _DropdownMenuItemButton<T>(
          route: widget.route,
          padding: widget.padding,
          buttonRect: widget.buttonRect,
          constraints: widget.constraints,
          itemIndex: itemIndex,
        ),
    ];

    return FadeTransition(
      opacity: _fadeOpacity,
      child: CustomPaint(
        painter: _DropdownMenuPainter(
          decoration: widget.backgroundDecoration,
          color: widget.dropdownColor ?? Theme.of(context).canvasColor,
          elevation: route.elevation,
          selectedIndex: route.selectedIndex,
          resize: _resize,
          radius: widget.radius,
          // This offset is passed as a callback, not a value, because it must
          // be retrieved at paint time (after layout), not at build time.
          getSelectedItemOffset: () => route.getItemOffset(route.selectedIndex),
        ),
        child: Semantics(
          scopesRoute: true,
          namesRoute: true,
          explicitChildNodes: true,
          label: localizations.popupMenuLabel,
          child: Material(
            type: MaterialType.transparency,
            textStyle: route.style,
            borderRadius: widget.radius,

            /// get clipping from widget.
            clipBehavior: widget.clipList,
            child: ScrollConfiguration(
              // Dropdown menus should never overscroll or display an overscroll indicator.
              // The default scrollbar platforms will apply.
              // Platform must use Theme and ScrollPhysics must be Clamping.
              behavior: ScrollConfiguration.of(context).copyWith(
                overscroll: false,
                physics: const ClampingScrollPhysics(),
                platform: Theme.of(context).platform,
              ),
              child: PrimaryScrollController(
                controller: widget.route.scrollController!,
                child: Scrollbar(
                  isAlwaysShown: true,
                  child: ListView(
                    padding: widget.listPadding ?? kMaterialListPadding,
                    shrinkWrap: true,
                    children: children,
                  ),
                ),
              ),
            ),
          ),
        ),
      ),
    );
  }
}

class _DropdownMenuRouteLayout<T> extends SingleChildLayoutDelegate {
  _DropdownMenuRouteLayout({
    required this.buttonRect,
    required this.route,
    required this.textDirection,
  });

  final Rect buttonRect;
  final _DropdownRoute<T> route;
  final TextDirection? textDirection;

  @override
  BoxConstraints getConstraintsForChild(BoxConstraints constraints) {
    // The maximum height of a simple menu should be one or more rows less than
    // the view height. This ensures a tappable area outside of the simple menu
    // with which to dismiss the menu.
    //   -- https://material.io/design/components/menus.html#usage
    double maxHeight =
        math.max(0.0, constraints.maxHeight - 2 * _kMenuItemHeight);
    if (route.menuMaxHeight != null && route.menuMaxHeight! <= maxHeight) {
      maxHeight = route.menuMaxHeight!;
    }
    // The width of a menu should be at most the view width. This ensures that
    // the menu does not extend past the left and right edges of the screen.
    final double width = math.min(constraints.maxWidth, buttonRect.width);
    return BoxConstraints(
      minWidth: width,
      maxWidth: width,
      minHeight: 0.0,
      maxHeight: maxHeight,
    );
  }

  @override
  Offset getPositionForChild(Size size, Size childSize) {
    final _MenuLimits menuLimits =
        route.getMenuLimits(buttonRect, size.height, route.selectedIndex);

    assert(() {
      final Rect container = Offset.zero & size;
      if (container.intersect(buttonRect) == buttonRect) {
        // If the button was entirely on-screen, then verify
        // that the menu is also on-screen.
        // If the button was a bit off-screen, then, oh well.
        assert(menuLimits.top >= 0.0);
        assert(menuLimits.top + menuLimits.height <= size.height);
      }
      return true;
    }());
    assert(textDirection != null);
    final double left;
    switch (textDirection!) {
      case TextDirection.rtl:
        left = buttonRect.right.clamp(0.0, size.width) - childSize.width;
        break;
      case TextDirection.ltr:
        left = buttonRect.left.clamp(0.0, size.width - childSize.width);
        break;
    }

    return Offset(left, menuLimits.top);
  }

  @override
  bool shouldRelayout(_DropdownMenuRouteLayout<T> oldDelegate) {
    return buttonRect != oldDelegate.buttonRect ||
        textDirection != oldDelegate.textDirection;
  }
}

// We box the return value so that the return value can be null. Otherwise,
// canceling the route (which returns null) would get confused with actually
// returning a real null value.
@immutable
class _DropdownRouteResult<T> {
  const _DropdownRouteResult(this.result);

  final T? result;

  @override
  bool operator ==(Object other) {
    return other is _DropdownRouteResult<T> && other.result == result;
  }

  @override
  int get hashCode => result.hashCode;
}

class _MenuLimits {
  const _MenuLimits(this.top, this.bottom, this.height, this.scrollOffset);
  final double top;
  final double bottom;
  final double height;
  final double scrollOffset;
}

class _DropdownRoute<T> extends PopupRoute<_DropdownRouteResult<T>> {
  _DropdownRoute({
    required this.items,
    required this.padding,
    required this.buttonRect,
    required this.selectedIndex,
    this.elevation = 8,
    required this.capturedThemes,
    required this.style,
    this.barrierLabel,
    this.itemHeight,
    this.dropdownColor,
    this.menuMaxHeight,
    this.radius,
    this.clipList = Clip.none,
    this.listPadding,
    this.backgroundDecoration,
  })  : assert(style != null),
        itemHeights = List<double>.filled(
            items.length, itemHeight ?? kMinInteractiveDimension);

  final List<_MenuItem<T>> items;
  final EdgeInsetsGeometry padding;
  final Rect buttonRect;
  final int selectedIndex;
  final int elevation;
  final CapturedThemes capturedThemes;
  final TextStyle style;
  final double? itemHeight;
  final Color? dropdownColor;
  final double? menuMaxHeight;
  final BorderRadius? radius;
  final EdgeInsets? listPadding;
  final BoxDecoration? backgroundDecoration;
  final Clip clipList;

  final List<double> itemHeights;
  ScrollController? scrollController;

  @override
  Duration get transitionDuration => _kDropdownMenuDuration;

  @override
  bool get barrierDismissible => true;

  @override
  Color? get barrierColor => null;

  @override
  final String? barrierLabel;

  @override
  Widget buildPage(BuildContext context, Animation<double> animation,
      Animation<double> secondaryAnimation) {
    return LayoutBuilder(
      builder: (BuildContext context, BoxConstraints constraints) {
        return _DropdownRoutePage<T>(
          route: this,
          constraints: constraints,
          items: items,
          clipList: clipList,
          listPadding: listPadding,
          backgroundDecoration: backgroundDecoration,
          radius: radius,
          padding: padding,
          buttonRect: buttonRect,
          selectedIndex: selectedIndex,
          elevation: elevation,
          capturedThemes: capturedThemes,
          style: style,
          dropdownColor: dropdownColor,
        );
      },
    );
  }

  void _dismiss() {
    if (isActive) {
      navigator?.removeRoute(this);
    }
  }

  double getItemOffset(int index) {
    double offset = kMaterialListPadding.top;
    if (items.isNotEmpty && index > 0) {
      assert(items.length == itemHeights.length);
      offset += itemHeights
          .sublist(0, index)
          .reduce((double total, double height) => total + height);
    }
    return offset;
  }

  // Returns the vertical extent of the menu and the initial scrollOffset
  // for the ListView that contains the menu items. The vertical center of the
  // selected item is aligned with the button's vertical center, as far as
  // that's possible given availableHeight.
  _MenuLimits getMenuLimits(
      Rect buttonRect, double availableHeight, int index) {
    final double maxMenuHeight = availableHeight - 2.0 * _kMenuItemHeight;
    final double buttonTop = buttonRect.top;
    final double buttonBottom = math.min(buttonRect.bottom, availableHeight);
    final double selectedItemOffset = getItemOffset(index);

    // If the button is placed on the bottom or top of the screen, its top or
    // bottom may be less than [_kMenuItemHeight] from the edge of the screen.
    // In this case, we want to change the menu limits to align with the top
    // or bottom edge of the button.
    final double topLimit = math.min(_kMenuItemHeight, buttonTop);
    final double bottomLimit =
        math.max(availableHeight - _kMenuItemHeight, buttonBottom);

    double menuTop = (buttonTop - selectedItemOffset) -
        (itemHeights[selectedIndex] - buttonRect.height) / 2.0;
    double preferredMenuHeight = kMaterialListPadding.vertical;
    if (items.isNotEmpty)
      preferredMenuHeight +=
          itemHeights.reduce((double total, double height) => total + height);

    // If there are too many elements in the menu, we need to shrink it down
    // so it is at most the maxMenuHeight.
    final double menuHeight = math.min(maxMenuHeight, preferredMenuHeight);
    double menuBottom = menuTop + menuHeight;

    // If the computed top or bottom of the menu are outside of the range
    // specified, we need to bring them into range. If the item height is larger
    // than the button height and the button is at the very bottom or top of the
    // screen, the menu will be aligned with the bottom or top of the button
    // respectively.
    if (menuTop < topLimit) menuTop = math.min(buttonTop, topLimit);

    if (menuBottom > bottomLimit) {
      menuBottom = math.max(buttonBottom, bottomLimit);
      menuTop = menuBottom - menuHeight;
    }

    double scrollOffset = 0;
    // If all of the menu items will not fit within availableHeight then
    // compute the scroll offset that will line the selected menu item up
    // with the select item. This is only done when the menu is first
    // shown - subsequently we leave the scroll offset where the user left
    // it. This scroll offset is only accurate for fixed height menu items
    // (the default).
    if (preferredMenuHeight > maxMenuHeight) {
      // The offset should be zero if the selected item is in view at the beginning
      // of the menu. Otherwise, the scroll offset should center the item if possible.
      scrollOffset = math.max(0.0, selectedItemOffset - (buttonTop - menuTop));
      // If the selected item's scroll offset is greater than the maximum scroll offset,
      // set it instead to the maximum allowed scroll offset.
      scrollOffset = math.min(scrollOffset, preferredMenuHeight - menuHeight);
    }

    return _MenuLimits(menuTop, menuBottom, menuHeight, scrollOffset);
  }
}

class _DropdownRoutePage<T> extends StatelessWidget {
  const _DropdownRoutePage({
    Key? key,
    required this.route,
    required this.constraints,
    this.items,
    this.radius,
    this.clipList = Clip.none,
    this.listPadding,
    this.backgroundDecoration,
    required this.padding,
    required this.buttonRect,
    required this.selectedIndex,
    this.elevation = 8,
    required this.capturedThemes,
    this.style,
    required this.dropdownColor,
  }) : super(key: key);

  final BorderRadius? radius;
  final EdgeInsets? listPadding;
  final Clip clipList;
  final _DropdownRoute<T> route;
  final BoxConstraints constraints;
  final List<_MenuItem<T>>? items;
  final EdgeInsetsGeometry padding;
  final Rect buttonRect;
  final int selectedIndex;
  final int elevation;
  final CapturedThemes capturedThemes;
  final TextStyle? style;
  final Color? dropdownColor;
  final BoxDecoration? backgroundDecoration;

  @override
  Widget build(BuildContext context) {
    assert(debugCheckHasDirectionality(context));

    // Computing the initialScrollOffset now, before the items have been laid
    // out. This only works if the item heights are effectively fixed, i.e. either
    // DropdownButton.itemHeight is specified or DropdownButton.itemHeight is null
    // and all of the items' intrinsic heights are less than kMinInteractiveDimension.
    // Otherwise the initialScrollOffset is just a rough approximation based on
    // treating the items as if their heights were all equal to kMinInteractveDimension.
    if (route.scrollController == null) {
      final _MenuLimits menuLimits =
          route.getMenuLimits(buttonRect, constraints.maxHeight, selectedIndex);
      route.scrollController =
          ScrollController(initialScrollOffset: menuLimits.scrollOffset);
    }

    final TextDirection? textDirection = Directionality.maybeOf(context);
    final Widget menu = _DropdownMenu<T>(
      route: route,
      radius: radius,
      backgroundDecoration: backgroundDecoration,
      clipList: clipList,
      listPadding: listPadding,
      padding: padding.resolve(textDirection),
      buttonRect: buttonRect,
      constraints: constraints,
      dropdownColor: dropdownColor,
    );

    return MediaQuery.removePadding(
      context: context,
      removeTop: true,
      removeBottom: true,
      removeLeft: true,
      removeRight: true,
      child: Builder(
        builder: (BuildContext context) {
          return CustomSingleChildLayout(
            delegate: _DropdownMenuRouteLayout<T>(
              buttonRect: buttonRect,
              route: route,
              textDirection: textDirection,
            ),
            child: capturedThemes.wrap(menu),
          );
        },
      ),
    );
  }
}

// This widget enables _DropdownRoute to look up the sizes of
// each menu item. These sizes are used to compute the offset of the selected
// item so that _DropdownRoutePage can align the vertical center of the
// selected item lines up with the vertical center of the dropdown button,
// as closely as possible.
class _MenuItem<T> extends SingleChildRenderObjectWidget {
  const _MenuItem({
    Key? key,
    required this.onLayout,
    required this.item,
  })  : assert(onLayout != null),
        super(key: key, child: item);

  final ValueChanged<Size> onLayout;
  final DropdownMenuItem<T>? item;

  @override
  RenderObject createRenderObject(BuildContext context) {
    return _RenderMenuItem(onLayout);
  }

  @override
  void updateRenderObject(
      BuildContext context, covariant _RenderMenuItem renderObject) {
    renderObject.onLayout = onLayout;
  }
}

class _RenderMenuItem extends RenderProxyBox {
  _RenderMenuItem(this.onLayout, [RenderBox? child])
      : assert(onLayout != null),
        super(child);

  ValueChanged<Size> onLayout;

  @override
  void performLayout() {
    super.performLayout();
    onLayout(size);
  }
}

// The container widget for a menu item created by a [DropdownButton]. It
// provides the default configuration for [DropdownMenuItem]s, as well as a
// [DropdownButton]'s hint and disabledHint widgets.
class _DropdownMenuItemContainer extends StatelessWidget {
  /// Creates an item for a dropdown menu.
  ///
  /// The [child] argument is required.
  const _DropdownMenuItemContainer({
    Key? key,
    required this.child,
  })  : assert(child != null),
        super(key: key);

  /// The widget below this widget in the tree.
  ///
  /// Typically a [Text] widget.
  final Widget child;

  @override
  Widget build(BuildContext context) {
    return Container(
      constraints: const BoxConstraints(minHeight: _kMenuItemHeight),
      alignment: AlignmentDirectional.centerStart,
      child: child,
    );
  }
}

/// A material design button for selecting from a list of items.
///
/// A dropdown button lets the user select from a number of items. The button
/// shows the currently selected item as well as an arrow that opens a menu for
/// selecting another item.
///
/// The type `T` is the type of the [value] that each dropdown item represents.
/// All the entries in a given menu must represent values with consistent types.
/// Typically, an enum is used. Each [DropdownMenuItem] in [items] must be
/// specialized with that same type argument.
///
/// The [onChanged] callback should update a state variable that defines the
/// dropdown's value. It should also call [State.setState] to rebuild the
/// dropdown with the new value.
///
/// {@tool dartpad --template=stateful_widget_scaffold_center}
///
/// This sample shows a `DropdownButton` with a large arrow icon,
/// purple text style, and bold purple underline, whose value is one of "One",
/// "Two", "Free", or "Four".
///
/// ![](https://flutter.github.io/assets-for-api-docs/assets/material/dropdown_button.png)
///
/// ```dart
/// String dropdownValue = 'One';
///
/// @override
/// Widget build(BuildContext context) {
///   return DropdownButton<String>(
///     value: dropdownValue,
///     icon: const Icon(Icons.arrow_downward),
///     iconSize: 24,
///     elevation: 16,
///     style: const TextStyle(
///       color: Colors.deepPurple
///     ),
///     underline: Container(
///       height: 2,
///       color: Colors.deepPurpleAccent,
///     ),
///     onChanged: (String? newValue) {
///       setState(() {
///         dropdownValue = newValue!;
///       });
///     },
///     items: <String>['One', 'Two', 'Free', 'Four']
///       .map<DropdownMenuItem<String>>((String value) {
///         return DropdownMenuItem<String>(
///           value: value,
///           child: Text(value),
///         );
///       })
///       .toList(),
///   );
/// }
/// ```
/// {@end-tool}
///
/// If the [onChanged] callback is null or the list of [items] is null
/// then the dropdown button will be disabled, i.e. its arrow will be
/// displayed in grey and it will not respond to input. A disabled button
/// will display the [disabledHint] widget if it is non-null. However, if
/// [disabledHint] is null and [hint] is non-null, the [hint] widget will
/// instead be displayed.
///
/// Requires one of its ancestors to be a [Material] widget.
///
/// See also:
///
///  * [DropdownMenuItem], the class used to represent the [items].
///  * [DropdownButtonHideUnderline], which prevents its descendant dropdown buttons
///    from displaying their underlines.
///  * [ElevatedButton], [TextButton], ordinary buttons that trigger a single action.
///  * <https://material.io/design/components/menus.html#dropdown-menu>
class CustomDropdownButton<T> extends StatefulWidget {
  /// Creates a dropdown button.
  ///
  /// The [items] must have distinct values. If [value] isn't null then it
  /// must be equal to one of the [DropdownMenuItem] values. If [items] or
  /// [onChanged] is null, the button will be disabled, the down arrow
  /// will be greyed out.
  ///
  /// If [value] is null and the button is enabled, [hint] will be displayed
  /// if it is non-null.
  ///
  /// If [value] is null and the button is disabled, [disabledHint] will be displayed
  /// if it is non-null. If [disabledHint] is null, then [hint] will be displayed
  /// if it is non-null.
  ///
  /// The [elevation] and [iconSize] arguments must not be null (they both have
  /// defaults, so do not need to be specified). The boolean [isDense] and
  /// [isExpanded] arguments must not be null.
  ///
  /// The [autofocus] argument must not be null.
  ///
  /// The [dropdownColor] argument specifies the background color of the
  /// dropdown when it is open. If it is null, the current theme's
  /// [ThemeData.canvasColor] will be used instead.
  CustomDropdownButton({
    Key? key,
    required this.items,
    this.selectedItemBuilder,
    this.value,
    this.hint,
    this.disabledHint,
    this.onChanged,
    this.onTap,
    this.elevation = 8,
    this.style,
    this.underline,
    this.icon,
    this.iconDisabledColor,
    this.iconEnabledColor,
    this.iconSize = 24.0,
    this.isDense = false,
    this.isExpanded = false,
    this.itemHeight = kMinInteractiveDimension,
    this.focusColor,
    this.focusNode,
    this.autofocus = false,
    this.dropdownColor,
    this.menuMaxHeight,
    this.menuRadius,
    this.menuPadding,
    this.itemPadding,
    this.menuClip = Clip.none,
    this.menuDecoration,
    // When adding new arguments, consider adding similar arguments to
    // DropdownButtonFormField.
  })  : 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(autofocus != null),
        assert(itemHeight == null || itemHeight >= kMinInteractiveDimension),
        super(key: key);

  /// The list of items the user can select.
  ///
  /// If the [onChanged] callback is null or the list of items is null
  /// then the dropdown button will be disabled, i.e. its arrow will be
  /// displayed in grey and it will not respond to input.
  final List<DropdownMenuItem<T>>? items;

  /// The value of the currently selected [DropdownMenuItem].
  ///
  /// If [value] is null and the button is enabled, [hint] will be displayed
  /// if it is non-null.
  ///
  /// If [value] is null and the button is disabled, [disabledHint] will be displayed
  /// if it is non-null. If [disabledHint] is null, then [hint] will be displayed
  /// if it is non-null.
  final T? value;

  /// A placeholder widget that is displayed by the dropdown button.
  ///
  /// If [value] is null and the dropdown is enabled ([items] and [onChanged] are non-null),
  /// this widget is displayed as a placeholder for the dropdown button's value.
  ///
  /// If [value] is null and the dropdown is disabled and [disabledHint] is null,
  /// this widget is used as the placeholder.
  final Widget? hint;

  /// A preferred placeholder widget that is displayed when the dropdown is disabled.
  ///
  /// If [value] is null, the dropdown is disabled ([items] or [onChanged] is null),
  /// this widget is displayed as a placeholder for the dropdown button's value.
  final Widget? disabledHint;

  /// {@template flutter.material.dropdownButton.onChanged}
  /// Called when the user selects an item.
  ///
  /// If the [onChanged] callback is null or the list of [CustomDropdownButton.items]
  /// is null then the dropdown button will be disabled, i.e. its arrow will be
  /// displayed in grey and it will not respond to input. A disabled button
  /// will display the [CustomDropdownButton.disabledHint] widget if it is non-null.
  /// If [CustomDropdownButton.disabledHint] is also null but [CustomDropdownButton.hint] is
  /// non-null, [CustomDropdownButton.hint] will instead be displayed.
  /// {@endtemplate}
  final ValueChanged<T?>? onChanged;

  /// Called when the dropdown button is tapped.
  ///
  /// This is distinct from [onChanged], which is called when the user
  /// selects an item from the dropdown.
  ///
  /// The callback will not be invoked if the dropdown button is disabled.
  final VoidCallback? onTap;

  /// A builder to customize the dropdown buttons corresponding to the
  /// [DropdownMenuItem]s in [items].
  ///
  /// When a [DropdownMenuItem] is selected, the widget that will be displayed
  /// from the list corresponds to the [DropdownMenuItem] of the same index
  /// in [items].
  ///
  /// {@tool dartpad --template=stateful_widget_scaffold}
  ///
  /// This sample shows a `DropdownButton` with a button with [Text] that
  /// corresponds to but is unique from [DropdownMenuItem].
  ///
  /// ```dart
  /// final List<String> items = <String>['1','2','3'];
  /// String selectedItem = '1';
  ///
  /// @override
  /// Widget build(BuildContext context) {
  ///   return Padding(
  ///     padding: const EdgeInsets.symmetric(horizontal: 12.0),
  ///     child: DropdownButton<String>(
  ///       value: selectedItem,
  ///       onChanged: (String? string) => setState(() => selectedItem = string!),
  ///       selectedItemBuilder: (BuildContext context) {
  ///         return items.map<Widget>((String item) {
  ///           return Text(item);
  ///         }).toList();
  ///       },
  ///       items: items.map((String item) {
  ///         return DropdownMenuItem<String>(
  ///           child: Text('Log $item'),
  ///           value: item,
  ///         );
  ///       }).toList(),
  ///     ),
  ///   );
  /// }
  /// ```
  /// {@end-tool}
  ///
  /// If this callback is null, the [DropdownMenuItem] from [items]
  /// that matches [value] will be displayed.
  final DropdownButtonBuilder? selectedItemBuilder;

  /// The z-coordinate at which to place the menu when open.
  ///
  /// The following elevations have defined shadows: 1, 2, 3, 4, 6, 8, 9, 12,
  /// 16, and 24. See [kElevationToShadow].
  ///
  /// Defaults to 8, the appropriate elevation for dropdown buttons.
  final int elevation;

  /// custom props.
  final BorderRadius? menuRadius;
  final EdgeInsets? itemPadding;
  final EdgeInsets? menuPadding;
  final Clip menuClip;
  final BoxDecoration? menuDecoration;

  /// The text style to use for text in the dropdown button and the dropdown
  /// menu that appears when you tap the button.
  ///
  /// To use a separate text style for selected item when it's displayed within
  /// the dropdown button, consider using [selectedItemBuilder].
  ///
  /// {@tool dartpad --template=stateful_widget_scaffold}
  ///
  /// This sample shows a `DropdownButton` with a dropdown button text style
  /// that is different than its menu items.
  ///
  /// ```dart
  /// List<String> options = <String>['One', 'Two', 'Free', 'Four'];
  /// String dropdownValue = 'One';
  ///
  /// @override
  /// Widget build(BuildContext context) {
  ///   return Container(
  ///     alignment: Alignment.center,
  ///     color: Colors.blue,
  ///     child: DropdownButton<String>(
  ///       value: dropdownValue,
  ///       onChanged: (String? newValue) {
  ///         setState(() {
  ///           dropdownValue = newValue!;
  ///         });
  ///       },
  ///       style: const TextStyle(color: Colors.blue),
  ///       selectedItemBuilder: (BuildContext context) {
  ///         return options.map((String value) {
  ///           return Text(
  ///             dropdownValue,
  ///             style: const TextStyle(color: Colors.white),
  ///           );
  ///         }).toList();
  ///       },
  ///       items: options.map<DropdownMenuItem<String>>((String value) {
  ///         return DropdownMenuItem<String>(
  ///           value: value,
  ///           child: Text(value),
  ///         );
  ///       }).toList(),
  ///     ),
  ///   );
  /// }
  /// ```
  /// {@end-tool}
  ///
  /// Defaults to the [TextTheme.subtitle1] value of the current
  /// [ThemeData.textTheme] of the current [Theme].
  final TextStyle? style;

  /// The widget to use for drawing the drop-down button's underline.
  ///
  /// Defaults to a 0.0 width bottom border with color 0xFFBDBDBD.
  final Widget? underline;

  /// The widget to use for the drop-down button's icon.
  ///
  /// Defaults to an [Icon] with the [Icons.arrow_drop_down] glyph.
  final Widget? icon;

  /// The color of any [Icon] descendant of [icon] if this button is disabled,
  /// i.e. if [onChanged] is null.
  ///
  /// Defaults to [MaterialColor.shade400] of [Colors.grey] when the theme's
  /// [ThemeData.brightness] is [Brightness.light] and to
  /// [Colors.white10] when it is [Brightness.dark]
  final Color? iconDisabledColor;

  /// The color of any [Icon] descendant of [icon] if this button is enabled,
  /// i.e. if [onChanged] is defined.
  ///
  /// Defaults to [MaterialColor.shade700] of [Colors.grey] when the theme's
  /// [ThemeData.brightness] is [Brightness.light] and to
  /// [Colors.white70] when it is [Brightness.dark]
  final Color? iconEnabledColor;

  /// The size to use for the drop-down button's down arrow icon button.
  ///
  /// Defaults to 24.0.
  final double iconSize;

  /// Reduce the button's height.
  ///
  /// By default this button's height is the same as its menu items' heights.
  /// If isDense is true, the button's height is reduced by about half. This
  /// can be useful when the button is embedded in a container that adds
  /// its own decorations, like [InputDecorator].
  final bool isDense;

  /// Set the dropdown's inner contents to horizontally fill its parent.
  ///
  /// By default this button's inner width is the minimum size of its contents.
  /// If [isExpanded] is true, the inner width is expanded to fill its
  /// surrounding container.
  final bool isExpanded;

  /// If null, then the menu item heights will vary according to each menu item's
  /// intrinsic height.
  ///
  /// The default value is [kMinInteractiveDimension], which is also the minimum
  /// height for menu items.
  ///
  /// If this value is null and there isn't enough vertical room for the menu,
  /// then the menu's initial scroll offset may not align the selected item with
  /// the dropdown button. That's because, in this case, the initial scroll
  /// offset is computed as if all of the menu item heights were
  /// [kMinInteractiveDimension].
  final double? itemHeight;

  /// The color for the button's [Material] when it has the input focus.
  final Color? focusColor;

  /// {@macro flutter.widgets.Focus.focusNode}
  final FocusNode? focusNode;

  /// {@macro flutter.widgets.Focus.autofocus}
  final bool autofocus;

  /// The background color of the dropdown.
  ///
  /// If it is not provided, the theme's [ThemeData.canvasColor] will be used
  /// instead.
  final Color? dropdownColor;

  /// The maximum height of the menu.
  ///
  /// The maximum height of the menu must be at least one row shorter than
  /// the height of the app's view. This ensures that a tappable area
  /// outside of the simple menu is present so the user can dismiss the menu.
  ///
  /// If this property is set above the maximum allowable height threshold
  /// mentioned above, then the menu defaults to being padded at the top
  /// and bottom of the menu by at one menu item's height.
  final double? menuMaxHeight;

  @override
  _CustomDropdownButtonState<T> createState() =>
      _CustomDropdownButtonState<T>();
}

class _CustomDropdownButtonState<T> extends State<CustomDropdownButton<T>>
    with WidgetsBindingObserver {
  int? _selectedIndex;
  _DropdownRoute<T>? _dropdownRoute;
  Orientation? _lastOrientation;
  FocusNode? _internalNode;
  FocusNode? get focusNode => widget.focusNode ?? _internalNode;
  bool _hasPrimaryFocus = false;
  late Map<Type, Action<Intent>> _actionMap;
  late FocusHighlightMode _focusHighlightMode;

  // Only used if needed to create _internalNode.
  FocusNode _createFocusNode() {
    return FocusNode(debugLabel: '${widget.runtimeType}');
  }

  @override
  void initState() {
    super.initState();
    _updateSelectedIndex();
    if (widget.focusNode == null) {
      _internalNode ??= _createFocusNode();
    }
    _actionMap = <Type, Action<Intent>>{
      ActivateIntent: CallbackAction<ActivateIntent>(
        onInvoke: (ActivateIntent intent) => _handleTap(),
      ),
      ButtonActivateIntent: CallbackAction<ButtonActivateIntent>(
        onInvoke: (ButtonActivateIntent intent) => _handleTap(),
      ),
    };
    focusNode!.addListener(_handleFocusChanged);
    final FocusManager focusManager = WidgetsBinding.instance!.focusManager;
    _focusHighlightMode = focusManager.highlightMode;
    focusManager.addHighlightModeListener(_handleFocusHighlightModeChange);
  }

  @override
  void dispose() {
    WidgetsBinding.instance!.removeObserver(this);
    _removeDropdownRoute();
    WidgetsBinding.instance!.focusManager
        .removeHighlightModeListener(_handleFocusHighlightModeChange);
    focusNode!.removeListener(_handleFocusChanged);
    _internalNode?.dispose();
    super.dispose();
  }

  void _removeDropdownRoute() {
    _dropdownRoute?._dismiss();
    _dropdownRoute = null;
    _lastOrientation = null;
  }

  void _handleFocusChanged() {
    if (_hasPrimaryFocus != focusNode!.hasPrimaryFocus) {
      setState(() {
        _hasPrimaryFocus = focusNode!.hasPrimaryFocus;
      });
    }
  }

  void _handleFocusHighlightModeChange(FocusHighlightMode mode) {
    if (!mounted) {
      return;
    }
    setState(() {
      _focusHighlightMode = mode;
    });
  }

  @override
  void didUpdateWidget(CustomDropdownButton<T> oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (widget.focusNode != oldWidget.focusNode) {
      oldWidget.focusNode?.removeListener(_handleFocusChanged);
      if (widget.focusNode == null) {
        _internalNode ??= _createFocusNode();
      }
      _hasPrimaryFocus = focusNode!.hasPrimaryFocus;
      focusNode!.addListener(_handleFocusChanged);
    }
    _updateSelectedIndex();
  }

  void _updateSelectedIndex() {
    if (widget.items == null ||
        widget.items!.isEmpty ||
        (widget.value == null &&
            widget.items!
                .where((DropdownMenuItem<T> item) => item.value == widget.value)
                .isEmpty)) {
      _selectedIndex = null;
      return;
    }

    assert(widget.items!
            .where((DropdownMenuItem<T> item) => item.value == widget.value)
            .length ==
        1);
    for (int itemIndex = 0; itemIndex < widget.items!.length; itemIndex++) {
      if (widget.items![itemIndex].value == widget.value) {
        _selectedIndex = itemIndex;
        return;
      }
    }
  }

  TextStyle? get _textStyle =>
      widget.style ?? Theme.of(context).textTheme.subtitle1;

  void _handleTap() {
    final TextDirection? textDirection = Directionality.maybeOf(context);
    final EdgeInsetsGeometry menuMargin =
        ButtonTheme.of(context).alignedDropdown
            ? _kAlignedMenuMargin
            : _kUnalignedMenuMargin;

    final List<_MenuItem<T>> menuItems = <_MenuItem<T>>[
      for (int index = 0; index < widget.items!.length; index += 1)
        _MenuItem<T>(
          item: widget.items![index],
          onLayout: (Size size) {
            // If [_dropdownRoute] is null and onLayout is called, this means
            // that performLayout was called on a _DropdownRoute that has not
            // left the widget tree but is already on its way out.
            //
            // Since onLayout is used primarily to collect the desired heights
            // of each menu item before laying them out, not having the _DropdownRoute
            // collect each item's height to lay out is fine since the route is
            // already on its way out.
            if (_dropdownRoute == null) return;

            _dropdownRoute!.itemHeights[index] = size.height;
          },
        ),
    ];

    final NavigatorState navigator = Navigator.of(context);
    assert(_dropdownRoute == null);
    final RenderBox itemBox = context.findRenderObject()! as RenderBox;
    final Rect itemRect = itemBox.localToGlobal(Offset.zero,
            ancestor: navigator.context.findRenderObject()) &
        itemBox.size;
    _dropdownRoute = _DropdownRoute<T>(
      items: menuItems,
      buttonRect: menuMargin.resolve(textDirection).inflateRect(itemRect),
      selectedIndex: _selectedIndex ?? 0,
      radius: widget.menuRadius,
      clipList: widget.menuClip,
      padding: (widget.itemPadding ?? _kMenuItemPadding).resolve(textDirection),
      listPadding: widget.menuPadding,
      backgroundDecoration: widget.menuDecoration,
      elevation: widget.elevation,
      capturedThemes:
          InheritedTheme.capture(from: context, to: navigator.context),
      style: _textStyle!,
      barrierLabel: MaterialLocalizations.of(context).modalBarrierDismissLabel,
      itemHeight: widget.itemHeight,
      dropdownColor: widget.dropdownColor,
      menuMaxHeight: widget.menuMaxHeight,
    );

    navigator
        .push(_dropdownRoute!)
        .then<void>((_DropdownRouteResult<T>? newValue) {
      _removeDropdownRoute();
      if (!mounted || newValue == null) return;
      widget.onChanged?.call(newValue.result);
    });

    widget.onTap?.call();
  }

  // When isDense is true, reduce the height of this button from _kMenuItemHeight to
  // _kDenseButtonHeight, but don't make it smaller than the text that it contains.
  // Similarly, we don't reduce the height of the button so much that its icon
  // would be clipped.
  double get _denseButtonHeight {
    final double fontSize = _textStyle!.fontSize ??
        Theme.of(context).textTheme.subtitle1!.fontSize!;
    return math.max(fontSize, math.max(widget.iconSize, _kDenseButtonHeight));
  }

  Color get _iconColor {
    // These colors are not defined in the Material Design spec.
    if (_enabled) {
      if (widget.iconEnabledColor != null) return widget.iconEnabledColor!;

      switch (Theme.of(context).brightness) {
        case Brightness.light:
          return Colors.grey.shade700;
        case Brightness.dark:
          return Colors.white70;
      }
    } else {
      if (widget.iconDisabledColor != null) return widget.iconDisabledColor!;

      switch (Theme.of(context).brightness) {
        case Brightness.light:
          return Colors.grey.shade400;
        case Brightness.dark:
          return Colors.white10;
      }
    }
  }

  bool get _enabled =>
      widget.items != null &&
      widget.items!.isNotEmpty &&
      widget.onChanged != null;

  Orientation _getOrientation(BuildContext context) {
    Orientation? result = MediaQuery.maybeOf(context)?.orientation;
    if (result == null) {
      // If there's no MediaQuery, then use the window aspect to determine
      // orientation.
      final Size size = window.physicalSize;
      result = size.width > size.height
          ? Orientation.landscape
          : Orientation.portrait;
    }
    return result;
  }

  bool get _showHighlight {
    switch (_focusHighlightMode) {
      case FocusHighlightMode.touch:
        return false;
      case FocusHighlightMode.traditional:
        return _hasPrimaryFocus;
    }
  }

  @override
  Widget build(BuildContext context) {
    assert(debugCheckHasMaterial(context));
    assert(debugCheckHasMaterialLocalizations(context));
    final Orientation newOrientation = _getOrientation(context);
    _lastOrientation ??= newOrientation;
    if (newOrientation != _lastOrientation) {
      _removeDropdownRoute();
      _lastOrientation = newOrientation;
    }

    // The width of the button and the menu are defined by the widest
    // item and the width of the hint.
    // We should explicitly type the items list to be a list of <Widget>,
    // otherwise, no explicit type adding items maybe trigger a crash/failure
    // when hint and selectedItemBuilder are provided.
    final List<Widget> items = widget.selectedItemBuilder == null
        ? (widget.items != null ? List<Widget>.from(widget.items!) : <Widget>[])
        : List<Widget>.from(widget.selectedItemBuilder!(context));

    int? hintIndex;
    if (widget.hint != null || (!_enabled && widget.disabledHint != null)) {
      Widget displayedHint =
          _enabled ? widget.hint! : widget.disabledHint ?? widget.hint!;
      if (widget.selectedItemBuilder == null)
        displayedHint = _DropdownMenuItemContainer(child: displayedHint);

      hintIndex = items.length;
      items.add(DefaultTextStyle(
        style: _textStyle!.copyWith(color: Theme.of(context).hintColor),
        child: IgnorePointer(
          ignoringSemantics: false,
          child: displayedHint,
        ),
      ));
    }

    final EdgeInsetsGeometry padding = ButtonTheme.of(context).alignedDropdown
        ? _kAlignedButtonPadding
        : _kUnalignedButtonPadding;

    // If value is null (then _selectedIndex is null) then we
    // display the hint or nothing at all.
    final Widget innerItemsWidget;
    if (items.isEmpty) {
      innerItemsWidget = Container();
    } else {
      innerItemsWidget = IndexedStack(
        index: _selectedIndex ?? hintIndex,
        alignment: AlignmentDirectional.centerStart,
        children: widget.isDense
            ? items
            : items.map((Widget item) {
                return widget.itemHeight != null
                    ? SizedBox(height: widget.itemHeight, child: item)
                    : Column(
                        mainAxisSize: MainAxisSize.min,
                        children: <Widget>[item]);
              }).toList(),
      );
    }

    const Icon defaultIcon = Icon(Icons.arrow_drop_down);

    Widget result = DefaultTextStyle(
      style: _enabled
          ? _textStyle!
          : _textStyle!.copyWith(color: Theme.of(context).disabledColor),
      child: Container(
        decoration: _showHighlight
            ? BoxDecoration(
                color: widget.focusColor ?? Theme.of(context).focusColor,
                borderRadius: const BorderRadius.all(Radius.circular(4.0)),
              )
            : null,
        padding: padding.resolve(Directionality.of(context)),
        height: widget.isDense ? _denseButtonHeight : null,
        child: Row(
          mainAxisAlignment: MainAxisAlignment.spaceBetween,
          mainAxisSize: MainAxisSize.min,
          children: <Widget>[
            if (widget.isExpanded)
              Expanded(child: innerItemsWidget)
            else
              innerItemsWidget,
            IconTheme(
              data: IconThemeData(
                color: _iconColor,
                size: widget.iconSize,
              ),
              child: widget.icon ?? defaultIcon,
            ),
          ],
        ),
      ),
    );

    if (!DropdownButtonHideUnderline.at(context)) {
      final double bottom =
          (widget.isDense || widget.itemHeight == null) ? 0.0 : 8.0;
      result = Stack(
        children: <Widget>[
          result,
          Positioned(
            left: 0.0,
            right: 0.0,
            bottom: bottom,
            child: widget.underline ??
                Container(
                  height: 1.0,
                  decoration: const BoxDecoration(
                    border: Border(
                      bottom: BorderSide(
                        color: Color(0xFFBDBDBD),
                        width: 0.0,
                      ),
                    ),
                  ),
                ),
          ),
        ],
      );
    }

    return Semantics(
      button: true,
      child: Actions(
        actions: _actionMap,
        child: Focus(
          canRequestFocus: _enabled,
          focusNode: focusNode,
          autofocus: widget.autofocus,
          child: GestureDetector(
            onTap: _enabled ? _handleTap : null,
            behavior: HitTestBehavior.opaque,
            child: result,
          ),
        ),
      ),
    );
  }
}

/// A convenience widget that makes a [CustomDropdownButton] into a [FormField].
class CustomDropdownButtonFormField<T> extends FormField<T> {
  /// Creates a [CustomDropdownButton] widget that is a [FormField], wrapped in an
  /// [InputDecorator].
  ///
  /// For a description of the `onSaved`, `validator`, or `autovalidateMode`
  /// parameters, see [FormField]. For the rest (other than [decoration]), see
  /// [CustomDropdownButton].
  ///
  /// The `items`, `elevation`, `iconSize`, `isDense`, `isExpanded`,
  /// `autofocus`, and `decoration`  parameters must not be null.
  CustomDropdownButtonFormField({
    Key? key,
    required List<DropdownMenuItem<T>>? items,
    DropdownButtonBuilder? selectedItemBuilder,
    T? value,
    Widget? hint,
    Widget? disabledHint,
    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,
    FormFieldSetter<T>? onSaved,
    FormFieldValidator<T>? validator,
    @Deprecated(
      'Use autovalidateMode parameter which provide more specific '
      'behaviour related to auto validation. '
      'This feature was deprecated after v1.19.0.',
    )
        bool autovalidate = false,
    AutovalidateMode? autovalidateMode,
    double? menuMaxHeight,
  })  : 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),
        assert(autovalidate != null),
        assert(
          autovalidate == false ||
              autovalidate == true && autovalidateMode == null,
          'autovalidate and autovalidateMode should not be used together.',
        ),
        decoration = decoration ?? InputDecoration(focusColor: focusColor),
        super(
          key: key,
          onSaved: onSaved,
          initialValue: value,
          validator: validator,
          autovalidateMode: autovalidate
              ? AutovalidateMode.always
              : (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,
            );
            // 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 InputDecorator(
                  decoration:
                      effectiveDecoration.copyWith(errorText: field.errorText),
                  isEmpty: state.value == null,
                  isFocused: Focus.of(context).hasFocus,
                  child: DropdownButtonHideUnderline(
                    child: CustomDropdownButton<T>(
                      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,
                    ),
                  ),
                );
              }),
            );
          },
        );

  /// {@macro flutter.material.dropdownButton.onChanged}
  final ValueChanged<T?>? onChanged;

  /// The decoration to show around the dropdown button form field.
  ///
  /// By default, draws a horizontal line under the dropdown button field but
  /// can be configured to show an icon, label, hint text, and error text.
  ///
  /// If not specified, an [InputDecorator] with the `focusColor` set to the
  /// supplied `focusColor` (if any) will be used.
  final InputDecoration decoration;

  @override
  FormFieldState<T> createState() => _DropdownButtonFormFieldState<T>();
}

class _DropdownButtonFormFieldState<T> extends FormFieldState<T> {
  @override
  CustomDropdownButtonFormField<T> get widget =>
      super.widget as CustomDropdownButtonFormField<T>;

  @override
  void didChange(T? value) {
    super.didChange(value);
    assert(widget.onChanged != null);
    widget.onChanged!(value);
  }

  @override
  void didUpdateWidget(CustomDropdownButtonFormField<T> oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.initialValue != widget.initialValue) {
      setValue(widget.initialValue);
    }
  }
}

sample_component.dart

/// sample:
MyAppDropdown(
  options: [
    'option 1',
    'option 2',
    'option 3',
    'option 4'
  ],
  onChanged: (value) {
    print('selected option: $value');
  },
)

/// widget:
class MyAppDropdown extends StatelessWidget {
  final List<String> options;
  final ValueChanged<String?>? onChanged;
  final String? selected;
  final String? label;

  const MyAppDropdown({
    Key? key,
    this.label = "Select a topic",
    required this.options,
    required this.onChanged,
    this.selected,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final items = options
        .map(
          (e) => DropdownMenuItem(
              child: Text(
                e,
                style: const TextStyle(
                  color: Color(0xffaeaeae),
                  fontSize: 14,
                  fontWeight: FontWeight.w400,
                  letterSpacing: 0.252,
                ),
              ),
              value: e),
        )
        .toList(growable: false);
    return Container(
      height: 60,
      padding: const EdgeInsets.symmetric(horizontal: 12.0, vertical: 6.0),
      decoration: BoxDecoration(
        color: const Color(0xff282835),
        border: Border.all(color: const Color(0xFF393A4B)),
        borderRadius: kBorderRadius8,
      ),
      child: CustomDropdownButton<String>(
        isExpanded: true,
        menuRadius: BorderRadius.circular(8),
        menuDecoration: BoxDecoration(
            color: Color(0xff282835),
            gradient:
                LinearGradient(colors: [Color(0xff282835), Colors.redAccent]),
            border: Border.all(color: Color(0xff393A4B), width: 2),
            boxShadow: [
              BoxShadow(
                color: Colors.redAccent.withOpacity(1),
                blurRadius: 24,
              ),
            ]),
        menuPadding: EdgeInsets.zero,
        menuClip: Clip.antiAlias,
        hint: Text(
          label ?? "Select a topic",
          style: const TextStyle(
            color: Color(0xffaeaeae),
            fontSize: 14,
            fontWeight: FontWeight.w400,
            letterSpacing: 0.252,
          ),
        ),
        underline: const SizedBox.shrink(),
        // dropdownColor: Color(0xff282835),
        items: items,
        value: selected,
        elevation: 4,
        onChanged: onChanged,
      ),
    );
  }
}