📚 Appendix

For your info

Exception has occurred.

PlatformException (PlatformException(Error performing getDocuments, FAILED_PRECONDITION: The query requires an index. You can create it here: https://console.firebase.google.com/v1/r/project/gdgflutterfirebasechat/firestore/indexes?create_composite=ClRwcm9qZWN0cy9nZGdmbHV0dGVyZmlyZWJhc2VjaGF0L2RhdGFiYXNlcy8oZGVmYXVsdCkvY29sbGVjdGlvbkdyb3Vwcy9jaGF0cy9pbmRleGVzL18QARoMCghzZW5kZXJJZBABGgwKCHRvVXNlcklkEAEaDQoJdGltZXN0YW1wEAIaDAoIX19uYW1lX18QAg, null))

Error with Firebase sdk add for Flutter 2.8

https://stackoverflow.com/questions/52060516/how-to-change-android-minsdkversion-in-flutter-project

updatelocal.properties

flutter.minSdkVersion=21
flutter.targetSdkVersion=30
flutter.compileSdkVersion=30

Change like this from build.gradle

...
  android {
    compileSdkVersion localProperties.getProperty('flutter.compileSdkVersion').toInteger()

defaultConfig {
    minSdkVersion localProperties.getProperty('flutter.minSdkVersion').toInteger()
    targetSdkVersion localProperties.getProperty('flutter.targetSdkVersion').toInteger()
    versionCode flutterVersionCode.toInteger()
    versionName flutterVersionName
}

Error with firebase and IOS

[!] Automatically assigning platform `iOS` with version `9.0` on target `Runner` because no platform was specified. Please specify a platform for this target in your Podfile. See `https://guides.cocoapods.org/syntax/podfile.html#platform

open [YourProject]/ios/Podfile and remove the # sign from platform :ios, '9.0' and try flutter run. If it doesn't work increase the version number for example to 11.0

delete podfile.lock and run these commands

flutter clean
cd ios
pod deintegrate
pod repo update
pod install

Explain 

Talk about Provider and StreamBuilder

runApp(MultiProvider(providers: [

      Provider<AuthService>(

        create: (_) => AuthService(),

      )

    ], child: MyApp()))

home: StreamBuilder<FirebaseUser>(
        stream: Provider.of<AuthService>(context,listen: false).user,
        builder: (BuildContext context, AsyncSnapshot snapshot) {
          if (snapshot.hasData) {
            return HomeScreen();
          } else {
            return LoginScreen();
          }
        },
      ),

Index for query

https://console.firebase.google.com/v1/r/project/flutterfirebasechat-622ea/firestore/indexes?create_composite=Cldwcm9qZWN0cy9mbHV0dGVyZmlyZWJhc2VjaGF0LTYyMmVhL2RhdGFiYXNlcy8oZGVmYXVsdCkvY29sbGVjdGlvbkdyb3Vwcy9jaGF0cy9pbmRleGVzL18QARoNCgltZW1iZXJJZHMYARoTCg9yZWNlbnRUaW1lc3RhbXAQAhoMCghfX25hbWVfXxAC

Additional Screenshots

Test mode and - eu-west

You should have node installed.

Open a terminal window and navigate to the directory for your code:

Initiate your project:

$ firebase init

Deploy your functions:

$ firebase deploy