return Scaffold(
  appBar: AppBar(
    title: const Text('Title'),
  ),
  body: const Center(
    child: Text('Test Page'),
  ),
);

class LogOutButton extends StatelessWidget {
    final String title;
    static final HexColor basicColor = HexColor('#E02B2B');
    const LogOutButton({Key? key, required this.title}) : super(key: key);

    @override
    Widget build(BuildContext context) {
        return Container(
            padding: EdgeInsets.symmetric(vertical: 20.0),
            alignment: Alignment.center,
            child: OutlinedButton(
                onPressed: (){},
                child: Row(
                    mainAxisAlignment: MainAxisAlignment.spaceBetween,
           
                    children: [
                        SizedBox(width: 24, height: 24,),
                        Text(
                            title,
                            style: GoogleFonts.roboto(
                                fontWeight: FontWeight.w500,
                                fontSize: 20.0,
                                color: basicColor,
                            ),
                        ),
                        Icon(
                            Icons.logout,
                            color: basicColor,
                        )
                    ],
                ),
                style: ButtonStyle(
                    maximumSize: MaterialStateProperty.all(Size(300.0, 50.0)),
                    minimumSize: MaterialStateProperty.all(Size(200.0, 50.0)),
                    overlayColor: MaterialStateProperty.all(Colors.transparent),
                    side: MaterialStateProperty.all(
                        BorderSide(
                            color: basicColor,
                            width: 2.0,
                        )
                    ),
                    shape: MaterialStateProperty.all(
                        RoundedRectangleBorder(
                            borderRadius: BorderRadius.circular(20.0),
                        )
                    )
                    
                )
            ),
        );
    }
}

 style: ButtonStyle(
                    maximumSize: MaterialStateProperty.all(Size(300.0, 50.0)),
                    minimumSize: MaterialStateProperty.all(Size(200.0, 50.0)),
                    overlayColor: MaterialStateProperty.all(Colors.transparent),
                    side: MaterialStateProperty.all(
                        BorderSide(
                            color: basicColor,
                            width: 2.0,
                        )
                    ),
                    shape: MaterialStateProperty.all(
                        RoundedRectangleBorder(
                            borderRadius: BorderRadius.circular(20.0),
                        )
                    )
                    
                )

sdk: ">=2.12.0 <3.0.0"


Then follow the steps:

Run flutter upgrade in the terminal to upgrade Flutter
Run dart migrate to run the dart migration tool.
Solve all errors which the migration tool shows.
Run flutter pub outdated --mode=null-safety to print all outdated packages.
Run flutter pub upgrade --null-safety to upgrade all packages automatically.
Check the code for errors and solve them (Very important).
Run dart migrate again and it should now be successful. Follow the link to checkout the proposed changes.
Press the "Apply Migration" button.
Check the code for errors again and fix them.

Future<File> moveFile(File sourceFile, String newPath) async {
  try {
    // prefer using rename as it is probably faster
    return await sourceFile.rename(newPath);
  } on FileSystemException catch (e) {
    // if rename fails, copy the source file and then delete it
    final newFile = await sourceFile.copy(newPath);
    await sourceFile.delete();
    return newFile;
  }
}

<uses-permission android:name="android.permission.MANAGE_EXTERNAL_STORAGE"
        tools:ignore="ScopedStorage" />

products.sort((a,b) => a.compareTo(b));

android {
    defaultConfig {
        ...
        multiDexEnabled true
    }
    ...
}

dependencies {
  implementation 'com.android.support:multidex:1.0.3'
}

String shakespeareQuote = "All the world's a stage, ...";

import 'package:path_provider/path_provider.dart';
Future<void> _deleteCacheDir() async {
final cacheDir = await getTemporaryDirectory();
if (cacheDir.existsSync()) {
cacheDir.deleteSync(recursive: true);}}
final appDir = await getApplicationSupportDirectory();
if (appDir.existsSync()) {
appDir.deleteSync(recursive: true);}}

                Container(
                  height: SizeConfig.safeBlockVertical * 24,
                  width: SizeConfig.safeBlockHorizontal * 45,
                  padding: EdgeInsets.only(
                      left: SizeConfig.safeBlockHorizontal * 5,
                      right: SizeConfig.safeBlockHorizontal * 4,
                      bottom: SizeConfig.safeBlockVertical * 2),
                  decoration: BoxDecoration(
                      borderRadius: BorderRadius.circular(20),
                      color: Colors.white,
                      gradient: RadialGradient(
                          focalRadius: 100,
                          radius: 1.5,
                          center: Alignment.bottomLeft,
                          colors: [
                            colorStyles["maslowGreen"]!.withOpacity(0.65),
                            colorStyles["maslowGreen"]!.withOpacity(0.65),
                          ],
                          stops: [
                            0.0,
                            1.0
                          ])),
                  child: _collectionName(),
                ),

InputDecoration(
   isDense: true,
   prefixIcon:Text("\$"),
   prefixIconConstraints: BoxConstraints(minWidth: 0, minHeight: 0),
),

RichText(
  text: TextSpan(
    text: 'Hello ',
    style: DefaultTextStyle.of(context).style,
    children: const <TextSpan>[
      TextSpan(text: 'bold', style: TextStyle(fontWeight: FontWeight.bold)),
      TextSpan(text: ' world!'),
    ],
  ),
)

pod install --repo-update 

@override
Widget build(BuildContext context) {

  return new FutureBuilder<User>(
      future: userInfo,
      builder: (BuildContext context, AsyncSnapshot<User> snapshot) 
    {
      switch(snapshot.connectionState) {
        case ConnectionState.none:
          _showDialog(context);
          return Container();
        case ConnectionState.waiting:
          return new Center(
            child: new CircularProgressIndicator());
        case ConnectionState.active:
          return new Text('');
        case ConnectionState.done:
          if(snapshot.hasError) {
            error = snapshot.error;
            _showDialog(context);
            return Container();
          } else {
            return Container();
          }
      }
  });

import 'dart:async';
import 'dart:isolate';
main() async {
  var receivePort = new ReceivePort();
  await Isolate.spawn(echo, receivePort.sendPort);
// The 'echo' isolate sends it's SendPort as the first message
  var sendPort = await receivePort.first;
var msg = await sendReceive(sendPort, "bye");
  print('received $msg');
  msg = await sendReceive(sendPort, "hello");
  print('received $msg');
}
// the entry point for the isolate
echo(SendPort sendPort) async {
  // Open the ReceivePort for incoming messages.
  var port = new ReceivePort();
// Notify any other isolates what port this isolate listens to.
  sendPort.send(port.sendPort);
await for (var msg in port) {
    var data = msg[0];
    SendPort replyTo = msg[1];
    replyTo.send(data);
    if (data == "hello") port.close();
  }
}
/// sends a message on a port, receives the response,
/// and returns the message
Future sendReceive(SendPort port, msg) {
  ReceivePort response = new ReceivePort();
  port.send([msg, response.sendPort]);
  return response.first;
}

taskkill /F /IM dart.exe

Future<String> readBase64(String filepath) async {
  File file = File(await getFilePath(filepath));
  String fileContent = await file.readAsString();
  return (fileContent);
}

ElevatedButton(
      child: Text('Button'),
      onPressed: () {},
      style: ElevatedButton.styleFrom({
           Color primary, // set the background color 
           Color onPrimary, 
           Color onSurface, 
           Color shadowColor, 
           double elevation, 
           TextStyle textStyle, 
           EdgeInsetsGeometry padding, 
           Size minimumSize, 
           BorderSide side, 
           OutlinedBorder shape, 
           MouseCursor enabledMouseCursor, 
           MouseCursor disabledMouseCursor, 
           VisualDensity visualDensity, 
           MaterialTapTargetSize tapTargetSize, 
           Duration animationDuration, 
           bool enableFeedback
     }),
),

Scaffold(
      appBar: AppBar(),
      body: Align(
        alignment: Alignment.topCenter,
        child: LayoutBuilder(
          builder: (BuildContext context, BoxConstraints constraints) {
            return Container(
              height: constraints.maxHeight / 2,
              width: MediaQuery.of(context).size.width / 2,
              color: Colors.red,
            );
          },
        ),
      ),
    );

TextField(
 enabled: false, // to trigger disabledBorder
 decoration: InputDecoration(
   filled: true,
   fillColor: Color(0xFFF2F2F2),
   focusedBorder: OutlineInputBorder(
     borderRadius: BorderRadius.all(Radius.circular(4)),
     borderSide: BorderSide(width: 1,color: Colors.red),
   ),
   disabledBorder: OutlineInputBorder(
     borderRadius: BorderRadius.all(Radius.circular(4)),
     borderSide: BorderSide(width: 1,color: Colors.orange),
   ),
   enabledBorder: OutlineInputBorder(
     borderRadius: BorderRadius.all(Radius.circular(4)),
     borderSide: BorderSide(width: 1,color: Colors.green),
   ),
   border: OutlineInputBorder(
     borderRadius: BorderRadius.all(Radius.circular(4)),
     borderSide: BorderSide(width: 1,)
   ),
   errorBorder: OutlineInputBorder(
     borderRadius: BorderRadius.all(Radius.circular(4)),
     borderSide: BorderSide(width: 1,color: Colors.black)
   ),
   focusedErrorBorder: OutlineInputBorder(
     borderRadius: BorderRadius.all(Radius.circular(4)),
     borderSide: BorderSide(width: 1,color: Colors.yellowAccent)
   ),
   hintText: "HintText",
   hintStyle: TextStyle(fontSize: 16,color: Color(0xFFB3B1B1)),
   errorText: snapshot.error,
 ),
 controller: _passwordController,
 onChanged: _authenticationFormBloc.onPasswordChanged,
                            obscureText: false,
),

class AppColors {
  static MaterialColor hex(String hex) =>
      AppColors._factoryColor(AppColors._getColorHexFromStr(hex));

  static MaterialColor _factoryColor(int color) {
    return MaterialColor(color, <int, Color>{
      50: Color(color),
      100: Color(color),
      200: Color(color),
      300: Color(color),
      400: Color(color),
      500: Color(color),
      600: Color(color),
      700: Color(color),
      800: Color(color),
      900: Color(color),
    });
  }

  static int _getColorHexFromStr(String colorStr) {
    colorStr = "FF" + colorStr;
    colorStr = colorStr.replaceAll("#", "");
    int val = 0;
    int len = colorStr.length;
    for (int i = 0; i < len; i++) {
      int hexDigit = colorStr.codeUnitAt(i);
      if (hexDigit >= 48 && hexDigit <= 57) {
        val += (hexDigit - 48) * (1 << (4 * (len - 1 - i)));
      } else if (hexDigit >= 65 && hexDigit <= 70) {
        // A..F
        val += (hexDigit - 55) * (1 << (4 * (len - 1 - i)));
      } else if (hexDigit >= 97 && hexDigit <= 102) {
        // a..f
        val += (hexDigit - 87) * (1 << (4 * (len - 1 - i)));
      } else {
        val = 0xFFFFFFFF;
      }
    }
    return val;
  }
}

// Example: In your app
// ...
class MyExample extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
    	height: 100.0,
      	width: 100.0,
      	decoration: BoxDecoration(
    		color: AppColors.hex('#000000'),	
    ),
    );
  }
}

Column( 
  crossAxisAlignment: CrossAxisAlignment.start, children: <Widget>[ Container( child: Text('${payment.name}',textAlign: TextAlign.left,), color: Colors.redAccent, ), ...... )

 decoration: BoxDecoration(
          border: Border(
            top: BorderSide(width: 16.0, color: Colors.lightBlue.shade600),
            bottom: BorderSide(width: 16.0, color: Colors.lightBlue.shade900),
          ),
          color: Colors.white,
        ),

extension StringExtension on String {
    String capitalize() {
      return "${this[0].toUpperCase()}${this.substring(1)}";
    }
}

appBar: AppBar(
        automaticallyImplyLeading: false,
        title: Text(signedIn ? 'Home' : 'Sign In'),
      ),

Container(
      height: 120.0,
      width: 120.0,
      decoration: BoxDecoration(
        image: DecorationImage(
          image: AssetImage(
              'assets/assets/alucard.jpg'),
          fit: BoxFit.fill,
        ),
        shape: BoxShape.circle,
      ),
    )

const color = const Color(0xffb74093); // Second `const` is optional in assignments.

Text(
  'Hello world',
  style: TextStyle(
    decoration: TextDecoration.underline,
  ),
)

Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(title: Text('IntrinsicWidth')),
    body: Center(
      child: IntrinsicWidth(
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            RaisedButton(
              onPressed: () {},
              child: Text('Short'),
            ),
            RaisedButton(
              onPressed: () {},
              child: Text('A bit Longer'),
            ),
            RaisedButton(
              onPressed: () {},
              child: Text('The Longest text button'),
            ),
          ],
        ),
      ),
    ),
  );
}

try {
      final result = await InternetAddress.lookup('google.com');
      if (result.isNotEmpty && result[0].rawAddress.isNotEmpty) {
          print(' connected');
      }
    } on SocketException catch (_) {
      //NOT connected to the internet
      print('not connected');
    }

  class MyBlinkingButton extends StatefulWidget {
    @override
    _MyBlinkingButtonState createState() => _MyBlinkingButtonState();
  }

  class _MyBlinkingButtonState extends State<MyBlinkingButton>
      with SingleTickerProviderStateMixin {
    AnimationController _animationController;

    @override
    void initState() {
      _animationController =
          new AnimationController(vsync: this, duration: Duration(seconds: 1));
      _animationController.repeat(reverse: true);
      super.initState();
    }

    @override
    Widget build(BuildContext context) {
      return FadeTransition(
        opacity: _animationController,
        child: MaterialButton(
          onPressed: () => null,
          child: Text("Text button"),
          color: Colors.green,
        ),
      );
    }

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

List<String> feed = List();

feed.add("AAAA");
feed.add("BBBB");
feed.add("CCCC");

ListView.builder(
  itemCount: feed.length,
  itemBuilder: (context, index) {
    return ListTile(title: feed[feed.length - 1 -index]);
  },
);

import 'dart:math';

void main() {
  print(getRandomString(5));  // 5GKjb
  print(getRandomString(10)); // LZrJOTBNGA
  print(getRandomString(15)); // PqokAO1BQBHyJVK
}

const _chars = 'AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz1234567890';
Random _rnd = Random();

String getRandomString(int length) => String.fromCharCodes(Iterable.generate(
    length, (_) => _chars.codeUnitAt(_rnd.nextInt(_chars.length))));

import 'dart:io';
import 'package:exif/exif.dart';
import 'package:image/image.dart' as img;

Future<File> fixExifRotation(String imagePath) async {
    final originalFile = File(imagePath);
    List<int> imageBytes = await originalFile.readAsBytes();

    final originalImage = img.decodeImage(imageBytes);

    final height = originalImage.height;
    final width = originalImage.width;

    // Let's check for the image size
    // This will be true also for upside-down photos but it's ok for me
    if (height >= width) {
      // I'm interested in portrait photos so
      // I'll just return here
      return originalFile;
    }

    // We'll use the exif package to read exif data
    // This is map of several exif properties
    // Let's check 'Image Orientation'
    final exifData = await readExifFromBytes(imageBytes);

    img.Image fixedImage;

    if (height < width) {
      logger.logInfo('Rotating image necessary');
      // rotate
      if (exifData['Image Orientation'].printable.contains('Horizontal')) {
        fixedImage = img.copyRotate(originalImage, 90);
      } else if (exifData['Image Orientation'].printable.contains('180')) {
        fixedImage = img.copyRotate(originalImage, -90);
      } else if (exifData['Image Orientation'].printable.contains('CCW')) {
        fixedImage = img.copyRotate(originalImage, 180);
      } else {
        fixedImage = img.copyRotate(originalImage, 0);
      }
    }

    // Here you can select whether you'd like to save it as png
    // or jpg with some compression
    // I choose jpg with 100% quality
    final fixedFile =
        await originalFile.writeAsBytes(img.encodeJpg(fixedImage));

    return fixedFile;
  }

//I think your understanding of Promises and async/await is not correct.
//When you make a function async, the await keyword before a promise make sure
//that code execution will pause until the promise fulfilled or rejected. 
//Your code can be simplified:

var currentPromise;

async function asyncFunc(promise){
    // Do some code here
    return await promise;
}

function anotherFunction(){
    asyncFunc(currentPromise);
}
//When we are calling return await promise, the execution engine waits in a
//non-blocking way until the promise fulfilled or rejected, and then it returns 
//the output to the anotherFunction function.

const placeholderServices = [
  PlaceholderService(
    name: 'Place Kitten',
    description:
        'Use the Internet first-class citizens as placeholders for your Website.',
    url: 'https://placekitten.com',
    pictureUrl: 'https://placekitten.com/1024/768',
    tags: ['Fun', 'Grayscale'],
  ),
  PlaceholderService(
    name: 'Fill Murray',
    description:
        'Iconic pictures of Bill Murray. Also by the same author: PlaceCage and Steven SeGallery.',
    url: 'http://www.fillmurray.com',
    pictureUrl: 'http://www.fillmurray.com/1200/900',
    tags: ['Fun', 'Grayscale', 'Open-source'],
  ),
  PlaceholderService(
    name: 'PlaceIMG',
    description:
        'Returns stock photos for a given theme, like "Architecture", "Tech" or "People". Also support sepia filter.',
    url: 'http://placeimg.com',
    pictureUrl: 'http://placeimg.com/1200/900/arch',
    tags: ['Stock photo', 'Grayscale', 'Sepia', 'Themes'],
  ),
  PlaceholderService(
    name: 'Bacon Mockup',
    description:
        'Bacon images, but not only. Also on display: briskets, pulled pork, drumsticks, ribs, etc.',
    url: 'https://baconmockup.com',
    pictureUrl: 'https://baconmockup.com/1200/900',
    tags: ['Fun'],
  ),
  PlaceholderService(
    name: 'Placemat',
    description:
        'Stock images categorized in 3 themes: "People", "Places" and "Things. Various options of customization: text, overlay, random seed.',
    url: 'https://placem.at',
    pictureUrl: 'https://placem.at/things?w=1200&h=900',
    tags: ['Stock photo', 'Text', 'Themes', 'Overlay'],
  ),
  PlaceholderService(
    name: 'Dynamic Dummy Image Generator',
    description:
        'More developer-oriented, this service returns plain mono-color images. Lots of handy options including shortcuts for different screen or ads resolutions.',
    url: 'https://dummyimage.com',
    pictureUrl: 'https://dummyimage.com/1200x900/bada55/ffffff',
    tags: ['Technical', 'Text', 'Formats'],
  ),
  PlaceholderService(
    name: 'lorempixel',
    description:
        'Another stock photo placeholder service. Note the "Abstract" category because sometimes you just want your image to represent nothing.',
    url: 'http://lorempixel.com',
    pictureUrl: 'http://lorempixel.com/1200/900/sports',
    tags: ['Stock photo', 'Text', 'Grayscale', 'Themes'],
  ),
];

Widget _signInButton() {




   return OutlineButton(


     splashColor: Colors.grey,


     onPressed: () {},


     shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(40)),


     highlightElevation: 0,


     borderSide: BorderSide(color: Colors.grey),


     child: Padding(


       padding: const EdgeInsets.fromLTRB(0, 10, 0, 10),


       child: Row(


         mainAxisSize: MainAxisSize.min,


         mainAxisAlignment: MainAxisAlignment.center,


         children: <Widget>[


           Image(image: AssetImage("assets/google_logo.png"), height: 35.0),


           Padding(


             padding: const EdgeInsets.only(left: 10),


             child: Text(


               'Sign in with Google',


               style: TextStyle(


                 fontSize: 20,


                 color: Colors.grey,


               ),


             ),


           )


         ],


       ),


     ),


   );


 }

Text('Click \u{2795} to add')
RichText( text: TextSpan( children: [ TextSpan( text: "Click ", ), WidgetSpan( child: Icon(Icons.add, size: 14), ), TextSpan( text: " to add", ), ], ), )

//no code

Center(
          child: new Container(
            padding: const EdgeInsets.all(8.0),
            height: 500.0,
            width: 500.0,
            child: new Stack(
              overflow: Overflow.visible,
              children: <Widget>[
                new Icon(Icons.pages, size: 36.0, color: Colors.red),
                new Positioned(
                  left: 20.0,
                  child: new Icon(Icons.pages, size: 36.0, color: Colors.green),
                ),
 
              ],
            ),
          ),
    )

main() {
  List customers = [];
  customers.add(Customer('Jack', 23));
  customers.add(Customer('Adam', 27));
  customers.add(Customer('Katherin', 25));

  customers.sort((a, b) => a.age.compareTo(b.age));
  print('Sort by Age: ' + customers.toString());

  customers.sort((a, b) => a.name.compareTo(b.name));
  print('Sort by Name: ' + customers.toString());
}

//Inside main.dart
return MaterialApp(
 initialRoute: WelcomeScreen.id,
 routes: {
   WelcomeScreen.id: (context) => WelcomeScreen(),
   LoginScreen.id: (context) => LoginScreen(),
   RegistrationScreen.id: (context) => RegistrationScreen(),
   ChatScreen.id: (context) => ChatScreen(),
 },
);

//Inside **** Class:
static const String id = '****';

//Inside onPressed
  onTap: () => Navigator.pushNamed(context, VideoScreen.id, arguments: {
                'chapter_name': 'Chapter $index',
              }),
              
 // In receiving side
 final  Map<String, String> received = ModalRoute.of(context).settings.arguments;
    final chapterName = received['chapter_name'];
 

import 'package:dynamicmathapp/services/auth.dart';
import 'package:firebase_database/firebase_database.dart';
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:dynamicmathapp/models/user.dart';
import 'package:cloud_firestore/cloud_firestore.dart';

class Home extends StatelessWidget {
  final AuthService _auth = AuthService();
  final ref = FirebaseDatabase.instance.reference();

  @override
  Widget build(BuildContext context) {
    User user = Provider.of<User>(context);

    return Scaffold(
      backgroundColor: Colors.brown[50],
      appBar: AppBar(
        title: Text('Welcome'),
        backgroundColor: Colors.brown[400],
        elevation: 0.0,
        actions: <Widget>[
          FlatButton.icon(
            icon: Icon(Icons.person),
            label: Text('logout'),
            onPressed: () async {
              await _auth.signOut();
            },
          ),
        ],
      ),
      body: Column(
        children: <Widget>[
          Container(
            child: Text(
              'ID:' + user.uid + 'email:' + user.email,
            ),
          ),
          RaisedButton(
            child: Text('Create Record'),
            onPressed: () {
              createRecord();
            },
          ),
          RaisedButton(
            child: Text('View Record'),
            onPressed: () {
              getData();
            },
          ),
          RaisedButton(
            child: Text('Update Record'),
            onPressed: () {
              updateData();
            },
          ),
          RaisedButton(
            child: Text('Delete Record'),
            onPressed: () {
              deleteData();
            },
          ),
        ],
      ),
    );
  }

  void createRecord() {
    ref
        .child('videos')
        .child('class_10')
        .child('chapter_1')
        .set([
      {"length": "1:3333", "title": "Introduction 1.1"},
      {"length": "2:30", "title": "Introduction 1.2"}
    ]);
  }

  void getData() {
    ref
        .child('videos')
        .child('class_10')
        .child('chapter_1')
        .once()
        .then((DataSnapshot data) {
      print(data..value);
      print(data.key);
    });
  }

  void updateData() {}

  void deleteData() {}
}

final ref = FirebaseDatabase.instance.reference();

 void getData() {
    ref
        .child('videos')
        .child('class_10')
        .child('chapter_1')
        .once()
        .then((DataSnapshot data) {
      print(data.value);
      print(data.key);
    });
  }
  
    void createRecord() {
    ref
        .child('videos')
        .child('class_10')
        .child('chapter_1')
        .set([
      {"length": "1:3333", "title": "Introduction 1.1"},
      {"length": "2:30", "title": "Introduction 1.2"}
    ]);
  }

   final databaseReference = Firestore.instance;

 
 void createRecord() async {
    Map data = {'length': '1:30:20', 'position': '1', 'title': 'Introduction 3.'};
    await databaseReference
        .collection("class_10")
        .document('chapter_4')
        .collection('videos')
        .document()
        .setData(
           data
);

//    await databaseReference.collection("classes")
//        .document('10')
//        .collection('chapter')
//        .document('1')
//    .collection('videos')
//    .document('1')
//        .setData({
//      'title': 'Introduction 1 Sets',
//      'length': '10:20'
//    });
  }

  void getData() {
    databaseReference
        .collection("class_10")
        .document('chapter_1')
        .collection('videos')
        .getDocuments()
        .then((QuerySnapshot snapshot) {
      snapshot.documents.forEach((f) => print('${f.data}}'));
    });
  }

  void updateData() {
    try {
      databaseReference
          .collection('books')
          .document('1')
          .updateData({'description': 'Head First Flutter'});
    } catch (e) {
      print(e.toString());
    }
    databaseReference
        .collection("classes")
        .document('10')
        .collection('chapter')
        .document('1')
        .collection('videos');
  }

  void deleteData() {
    try {
      databaseReference.collection('books').document('1').delete();
    } catch (e) {
      print(e.toString());
    }
  }

MaterialApp(
 debugShowCheckedModeBanner: false
)

bool isAnagram(String str1, String str2) {
  String normalize(String str) => (str
          .toLowerCase()
          .replaceAll(RegExp(r'[^a-z0-9]', caseSensitive: false), '')
          .split('')
            ..sort())
      .join('');
  return normalize(str1) == normalize(str2);
}

Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(title: Text('IntrinsicWidth')),
    body: Center(
      child: IntrinsicHeight(
        child: Row(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            RaisedButton(
              onPressed: () {},
              child: Text('Short'),
            ),
            RaisedButton(
              onPressed: () {},
              child: Text('A bit Longer'),
            ),
            RaisedButton(
              onPressed: () {},
              child: Text('The Longest text button'),
            ),
          ],
        ),
      ),
    ),
  );
}

Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(title: Text('IntrinsicWidth')),
    body: Center(
      child: IntrinsicWidth(
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            RaisedButton(
              onPressed: () {},
              child: Text('Short'),
            ),
            RaisedButton(
              onPressed: () {},
              child: Text('A bit Longer'),
            ),
            RaisedButton(
              onPressed: () {},
              child: Text('The Longest text button'),
            ),
          ],
        ),
      ),
    ),
  );
}

import 'package:flutter/material.dart';

void main() {
  runApp(
    new MaterialApp(
      title: 'Hello World App',
      home: new myApp(),
    )
  );
}

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

Text("Border test",
    style: TextStyle(
      inherit: true,
      fontSize: 48.0,
      color: Colors.pink,
      shadows: [
        Shadow( // bottomLeft
          offset: Offset(-1.5, -1.5),
          color: Colors.white
        ),
        Shadow( // bottomRight
          offset: Offset(1.5, -1.5),
          color: Colors.white
        ),
        Shadow( // topRight
          offset: Offset(1.5, 1.5),
          color: Colors.white
        ),
        Shadow( // topLeft
          offset: Offset(-1.5, 1.5),
          color: Colors.white
        ),
      ]
    ),
);

String capitalize(String s) {
  if (s == null || s.isEmpty) {
    return s;
  }
  return s.length < 1 ? s.toUpperCase() : s[0].toUpperCase() + s.substring(1);
}