my_dropdown.dart

// 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/rendering.dart';
import 'package:flutter/services.dart';
import 'package:flutter/widgets.dart';

import 'package:flutter/material.dart'
    hide DropdownButton, DropdownMenuItem, DropdownButtonHideUnderline;

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);

typedef DropdownButtonBuilder = List<Widget> Function(BuildContext context);

class _DropdownMenuPainter extends CustomPainter {
  _DropdownMenuPainter({
    this.color,
    this.elevation,
    this.selectedIndex,
    this.resize,
    this.getSelectedItemOffset,
  })  : _painter = 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.
          color: color,
          borderRadius: BorderRadius.circular(15.0),
          boxShadow: kElevationToShadow[elevation],
        ).createBoxPainter(),
        super(repaint: resize);

  final Color color;
  final int elevation;
  final int selectedIndex;
  final Animation<double> resize;
  final ValueGetter<double> getSelectedItemOffset;
  final BoxPainter _painter;

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

    final Tween<double> bottom = Tween<double>(
      begin: (top.begin + _kMenuItemHeight).clamp(_kMenuItemHeight, size.height)
          as double,
      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.elevation != elevation ||
        oldPainter.selectedIndex != selectedIndex ||
        oldPainter.resize != resize;
  }
}

// Do not use the platform-specific default scroll configuration.
// Dropdown menus should never overscroll or display an overscroll indicator.
class _DropdownScrollBehavior extends ScrollBehavior {
  const _DropdownScrollBehavior();

  @override
  TargetPlatform getPlatform(BuildContext context) =>
      Theme.of(context).platform;

  @override
  Widget buildViewportChrome(
          BuildContext context, Widget child, AxisDirection axisDirection) =>
      child;

  @override
  ScrollPhysics getScrollPhysics(BuildContext context) =>
      const ClampingScrollPhysics();
}

// The widget that is the button wrapping the menu items.
class _DropdownMenuItemButton<T> extends StatefulWidget {
  const _DropdownMenuItemButton({
    Key key,
    @required 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) {
    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;

    if (dropdownMenuItem.onTap != null) {
      dropdownMenuItem.onTap();
    }

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

  static final Map<LogicalKeySet, Intent> _webShortcuts =
      <LogicalKeySet, Intent>{
    LogicalKeySet(LogicalKeyboardKey.enter): const ActivateIntent(),
  };

  @override
  Widget build(BuildContext context) {
    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) as double;
      final double end = (start + 1.5 * unit).clamp(0.0, 1.0) as double;
      opacity = CurvedAnimation(
          parent: widget.route.animation, curve: Interval(start, end));
    }
    Widget child = FadeTransition(
      opacity: opacity,
      child: InkWell(
        autofocus: widget.itemIndex == widget.route.selectedIndex,
        child: Container(
          padding: widget.padding,
          child: widget.route.items[widget.itemIndex],
        ),
        onTap: _handleOnTap,
        onFocusChange: _handleFocusChange,
      ),
    );
    if (kIsWeb) {
      // On the web, enter doesn't select things, *except* in a <select>
      // element, which is what a dropdown emulates.
      child = Shortcuts(
        shortcuts: _webShortcuts,
        child: child,
      );
    }
    return child;
  }
}

class _DropdownMenu<T> extends StatefulWidget {
  const _DropdownMenu({
    Key key,
    this.padding,
    this.route,
    this.buttonRect,
    this.constraints,
    this.dropdownColor,
  }) : super(key: key);

  final _DropdownRoute<T> route;
  final EdgeInsets padding;
  final Rect buttonRect;
  final BoxConstraints constraints;
  final Color dropdownColor;

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

class _DropdownMenuState<T> extends State<_DropdownMenu<T>> {
  CurvedAnimation _fadeOpacity;
  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(
          color: widget.dropdownColor ?? Theme.of(context).canvasColor,
          elevation: route.elevation,
          selectedIndex: route.selectedIndex,
          resize: _resize,
          // 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,
            child: ScrollConfiguration(
              behavior: const _DropdownScrollBehavior(),
              child: Scrollbar(
                child: ListView(
                  controller: widget.route.scrollController,
                  padding: 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
    final double maxHeight =
        math.max(0.0, constraints.maxHeight - 2 * _kMenuItemHeight);
    // 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);
    double left;
    switch (textDirection) {
      case TextDirection.rtl:
        left = (buttonRect.right.clamp(0.0, size.width) as double) -
            childSize.width;
        break;
      case TextDirection.ltr:
        left =
            buttonRect.left.clamp(0.0, size.width - childSize.width) as double;
        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({
    this.items,
    this.padding,
    this.buttonRect,
    this.selectedIndex,
    this.elevation = 8,
    this.theme,
    @required this.style,
    this.barrierLabel,
    this.itemHeight,
    this.dropdownColor,
  })  : 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 ThemeData theme;
  final TextStyle style;
  final double itemHeight;
  final Color dropdownColor;

  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,
        padding: padding,
        buttonRect: buttonRect,
        selectedIndex: selectedIndex,
        elevation: elevation,
        theme: theme,
        style: style,
        dropdownColor: dropdownColor,
      );
    });
  }

  void _dismiss() {
    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;
    }

    // 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).
    final double scrollOffset = preferredMenuHeight <= maxMenuHeight
        ? 0
        : math.max(0.0, selectedItemOffset - (buttonTop - menuTop));

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

class _DropdownRoutePage<T> extends StatelessWidget {
  const _DropdownRoutePage({
    Key key,
    this.route,
    this.constraints,
    this.items,
    this.padding,
    this.buttonRect,
    this.selectedIndex,
    this.elevation = 8,
    this.theme,
    this.style,
    this.dropdownColor,
  }) : super(key: key);

  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 ThemeData theme;
  final TextStyle style;
  final Color dropdownColor;

  @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.of(context);
    Widget menu = _DropdownMenu<T>(
      route: route,
      padding: padding.resolve(textDirection),
      buttonRect: buttonRect,
      constraints: constraints,
      dropdownColor: dropdownColor,
    );

    if (theme != null) menu = Theme(data: theme, child: menu);

    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: 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,
    );
  }
}

/// An item in a menu created by a [DropdownButton].
///
/// The type `T` is the type of the value the entry represents. All the entries
/// in a given menu must represent values with consistent types.
class DropdownMenuItem<T> extends _DropdownMenuItemContainer {
  /// Creates an item for a dropdown menu.
  ///
  /// The [child] argument is required.
  const DropdownMenuItem({
    Key key,
    this.onTap,
    this.value,
    @required Widget child,
  })  : assert(child != null),
        super(key: key, child: child);

  /// Called when the dropdown menu item is tapped.
  final VoidCallback onTap;

  /// The value to return if the user selects this menu item.
  ///
  /// Eventually returned in a call to [DropdownButton.onChanged].
  final T value;
}

/// An inherited widget that causes any descendant [DropdownButton]
/// widgets to not include their regular underline.
///
/// This is used by [DataTable] to remove the underline from any
/// [DropdownButton] widgets placed within material data tables, as
/// required by the material design specification.
class DropdownButtonHideUnderline extends InheritedWidget {
  /// Creates a [DropdownButtonHideUnderline]. A non-null [child] must
  /// be given.
  const DropdownButtonHideUnderline({
    Key key,
    @required Widget child,
  })  : assert(child != null),
        super(key: key, child: child);

  /// Returns whether the underline of [DropdownButton] widgets should
  /// be hidden.
  static bool at(BuildContext context) {
    return context.dependOnInheritedWidgetOfExactType<
            DropdownButtonHideUnderline>() !=
        null;
  }

  @override
  bool updateShouldNotify(DropdownButtonHideUnderline oldWidget) => false;
}

/// 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: Icon(Icons.arrow_downward),
///     iconSize: 24,
///     elevation: 16,
///     style: 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.
///  * [RaisedButton], [FlatButton], ordinary buttons that trigger a single action.
///  * <https://material.io/design/components/menus.html#dropdown-menu>
class DropdownButton<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, and the [disabledHint] will be shown (if provided).
  /// If [disabledHint] is null and [hint] is non-null, [hint] will instead be
  /// shown.
  ///
  /// 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 [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.
  DropdownButton({
    Key key,
    @required this.items,
    this.selectedItemBuilder,
    this.value,
    this.hint,
    this.disabledHint,
    @required 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,
  })  : 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. A disabled button
  /// will display the [disabledHint] widget if it is non-null. If
  /// [disabledHint] is also null but [hint] is non-null, [hint] will instead
  /// be displayed.
  final List<DropdownMenuItem<T>> items;

  /// The value of the currently selected [DropdownMenuItem].
  ///
  /// If [value] is null and [hint] is non-null, the [hint] widget is
  /// displayed as a placeholder for the dropdown button's value.
  final T value;

  /// A placeholder widget that is displayed by the dropdown button.
  ///
  /// If [value] is null, this widget is displayed as a placeholder for
  /// the dropdown button's value. This widget is also displayed if the button
  /// is disabled ([items] or [onChanged] is null) and [disabledHint] is null.
  final Widget hint;

  /// A message to show when the dropdown is disabled.
  ///
  /// Displayed if [items] or [onChanged] is null. If [hint] is non-null and
  /// [disabledHint] is null, the [hint] widget will be displayed instead.
  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 [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. If
  /// [disabledHint] is also null but [hint] is non-null, [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;

  /// 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: TextStyle(color: Colors.blue),
  ///       selectedItemBuilder: (BuildContext context) {
  ///         return options.map((String value) {
  ///           return Text(
  ///             dropdownValue,
  ///             style: 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 [Colors.grey.shade400] 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 [Colors.grey.shade700] 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;

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

class _DropdownButtonState<T> extends State<DropdownButton<T>>
    with WidgetsBindingObserver {
  int _selectedIndex;
  _DropdownRoute<T> _dropdownRoute;
  Orientation _lastOrientation;
  FocusNode _internalNode;
  FocusNode get focusNode => widget.focusNode ?? _internalNode;
  bool _hasPrimaryFocus = false;
  Map<Type, Action<Intent>> _actionMap;
  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(),
      ),
    };
    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(DropdownButton<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 (!_enabled) {
      return;
    }

    assert(widget.value == null ||
        widget.items
                .where((DropdownMenuItem<T> item) => item.value == widget.value)
                .length ==
            1);
    _selectedIndex = null;
    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 RenderBox itemBox = context.findRenderObject() as RenderBox;
    final Rect itemRect = itemBox.localToGlobal(Offset.zero) & itemBox.size;
    final TextDirection textDirection = Directionality.of(context);
    final EdgeInsetsGeometry menuMargin =
        ButtonTheme.of(context).alignedDropdown
            ? _kAlignedMenuMargin
            : _kUnalignedMenuMargin;

    final List<_MenuItem<T>> menuItems =
        List<_MenuItem<T>>(widget.items.length);
    for (int index = 0; index < widget.items.length; index += 1) {
      menuItems[index] = _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;
        },
      );
    }

    assert(_dropdownRoute == null);
    _dropdownRoute = _DropdownRoute<T>(
      items: menuItems,
      buttonRect: menuMargin.resolve(textDirection).inflateRect(itemRect),
      padding: _kMenuItemPadding.resolve(textDirection),
      selectedIndex: _selectedIndex ?? 0,
      elevation: widget.elevation,
      theme: Theme.of(context, shadowThemeOnly: true),
      style: _textStyle,
      barrierLabel: MaterialLocalizations.of(context).modalBarrierDismissLabel,
      itemHeight: widget.itemHeight,
      dropdownColor: widget.dropdownColor,
    );

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

    if (widget.onTap != null) {
      widget.onTap();
    }
  }

  // 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;
      }
    }

    assert(false);
    return null;
  }

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

  Orientation _getOrientation(BuildContext context) {
    Orientation result = MediaQuery.of(context, nullOk: true)?.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;
    }
    return null;
  }

  @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.
    List<Widget> items;
    if (_enabled) {
      items = widget.selectedItemBuilder == null
          ? List<Widget>.from(widget.items)
          : widget.selectedItemBuilder(context);
    } else {
      items = widget.selectedItemBuilder == null
          ? <Widget>[]
          : 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) or if disabled then we
    // display the hint or nothing at all.
    final int index = _enabled ? (_selectedIndex ?? hintIndex) : hintIndex;
    Widget innerItemsWidget;
    if (items.isEmpty) {
      innerItemsWidget = Container();
    } else {
      innerItemsWidget = IndexedStack(
        index: index,
        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: _textStyle,
      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 wraps a [DropdownButton] in a [FormField].
class DropdownButtonFormField<T> extends FormField<T> {
  /// Creates a [DropdownButton] widget wrapped in an [InputDecorator] and
  /// [FormField].
  ///
  /// The [DropdownButton] [items] parameters must not be null.
  DropdownButtonFormField({
    Key key,
    T value,
    @required List<DropdownMenuItem<T>> items,
    DropdownButtonBuilder selectedItemBuilder,
    Widget hint,
    @required this.onChanged,
    VoidCallback onTap,
    this.decoration = const InputDecoration(),
    FormFieldSetter<T> onSaved,
    FormFieldValidator<T> validator,
    bool autovalidate = false,
    Widget disabledHint,
    int elevation = 8,
    TextStyle style,
    Widget icon,
    Color iconDisabledColor,
    Color iconEnabledColor,
    double iconSize = 24.0,
    bool isDense = true,
    bool isExpanded = false,
    double itemHeight,
  })  : 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(decoration != null),
        assert(elevation != null),
        assert(iconSize != null),
        assert(isDense != null),
        assert(isExpanded != null),
        assert(itemHeight == null || itemHeight > 0),
        super(
          key: key,
          onSaved: onSaved,
          initialValue: value,
          validator: validator,
          autovalidate: autovalidate,
          builder: (FormFieldState<T> field) {
            final _DropdownButtonFormFieldState<T> state =
                field as _DropdownButtonFormFieldState<T>;
            final InputDecoration effectiveDecoration =
                decoration.applyDefaults(
              Theme.of(field.context).inputDecorationTheme,
            );
            return InputDecorator(
              decoration:
                  effectiveDecoration.copyWith(errorText: field.errorText),
              isEmpty: state.value == null,
              child: DropdownButtonHideUnderline(
                child: DropdownButton<T>(
                  value: state.value,
                  items: items,
                  selectedItemBuilder: selectedItemBuilder,
                  hint: hint,
                  onChanged: onChanged == null ? null : state.didChange,
                  onTap: onTap,
                  disabledHint: disabledHint,
                  elevation: elevation,
                  style: style,
                  icon: icon,
                  iconDisabledColor: iconDisabledColor,
                  iconEnabledColor: iconEnabledColor,
                  iconSize: iconSize,
                  isDense: isDense,
                  isExpanded: isExpanded,
                  itemHeight: itemHeight,
                ),
              ),
            );
          },
        );

  /// {@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.
  ///
  /// Specify null to remove the decoration entirely (including the
  /// extra padding introduced by the decoration to save space for the labels).
  final InputDecoration decoration;

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

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

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

usage.dart

import 'package:flutter/material.dart'
    hide DropdownButton, DropdownMenuItem, DropdownButtonHideUnderline;
import 'my_dropdown.dart';

void main() => runApp(App());

class App extends StatefulWidget {
  const App({Key key}) : super(key: key);

  @override
  _AppState createState() => _AppState();
}

class _AppState extends State<App> {
  String selectedValue;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: Container(
            width: 200,
            child: DropdownButtonHideUnderline(
              child: DropdownButton<String>(
                style: TextStyle(
                  fontWeight: FontWeight.w600,
                  fontSize: 18,
                  color: Colors.black,
                ),
                isExpanded: true,
                value: selectedValue, //var that
                items: [
                  DropdownMenuItem(child: Text('Apple')),
                  DropdownMenuItem(child: Text('Google')),
                  DropdownMenuItem(child: Text('Samsung')),
                  DropdownMenuItem(child: Text('Sony')),
                  DropdownMenuItem(child: Text('LG')),
                ], //simple function that holds the values of all the items
                onChanged: (String value) {
                  setState(() {
                    selectedValue = value;
                  });
                },
              ),
            ),
          ),
        ),
      ),
    );
  }
}