samples/material_3_demo/lib/component_screen.dart

import 'package:flutter/material.dart';

class ComponentScreen extends StatelessWidget {
  const ComponentScreen({Key? key, required this.showNavBottomBar})
      : super(key: key);

  final bool showNavBottomBar;

  @override
  Widget build(BuildContext context) {
    return Expanded(
      child: Padding(
        padding: const EdgeInsets.symmetric(horizontal: 10),
        child: Align(
          alignment: Alignment.topCenter,
          child: SizedBox(
            width: _maxWidthConstraint,
            child: ListView(
              shrinkWrap: true,
              children: [
                _colDivider,
                _colDivider,
                const Buttons(),
                _colDivider,
                const FloatingActionButtons(),
                _colDivider,
                const Cards(),
                _colDivider,
                const Dialogs(),
                _colDivider,
                showNavBottomBar
                    ? const NavigationBars(
                        selectedIndex: 0,
                        isExampleBar: true,
                      )
                    : Container(),
              ],
            ),
          ),
        ),
      ),
    );
  }
}

const _rowDivider = SizedBox(width: 10);
const _colDivider = SizedBox(height: 10);
const double _cardWidth = 115;
const double _maxWidthConstraint = 400;

void Function()? handlePressed(
    BuildContext context, bool isDisabled, String buttonName) {
  return isDisabled
      ? null
      : () {
          final snackBar = SnackBar(
            content: Text(
              'Yay! $buttonName is clicked!',
              style: TextStyle(color: Theme.of(context).colorScheme.surface),
            ),
            action: SnackBarAction(
              textColor: Theme.of(context).colorScheme.surface,
              label: 'Close',
              onPressed: () {},
            ),
          );

          ScaffoldMessenger.of(context).showSnackBar(snackBar);
        };
}

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

  @override
  State<Buttons> createState() => _ButtonsState();
}

class _ButtonsState extends State<Buttons> {
  @override
  Widget build(BuildContext context) {
    return Wrap(
      alignment: WrapAlignment.spaceEvenly,
      children: const <Widget>[
        ButtonsWithoutIcon(isDisabled: false),
        _rowDivider,
        ButtonsWithIcon(),
        _rowDivider,
        ButtonsWithoutIcon(isDisabled: true),
      ],
    );
  }
}

class ButtonsWithoutIcon extends StatelessWidget {
  final bool isDisabled;

  const ButtonsWithoutIcon({Key? key, required this.isDisabled})
      : super(key: key);

  @override
  Widget build(BuildContext context) {
    return IntrinsicWidth(
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: <Widget>[
          ElevatedButton(
            onPressed: handlePressed(context, isDisabled, "ElevatedButton"),
            child: const Text("Elevated"),
          ),
          _colDivider,
          ElevatedButton(
            style: ElevatedButton.styleFrom(
              // Foreground color
              onPrimary: Theme.of(context).colorScheme.onPrimary,
              // Background color
              primary: Theme.of(context).colorScheme.primary,
            ).copyWith(elevation: ButtonStyleButton.allOrNull(0.0)),
            onPressed: handlePressed(context, isDisabled, "FilledButton"),
            child: const Text('Filled'),
          ),
          _colDivider,
          ElevatedButton(
            style: ElevatedButton.styleFrom(
              // Foreground color
              onPrimary: Theme.of(context).colorScheme.onSecondaryContainer,
              // Background color
              primary: Theme.of(context).colorScheme.secondaryContainer,
            ).copyWith(elevation: ButtonStyleButton.allOrNull(0.0)),
            onPressed: handlePressed(context, isDisabled, "FilledTonalButton"),
            child: const Text('Filled Tonal'),
          ),
          _colDivider,
          OutlinedButton(
            onPressed: handlePressed(context, isDisabled, "OutlinedButton"),
            child: const Text("Outlined"),
          ),
          _colDivider,
          TextButton(
              onPressed: handlePressed(context, isDisabled, "TextButton"),
              child: const Text("Text")),
        ],
      ),
    );
  }
}

class ButtonsWithIcon extends StatelessWidget {
  const ButtonsWithIcon({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return IntrinsicWidth(
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: <Widget>[
          ElevatedButton.icon(
            onPressed:
                handlePressed(context, false, "ElevatedButton with Icon"),
            icon: const Icon(Icons.add),
            label: const Text("Icon"),
          ),
          _colDivider,
          ElevatedButton.icon(
            style: ElevatedButton.styleFrom(
              // Foreground color
              onPrimary: Theme.of(context).colorScheme.onPrimary,
              // Background color
              primary: Theme.of(context).colorScheme.primary,
            ).copyWith(elevation: ButtonStyleButton.allOrNull(0.0)),
            onPressed: handlePressed(context, false, "FilledButton with Icon"),
            label: const Text('Icon'),
            icon: const Icon(Icons.add),
          ),
          _colDivider,
          ElevatedButton.icon(
            style: ElevatedButton.styleFrom(
              // Foreground color
              onPrimary: Theme.of(context).colorScheme.onSecondaryContainer,
              // Background color
              primary: Theme.of(context).colorScheme.secondaryContainer,
            ).copyWith(elevation: ButtonStyleButton.allOrNull(0.0)),
            onPressed:
                handlePressed(context, false, "FilledTonalButton with Icon"),
            label: const Text('Icon'),
            icon: const Icon(Icons.add),
          ),
          _colDivider,
          OutlinedButton.icon(
            onPressed:
                handlePressed(context, false, "OutlinedButton with Icon"),
            icon: const Icon(Icons.add),
            label: const Text("Icon"),
          ),
          _colDivider,
          TextButton.icon(
            onPressed: handlePressed(context, false, "TextButton with Icon"),
            icon: const Icon(Icons.add),
            label: const Text("Icon"),
          )
        ],
      ),
    );
  }
}

class FloatingActionButtons extends StatelessWidget {
  const FloatingActionButtons({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.symmetric(vertical: 10),
      child: Wrap(
        alignment: WrapAlignment.spaceEvenly,
        crossAxisAlignment: WrapCrossAlignment.center,
        children: [
          FloatingActionButton.small(
            onPressed: () {},
            child: const Icon(Icons.add),
          ),
          _rowDivider,
          FloatingActionButton(
            onPressed: () {},
            child: const Icon(Icons.add),
          ),
          _rowDivider,
          FloatingActionButton.extended(
            onPressed: () {},
            icon: const Icon(Icons.add),
            label: const Text("Create"),
          ),
          _rowDivider,
          FloatingActionButton.large(
            onPressed: () {},
            child: const Icon(Icons.add),
          ),
        ],
      ),
    );
  }
}

class Cards extends StatelessWidget {
  const Cards({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.symmetric(vertical: 10),
      child: Wrap(
        alignment: WrapAlignment.spaceEvenly,
        children: [
          SizedBox(
            width: _cardWidth,
            child: Card(
              child: Container(
                padding: const EdgeInsets.all(10),
                child: Column(
                  children: const [
                    Align(
                      alignment: Alignment.topRight,
                      child: Icon(Icons.more_vert),
                    ),
                    SizedBox(height: 35),
                    Align(
                      alignment: Alignment.bottomLeft,
                      child: Text("Elevated"),
                    )
                  ],
                ),
              ),
            ),
          ),
          SizedBox(
            width: _cardWidth,
            child: Card(
              color: Theme.of(context).colorScheme.surfaceVariant,
              elevation: 0,
              child: Container(
                padding: const EdgeInsets.all(10),
                child: Column(
                  children: const [
                    Align(
                      alignment: Alignment.topRight,
                      child: Icon(Icons.more_vert),
                    ),
                    SizedBox(height: 35),
                    Align(
                      alignment: Alignment.bottomLeft,
                      child: Text("Filled"),
                    )
                  ],
                ),
              ),
            ),
          ),
          SizedBox(
            width: _cardWidth,
            child: Card(
              elevation: 0,
              shape: RoundedRectangleBorder(
                side: BorderSide(
                  color: Theme.of(context).colorScheme.outline,
                ),
                borderRadius: const BorderRadius.all(Radius.circular(12)),
              ),
              child: Container(
                padding: const EdgeInsets.all(10),
                child: Column(
                  children: const [
                    Align(
                      alignment: Alignment.topRight,
                      child: Icon(Icons.more_vert),
                    ),
                    SizedBox(height: 35),
                    Align(
                      alignment: Alignment.bottomLeft,
                      child: Text("Outlined"),
                    )
                  ],
                ),
              ),
            ),
          ),
        ],
      ),
    );
  }
}

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

  @override
  State<Dialogs> createState() => _DialogsState();
}

class _DialogsState extends State<Dialogs> {
  void openDialog(BuildContext context) {
    showDialog<void>(
      context: context,
      builder: (context) => AlertDialog(
        title: const Text("Basic Dialog Title"),
        content: const Text(
            "A dialog is a type of modal window that appears in front of app content to provide critical information, or prompt for a decision to be made."),
        actions: <Widget>[
          TextButton(
            child: const Text('Dismiss'),
            onPressed: () => Navigator.of(context).pop(),
          ),
          TextButton(
            child: const Text('Action'),
            onPressed: () => Navigator.of(context).pop(),
          ),
        ],
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.symmetric(vertical: 10),
      child: TextButton(
        child: const Text(
          "Open Dialog",
          style: TextStyle(fontWeight: FontWeight.bold),
        ),
        onPressed: () => openDialog(context),
      ),
    );
  }
}

const List<NavigationDestination> appBarDestinations = [
  NavigationDestination(
    tooltip: "",
    icon: Icon(Icons.widgets_outlined),
    label: 'Components',
    selectedIcon: Icon(Icons.widgets),
  ),
  NavigationDestination(
    tooltip: "",
    icon: Icon(Icons.format_paint_outlined),
    label: 'Color',
    selectedIcon: Icon(Icons.format_paint),
  ),
  NavigationDestination(
    tooltip: "",
    icon: Icon(Icons.text_snippet_outlined),
    label: 'Typography',
    selectedIcon: Icon(Icons.text_snippet),
  ),
  NavigationDestination(
    tooltip: "",
    icon: Icon(Icons.invert_colors_on_outlined),
    label: 'Elevation',
    selectedIcon: Icon(Icons.opacity),
  )
];

final List<NavigationRailDestination> navRailDestinations = appBarDestinations
    .map(
      (destination) => NavigationRailDestination(
        icon: Tooltip(
          message: destination.label,
          child: destination.icon,
        ),
        selectedIcon: Tooltip(
          message: destination.label,
          child: destination.selectedIcon,
        ),
        label: Text(destination.label),
      ),
    )
    .toList();

const List<Widget> exampleBarDestinations = [
  NavigationDestination(
    tooltip: "",
    icon: Icon(Icons.explore_outlined),
    label: 'Explore',
    selectedIcon: Icon(Icons.explore),
  ),
  NavigationDestination(
    tooltip: "",
    icon: Icon(Icons.pets_outlined),
    label: 'Pets',
    selectedIcon: Icon(Icons.pets),
  ),
  NavigationDestination(
    tooltip: "",
    icon: Icon(Icons.account_box_outlined),
    label: 'Account',
    selectedIcon: Icon(Icons.account_box),
  )
];

class NavigationBars extends StatefulWidget {
  final void Function(int)? onSelectItem;
  final int selectedIndex;
  final bool isExampleBar;

  const NavigationBars(
      {Key? key,
      this.onSelectItem,
      required this.selectedIndex,
      required this.isExampleBar})
      : super(key: key);

  @override
  State<NavigationBars> createState() => _NavigationBarsState();
}

class _NavigationBarsState extends State<NavigationBars> {
  int _selectedIndex = 0;

  @override
  void initState() {
    super.initState();
    _selectedIndex = widget.selectedIndex;
  }

  @override
  Widget build(BuildContext context) {
    return NavigationBar(
      selectedIndex: _selectedIndex,
      onDestinationSelected: (index) {
        setState(() {
          _selectedIndex = index;
        });
        if (!widget.isExampleBar) widget.onSelectItem!(index);
      },
      destinations:
          widget.isExampleBar ? exampleBarDestinations : appBarDestinations,
    );
  }
}

class NavigationRailSection extends StatefulWidget {
  final void Function(int) onSelectItem;
  final int selectedIndex;

  const NavigationRailSection(
      {Key? key, required this.onSelectItem, required this.selectedIndex})
      : super(key: key);

  @override
  State<NavigationRailSection> createState() => _NavigationRailSectionState();
}

class _NavigationRailSectionState extends State<NavigationRailSection> {
  int _selectedIndex = 0;

  @override
  void initState() {
    super.initState();
    _selectedIndex = widget.selectedIndex;
  }

  @override
  Widget build(BuildContext context) {
    return NavigationRail(
      minWidth: 50,
      destinations: navRailDestinations,
      selectedIndex: _selectedIndex,
      useIndicator: true,
      onDestinationSelected: (index) {
        setState(() {
          _selectedIndex = index;
        });
        widget.onSelectItem(index);
      },
    );
  }
}