面向 React Native 开发者的 Flutter

/// Dart
void main() {}

/// Dart
print('Hello world!');

/// Dart
/// Both variables are acceptable.
String name = 'dart'; // Explicitly typed as a [String].
var otherName = 'Dart'; // Inferred [String] type.

// Dart
var name; // == null; raises a linter warning
int? x; // == null

/// Dart
var myNull;
var zero = 0;
if (zero == 0) {
  print('use "== 0" to check zero');
}

/// Dart
/// You can explicitly define the return type.
bool fn() {
  return true;
}

// Dart
import 'dart:convert';

import 'package:http/http.dart' as http;

class Example {
  Future<String> _getIPAddress() {
    final url = Uri.https('httpbin.org', '/ip');
    return http.get(url).then((response) {
      final ip = jsonDecode(response.body)['origin'] as String;
      return ip;
    });
  }
}

void main() {
  final example = Example();
  example
      ._getIPAddress()
      .then((ip) => print(ip))
      .catchError((error) => print(error));
}

// Dart
import 'dart:convert';

import 'package:http/http.dart' as http;

class Example {
  Future<String> _getIPAddress() async {
    final url = Uri.https('httpbin.org', '/ip');
    final response = await http.get(url);
    final ip = jsonDecode(response.body)['origin'] as String;
    return ip;
  }
}

/// An async function returns a `Future`.
/// It can also return `void`, unless you use
/// the `avoid_void_async` lint. In that case,
/// return `Future<void>`.
void main() async {
  final example = Example();
  try {
    final ip = await example._getIPAddress();
    print(ip);
  } catch (error) {
    print(error);
  }
}

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';
import 'package:my_widgets/my_widgets.dart';

// Flutter
import 'package:flutter/material.dart';

void main() {
  runApp(
    const Center(
      child: Text(
        'Hello, world!',
        textDirection: TextDirection.ltr,
      ),
    ),
  );
}

// Flutter
import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Welcome to Flutter',
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Welcome to Flutter'),
        ),
        body: const Center(
          child: Text('Hello world'),
        ),
      ),
    );
  }
}

/// Flutter
class CustomCard extends StatelessWidget {
  const CustomCard({
    super.key,
    required this.index,
    required this.onPress,
  });

  final int index;
  final void Function() onPress;

  @override
  Widget build(BuildContext context) {
    return Card(
      child: Column(
        children: <Widget>[
          Text('Card $index'),
          TextButton(
            onPressed: onPress,
            child: const Text('Press'),
          ),
        ],
      ),
    );
  }
}

class UseCard extends StatelessWidget {
  const UseCard({super.key, required this.index});

  final int index;

  @override
  Widget build(BuildContext context) {
    /// Usage
    return CustomCard(
      index: index,
      onPress: () {
        print('Card $index');
      },
    );
  }
}

// Dart
void main() {
  print('Hello, this is the main function.');
}

Image.asset('assets/background.png');

Image.network('https://docs.fluttercn.cn/assets/images/docs/owl.jpg');

import 'package:flutter/material.dart';

var data = [
  'Hello',
  'World',
];
return ListView.builder(
  itemCount: data.length,
  itemBuilder: (context, index) {
    return Text(data[index]);
  },
);

class MyCanvasPainter extends CustomPainter {
  const MyCanvasPainter();

  @override
  void paint(Canvas canvas, Size size) {
    final Paint paint = Paint()..color = Colors.amber;
    canvas.drawCircle(const Offset(100, 200), 40, paint);
    final Paint paintRect = Paint()..color = Colors.lightBlue;
    final Rect rect = Rect.fromPoints(
      const Offset(150, 300),
      const Offset(300, 400),
    );
    canvas.drawRect(rect, paintRect);
  }

  @override
  bool shouldRepaint(MyCanvasPainter oldDelegate) => false;
}

class MyCanvasWidget extends StatelessWidget {
  const MyCanvasWidget({super.key});

  @override
  Widget build(BuildContext context) {
    return const Scaffold(
      body: CustomPaint(painter: MyCanvasPainter()),
    );
  }
}

@override
Widget build(BuildContext context) {
  return Center(
    child: Column(
      children: <Widget>[
        Container(
          color: Colors.red,
          width: 100,
          height: 100,
        ),
        Container(
          color: Colors.blue,
          width: 100,
          height: 100,
        ),
        Container(
          color: Colors.green,
          width: 100,
          height: 100,
        ),
      ],
    ),
  );

@override
Widget build(BuildContext context) {
  return Stack(
    alignment: const Alignment(0.6, 0.6),
    children: <Widget>[
      const CircleAvatar(
        backgroundImage: NetworkImage(
          'https://avatars3.githubusercontent.com/u/14101776?v=4',
        ),
      ),
      Container(
        color: Colors.black45,
        child: const Text('Flutter'),
      ),
    ],
  );

const TextStyle textStyle = TextStyle(
  color: Colors.cyan,
  fontSize: 32,
  fontWeight: FontWeight.w600,
);

return const Center(
  child: Column(
    children: <Widget>[
      Text('Sample text', style: textStyle),
      Padding(
        padding: EdgeInsets.all(20),
        child: Icon(
          Icons.lightbulb_outline,
          size: 48,
          color: Colors.redAccent,
        ),
      ),
    ],
  ),
);

return const Icon(Icons.lightbulb_outline, color: Colors.redAccent);

class SampleApp extends StatelessWidget {
  const SampleApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Sample App',
      theme: ThemeData(
          colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
          textSelectionTheme:
              const TextSelectionThemeData(selectionColor: Colors.red)),
      home: const SampleAppPage(),
    );
  }
}

@override
Widget build(BuildContext context) {
  return MaterialApp(
    theme: ThemeData(
      primaryColor: Colors.cyan,
      brightness: Brightness.dark,
    ),
    home: const StylingPage(),
  );
}

@override
Widget build(BuildContext context) {
  return Theme(
    data: ThemeData(
      primaryColor: Colors.cyan,
      brightness: brightness,
    ),
    child: Scaffold(
      backgroundColor: Theme.of(context).primaryColor,
      //...
    ),
  );
}

import 'package:flutter/material.dart';

void main() => runApp(
      const MyStatelessWidget(
        text: 'StatelessWidget Example to show immutable data',
      ),
    );

class MyStatelessWidget extends StatelessWidget {
  const MyStatelessWidget({
    super.key,
    required this.text,
  });

  final String text;

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Text(
        text,
        textDirection: TextDirection.ltr,
      ),
    );
  }
}

class MyStatefulWidget extends StatefulWidget {
  const MyStatefulWidget({
    super.key,
    required this.title,
  });

  final String title;

  @override
  State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
}

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  bool showText = true;
  bool toggleState = true;
  Timer? t2;

  void toggleBlinkState() {
    setState(() {
      toggleState = !toggleState;
    });
    if (!toggleState) {
      t2 = Timer.periodic(const Duration(milliseconds: 1000), (t) {
        toggleShowText();
      });
    } else {
      t2?.cancel();
    }
  }

  void toggleShowText() {
    setState(() {
      showText = !showText;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          children: <Widget>[
            if (showText)
              const Text(
                'This execution will be done before you can blink.',
              ),
            Padding(
              padding: const EdgeInsets.only(top: 70),
              child: ElevatedButton(
                onPressed: toggleBlinkState,
                child: toggleState
                    ? const Text('Blink')
                    : const Text('Stop Blinking'),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

class MyStatefulWidget extends StatefulWidget {
  const MyStatefulWidget({
    super.key,
    required this.title,
  });

  final String title;
  @override
  State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
}

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  @override
  Widget build(BuildContext context) {
    //...
  }
}

class MyStatelessWidget extends StatelessWidget {
  // This widget is the root of your application.
  const MyStatelessWidget({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Flutter Demo',
      home: MyStatefulWidget(title: 'State Change Demo'),
    );
  }
}

/// Flutter
class CustomCard extends StatelessWidget {
  const CustomCard({
    super.key,
    required this.index,
    required this.onPress,
  });

  final int index;
  final void Function() onPress;

  @override
  Widget build(BuildContext context) {
    return Card(
      child: Column(
        children: <Widget>[
          Text('Card $index'),
          TextButton(
            onPressed: onPress,
            child: const Text('Press'),
          ),
        ],
      ),
    );
  }
}

class UseCard extends StatelessWidget {
  const UseCard({super.key, required this.index});

  final int index;

  @override
  Widget build(BuildContext context) {
    /// Usage
    return CustomCard(
      index: index,
      onPress: () {
        print('Card $index');
      },
    );
  }
}

import 'package:shared_preferences/shared_preferences.dart';

Future<void> updateCounter() async {
  final prefs = await SharedPreferences.getInstance();
  int? counter = prefs.getInt('counter');
  if (counter is int) {
    await prefs.setInt('counter', ++counter);
  }
  setState(() {
    _counter = counter;
  });
}

class NavigationApp extends StatelessWidget {
  // This widget is the root of your application.
  const NavigationApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      //...
      routes: <String, WidgetBuilder>{
        '/a': (context) => const UsualNavScreen(),
        '/b': (context) => const DrawerNavScreen(),
      },
      //...
    );
  }
}

Navigator.of(context).pushNamed('/a');

Navigator.push(
  context,
  MaterialPageRoute(
    builder: (context) => const UsualNavScreen(),
  ),
);

class _MyAppState extends State<MyApp> with SingleTickerProviderStateMixin {
  late TabController controller = TabController(length: 2, vsync: this);

  @override
  Widget build(BuildContext context) {
    return TabBar(
      controller: controller,
      tabs: const <Tab>[
        Tab(icon: Icon(Icons.person)),
        Tab(icon: Icon(Icons.email)),
      ],
    );
  }
}

class _NavigationHomePageState extends State<NavigationHomePage>
    with SingleTickerProviderStateMixin {
  late TabController controller = TabController(length: 2, vsync: this);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        bottomNavigationBar: Material(
          color: Colors.blue,
          child: TabBar(
            tabs: const <Tab>[
              Tab(
                icon: Icon(Icons.person),
              ),
              Tab(
                icon: Icon(Icons.email),
              ),
            ],
            controller: controller,
          ),
        ),
        body: TabBarView(
          controller: controller,
          children: const <Widget>[HomeScreen(), TabScreen()],
        ));
  }
}

@override
Widget build(BuildContext context) {
  return Drawer(
    elevation: 20,
    child: ListTile(
      leading: const Icon(Icons.change_history),
      title: const Text('Screen2'),
      onTap: () {
        Navigator.of(context).pushNamed('/b');
      },
    ),
  );
}

@override
Widget build(BuildContext context) {
  return Scaffold(
    drawer: Drawer(
      elevation: 20,
      child: ListTile(
        leading: const Icon(Icons.change_history),
        title: const Text('Screen2'),
        onTap: () {
          Navigator.of(context).pushNamed('/b');
        },
      ),
    ),
    appBar: AppBar(title: const Text('Home')),
    body: Container(),
  );
}

@override
Widget build(BuildContext context) {
  return GestureDetector(
    child: Scaffold(
      appBar: AppBar(title: const Text('Gestures')),
      body: const Center(
          child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          Text('Tap, Long Press, Swipe Horizontally or Vertically'),
        ],
      )),
    ),
    onTap: () {
      print('Tapped');
    },
    onLongPress: () {
      print('Long Pressed');
    },
    onVerticalDragEnd: (value) {
      print('Swiped Vertically');
    },
    onHorizontalDragEnd: (value) {
      print('Swiped Horizontally');
    },
  );
}

import 'dart:io';

final url = Uri.parse('https://httpbin.org/ip');
final httpClient = HttpClient();

Future<void> getIPAddress() async {
  final request = await httpClient.getUrl(url);
  final response = await request.close();
  final responseBody = await response.transform(utf8.decoder).join();
  final ip = jsonDecode(responseBody)['origin'] as String;
  setState(() {
    _ipAddress = ip;
  });
}

final TextEditingController _controller = TextEditingController();

@override
Widget build(BuildContext context) {
  return Column(children: [
    TextField(
      controller: _controller,
      decoration: const InputDecoration(
        hintText: 'Type something',
        labelText: 'Text Field',
      ),
    ),
    ElevatedButton(
      child: const Text('Submit'),
      onPressed: () {
        showDialog(
            context: context,
            builder: (context) {
              return AlertDialog(
                title: const Text('Alert'),
                content: Text('You typed ${_controller.text}'),
              );
            });
      },
    ),
  ]);
}

@override
Widget build(BuildContext context) {
  return Form(
    key: formKey,
    child: Column(
      children: <Widget>[
        TextFormField(
          validator: (value) {
            if (value != null && value.contains('@')) {
              return null;
            }
            return 'Not a valid email.';
          },
          onSaved: (val) {
            _email = val;
          },
          decoration: const InputDecoration(
            hintText: 'Enter your email',
            labelText: 'Email',
          ),
        ),
        ElevatedButton(
          onPressed: _submit,
          child: const Text('Login'),
        ),
      ],
    ),
  );
}

void _submit() {
  final form = formKey.currentState;
  if (form != null && form.validate()) {
    form.save();
    showDialog(
      context: context,
      builder: (context) {
        return AlertDialog(
            title: const Text('Alert'),
            content: Text('Email: $_email, password: $_password'));
      },
    );
  }
}

final platform = Theme.of(context).platform;
if (platform == TargetPlatform.iOS) {
  return 'iOS';
}
if (platform == TargetPlatform.android) {
  return 'android';
}
if (platform == TargetPlatform.fuchsia) {
  return 'fuchsia';
}
return 'not recognized ';

import 'package:flutter/material.dart';

void main() {
  runApp(const Center(child: LogoFade()));
}

class LogoFade extends StatefulWidget {
  const LogoFade({super.key});

  @override
  State<LogoFade> createState() => _LogoFadeState();
}

class _LogoFadeState extends State<LogoFade>
    with SingleTickerProviderStateMixin {
  late Animation<double> animation;
  late AnimationController controller;

  @override
  void initState() {
    super.initState();
    controller = AnimationController(
      duration: const Duration(milliseconds: 3000),
      vsync: this,
    );
    final CurvedAnimation curve = CurvedAnimation(
      parent: controller,
      curve: Curves.easeIn,
    );
    animation = Tween(begin: 0.0, end: 1.0).animate(curve);
    controller.forward();
  }

  @override
  void dispose() {
    controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return FadeTransition(
      opacity: animation,
      child: const SizedBox(
        height: 300,
        width: 300,
        child: FlutterLogo(),
      ),
    );
  }
}

return Dismissible(
  key: Key(widget.key.toString()),
  onDismissed: (dismissDirection) {
    cards.removeLast();
  },
  child: Container(
      //...
      ),
);