void main() {  
  for (int i = 0; i < 5; i++) {  
    print('hello ${i + 1}');  
  }  
}​

Future<int> sumStream(Stream<int> stream) async {  
    var sum = 0;  
    await for (var value in stream) {  
      sum = sum + value;  
    }  
    return sum;  
  }

int a;                           // a is initialized with null value.    
a ??= 10;     
print(a);                      // It will print 10.    ​

print(5 ?? 10);            // It will print 5.   
print(null ?? 10);       // It will print 10.  ​

obj?.child?.child?.getter       //The expression returns null if obj, child1, or child2 are null. If not, the getter is called and returned.​

import 'package:flutter/foundation.dart' as Foundation;  ​

if (Foundation.kReleaseMode){ // is Release Mode ??  
    print('release mode');  
} else {  
    print('debug mode');  
} ​

Future<String> fetchData() async {
  // perform asynchronous operation
  final response = await http.get('https://jsonplaceholder.typicode.com/posts/1');
  // return result
  return response.body;
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return FutureBuilder<String>(
      future: fetchData(),
      builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
        if (snapshot.connectionState == ConnectionState.waiting) {
          return CircularProgressIndicator();
        } else if (snapshot.hasError) {
          return Text('Error: ${snapshot.error}');
        } else {
          return Text('Data: ${snapshot.data}');
        }
      },
    );
  }
}​

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Counter')),
      body: Center(
        child: Text('$_counter'),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        child: Icon(Icons.add),
      ),
    );
  }
}

In this example, `_counter` is a variable that holds the current value of the counter. The `_incrementCounter()` method is called when the user taps the floating action button, and it calls `setState()` to update the `_counter` variable and trigger a rebuild of the widget. The `build()` method returns a `Text` widget that displays the current value of `_counter`, and a `FloatingActionButton` that calls `_incrementCounter()` when pressed.

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('LayoutBuilder')),
      body: Center(
        child: Container(
          width: 200,
          height: 200,
          color: Colors.blue,
          child: LayoutBuilder(
            builder: (BuildContext context, BoxConstraints constraints) {
              return Container(
                width: constraints.maxWidth * 0.5,
                height: constraints.maxHeight * 0.5,
                color: Colors.red,
              );
            },
          ),
        ),
      ),
    );
  }
}

In this example, `LayoutBuilder` is used to get the maximum width and height of the parent `Container` widget, and then use this information to size and position a child `Container` widget. The child `Container` widget is half the width and half the height of the parent `Container`, and is colored red to make it visible.

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Image Widget')),
      body: Center(
        child: Image.network(
          'https://example.com/images/my_image.jpg',
          width: 200,
          height: 200,
          fit: BoxFit.cover,
        ),
      ),
    );
  }
}​

import 'package:flutter/material.dart';  
  
void main() => runApp(MyApp());  
  
class MyApp extends StatelessWidget {  
  @override  
  Widget build(BuildContext context) {  
    final appTitle = 'Flutter Form Demo';  
    return MaterialApp(  
      title: appTitle,  
      home: Scaffold(  
        appBar: AppBar(  
          title: Text(appTitle),  
        ),  
        body: MyCustomForm(),  
      ),  
    );  
  }  
}  
// Create a Form widget.  
class MyCustomForm extends StatefulWidget {  
  @override  
  MyCustomFormState createState() {  
    return MyCustomFormState();  
  }  
}  
// Create a corresponding State class. This class holds data related to the form.  
class MyCustomFormState extends State<MyCustomForm> {  
  // Create a global key that uniquely identifies the Form widget  
  // and allows validation of the form.  
  final _formKey = GlobalKey<FormState>();  
  
  @override  
  Widget build(BuildContext context) {  
    // Build a Form widget using the _formKey created above.  
    return Form(  
      key: _formKey,  
      child: Column(  
        crossAxisAlignment: CrossAxisAlignment.start,  
        children: <Widget>[  
          TextFormField(  
            decoration: const InputDecoration(  
              icon: const Icon(Icons.person),  
              hintText: 'Enter your name',  
              labelText: 'Name',  
            ),  
          ),  
          TextFormField(  
            decoration: const InputDecoration(  
              icon: const Icon(Icons.phone),  
              hintText: 'Enter a phone number',  
              labelText: 'Phone',  
            ),  
          ),  
          TextFormField(  
            decoration: const InputDecoration(  
            icon: const Icon(Icons.calendar_today),  
            hintText: 'Enter your date of birth',  
            labelText: 'Dob',  
            ),  
           ),  
          new Container(  
              padding: const EdgeInsets.only(left: 150.0, top: 40.0),  
              child: new RaisedButton(  
                child: const Text('Submit'),  
                  onPressed: null,  
              )),  
        ],  
      ),  
    );  
  }  
}  

import 'package:flutter/material.dart';

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final mediaQuery = MediaQuery.of(context);

    return Scaffold(
      body: Container(
        width: mediaQuery.size.width,
        height: mediaQuery.size.height,
        child: Center(
          child: Text('Hello, world!'),
        ),
      ),
    );
  }
}

In this example, we use the `MediaQuery.of()` method to retrieve a reference to the `MediaQueryData` object, which contains information about the device's display. We then use the `size` property of this object to set the width and height of a `Container` widget, which is used as the background for the page.

import 'package:flutter/material.dart';

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('My App'),
      ),
      body: Center(
        child: Text('Hello, world!'),
      ),
    );
  }
}​

Future<String> fetchData() async {
  await Future.delayed(Duration(seconds: 2)); // Simulate a network request delay
  return 'Data loaded successfully';
}​

import 'dart:async';

void main() {
  final stream = Stream<int>.periodic(Duration(seconds: 1), (count) => count).take(5);

  stream.listen((value) => print(value), onError: (error) => print(error), onDone: () => print('Done!'));
}​

import 'package:flutter/material.dart';

class MyButton extends StatelessWidget {
  final String text;
  final VoidCallback onPressed;

  MyButton({required this.text, required this.onPressed});

  @override
  Widget build(BuildContext context) {
    return ElevatedButton(
      onPressed: onPressed,
      child: Text(text),
    );
  }
}​

import 'package:flutter/material.dart';

class CounterWidget extends StatefulWidget {
  @override
  _CounterWidgetState createState() => _CounterWidgetState();
}

class _CounterWidgetState extends State<CounterWidget> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Text('Counter: $_counter'),
        ElevatedButton(
          onPressed: _incrementCounter,
          child: Text('Increment'),
        ),
      ],
    );
  }
}​

Navigator.push(
  context,
  MaterialPageRoute(builder: (context) => MyNewRoute()),
);​

Navigator.pop(context);​

// Push a new route onto the stack
Navigator.push(
  context,
  MaterialPageRoute(builder: (context) => MyNewRoute()),
);

// Pop the current route off the stack
Navigator.pop(context);

In this example, `MyNewRoute()` is a widget that represents the new screen to be displayed when `Navigator.push()` is called. When `Navigator.pop()` is called, it removes the current route from the stack and returns to the previous route.

var routes = {
  '/home': (context) => HomeScreen(),
  '/settings': (context) => SettingsScreen(),
  '/profile': (context) => ProfileScreen(),
};​

Navigator.pushNamed(context, '/settings');​

Navigator.pushNamed(context, '/profile', arguments: {'userId': '123'});​

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      child: Text("Hello World"),
    );
  }
}

In the `build` method, the `context` argument holds the reference to the current `BuildContext` object. The argument could have been named anything, but `BuildContext context` is a commonly used convention in Flutter development.

Here's an example of how to use a hero animation in Flutter:

class FirstScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: GestureDetector(
        onTap: () {
          Navigator.push(
            context,
            MaterialPageRoute(builder: (_) => SecondScreen()),
          );
        },
        child: Hero(
          tag: 'myHeroTag',
          child: Image.asset('assets/my_image.png'),
        ),
      ),
    );
  }
}

class SecondScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: GestureDetector(
        onTap: () {
          Navigator.pop(context);
        },
        child: Center(
          child: Hero(
            tag: 'myHeroTag',
            child: Image.asset('assets/my_image.png'),
          ),
        ),
      ),
    );
  }
}

In this example, the hero widget is an image that is displayed on both the first and second screen. The `GestureDetector` widget is used to detect taps on the widget, and the `Navigator` widget is used to navigate between the two screens. The `Hero` widget is used to define the hero animation, and the tag is set to the same string on both screens. When the user taps on the image, the app navigates to the second screen, and the hero animation is automatically triggered, smoothly transitioning the image from the first to the second screen.

...Data_structure
Example:-
var a = [0,1,2,3,4];
var b = [6,7,8,9];
var c = [...a,5,...b];​

print(c);  // prints: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

...?Data_structure
Exmaple :-
List<int> l1 = [1, 2, 3];
List<int> nullList = null;
List<int> result = [...l1, ...?nullList];
print(result); // prints:   [1, 2, 3] ​