Flutter Adaptive Ui

import 'package:flutter/material.dart';

/// Example of Capability and Policy classes
/// for handling platform-specific behavior.
class CapabilityPolicyExample extends StatelessWidget {
  const CapabilityPolicyExample({
    super.key,
    this.capability = const Capability(),
    this.policy = const Policy(),
  });

  final Capability capability;
  final Policy policy;

  @override
  Widget build(BuildContext context) {
    final canOpenExternalPurchase = capability.canOpenExternalPurchase();
    final shouldShowExternalPurchase = policy.shouldShowExternalPurchase();

    return Scaffold(
      appBar: AppBar(title: const Text('Capability & Policy Example')),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            if (canOpenExternalPurchase && shouldShowExternalPurchase)
              ElevatedButton(
                onPressed: capability.openExternalPurchase,
                child: const Text('Buy in Browser'),
              )
            else
              const Text('Purchase not available on this platform'),
          ],
        ),
      ),
    );
  }
}

/// Capability class - defines what the code CAN do
class Capability {
  const Capability();

  /// Check whether the app has an implementation for opening purchases.
  bool canOpenExternalPurchase() {
    return true;
  }

  /// Open purchase flow using the target app's URL launcher or service.
  void openExternalPurchase() {
    debugPrint('Opening purchase flow');
  }
}

/// Policy class - defines what the code SHOULD do
class Policy {
  const Policy({this.externalPurchaseAllowed = true});

  final bool externalPurchaseAllowed;

  /// Policy: decide whether the external purchase entry point is allowed.
  bool shouldShowExternalPurchase() {
    return externalPurchaseAllowed;
  }
}


import 'package:flutter/material.dart';

/// Example of responsive navigation that switches between
/// NavigationBar (bottom) and NavigationRail (side)
/// based on window width.
class ResponsiveNavigationExample extends StatefulWidget {
  const ResponsiveNavigationExample({super.key});

  @override
  State<ResponsiveNavigationExample> createState() =>
      _ResponsiveNavigationExampleState();
}

class _ResponsiveNavigationExampleState
    extends State<ResponsiveNavigationExample> {
  static const _destinations = [
    _AdaptiveDestination(Icons.home, 'Home'),
    _AdaptiveDestination(Icons.search, 'Search'),
    _AdaptiveDestination(Icons.person, 'Profile'),
  ];

  int _selectedIndex = 0;

  void _selectDestination(int index) {
    setState(() {
      _selectedIndex = index;
    });
  }

  @override
  Widget build(BuildContext context) {
    final width = MediaQuery.sizeOf(context).width;

    return width >= 600 ? _buildLargeLayout() : _buildSmallLayout();
  }

  /// Layout for small screens - bottom navigation
  Widget _buildSmallLayout() {
    return Scaffold(
      appBar: AppBar(title: Text(_destinations[_selectedIndex].label)),
      body: _DestinationBody(label: _destinations[_selectedIndex].label),
      bottomNavigationBar: NavigationBar(
        selectedIndex: _selectedIndex,
        onDestinationSelected: _selectDestination,
        destinations: [
          for (final destination in _destinations)
            NavigationDestination(
              icon: Icon(destination.icon),
              label: destination.label,
            ),
        ],
      ),
    );
  }

  /// Layout for large screens - side navigation rail
  Widget _buildLargeLayout() {
    return Scaffold(
      body: Row(
        children: [
          NavigationRail(
            selectedIndex: _selectedIndex,
            onDestinationSelected: _selectDestination,
            labelType: NavigationRailLabelType.all,
            destinations: [
              for (final destination in _destinations)
                NavigationRailDestination(
                  icon: Icon(destination.icon),
                  label: Text(destination.label),