In-App-Purchase In Flutter With Revenuecat

This is straight forward integration guide

Note: If you have not done revenuecat setup with Apple App Store & Google Play Store.
You can follow the below article for the initial setup.
https://devkrest.com/blog/how-to-setup-a-revenuecat-with-google-&-apple-for-in-app-purchase

Step - 1: Add Dependency in pubspec.yaml

purchases_flutter: ^6.24.0

Step - 2: Create StoreConfig Class

import 'package:purchases_flutter/purchases_flutter.dart';

class StoreConfig {
  final Store store;
  final String apiKey;
  static StoreConfig? _instance;

  factory StoreConfig({required Store store, required String apiKey}) {
    _instance ??= StoreConfig._internal(store, apiKey);
    return _instance!;
  }

  StoreConfig._internal(this.store, this.apiKey);

  static StoreConfig get instance {
    return _instance!;
  }

  static bool isForAppleStore() => instance.store == Store.appStore
      || instance.store == Store.macAppStore;

  static bool isForGooglePlay() => instance.store == Store.playStore;

  static bool isForAmazonAppstore() =>
      instance.store == Store.amazon;
}

Create StoreConfig File

Step - 2: Create RevenueCat Service - (We consider only one plan - premium)

import 'package:purchases_flutter/purchases_flutter.dart';
import 'store_config.dart';

class RevenueCatService {
 static const planEntitlementKey = 'premium';
  //It Will Be Singleton Class
  static final RevenueCatService _singleton = RevenueCatService._internal();
  factory RevenueCatService() {
    return _singleton;
  }
  RevenueCatService._internal();

  bool get isPremium => customerInfo()
              ?.entitlements
              .active
              .containsKey(AppCredential.entitlementKey) ??
          false
      ? isExpired
      : false;

  bool get isExpired {
    var list = customerInfo()?.entitlements.active.entries.where((element) {
      // logger.i(element.value.expirationDate);
      return element.value.expirationDate != null
          ? DateTime.parse(element.value.expirationDate!)
                  .microsecondsSinceEpoch <
              DateTime.now().microsecondsSinceEpoch
          : false;
    });
    return list?.isEmpty ?? false;
  }

  Offering? defaultOffering;
  CustomerInfo? customerInfo ;


  //Call this method in the main 
  Future<void> init() async {
    try {
      if (Platform.isIOS || Platform.isMacOS) {
        StoreConfig(
          store: Store.appStore,
          apiKey: {your apple key from revenuecat},
        );
      } else if (Platform.isAndroid) {
        // Run the app passing --dart-define=AMAZON=true
        const useAmazon = bool.fromEnvironment("amazon");
        StoreConfig(
          store: useAmazon ? Store.amazon : Store.playStore,
          apiKey: useAmazon
              ? "{Your AmazonKey from revenuecat}"
              : {Your google play key from revenuecat},
        );
      }
    } catch (e) {
      logger.e(e);
    }
  }

  Future<void> _configureSDK() async {
    await Purchases.setLogLevel(LogLevel.debug);

    PurchasesConfiguration configuration;
    if (StoreConfig.isForAmazonAppstore()) {
      configuration = AmazonConfiguration(StoreConfig.instance.apiKey);
    } else {
      configuration = PurchasesConfiguration(StoreConfig.instance.apiKey);
    }
    configuration.entitlementVerificationMode =
        EntitlementVerificationMode.informational;
    await Purchases.configure(configuration);
    await Purchases.enableAdServicesAttributionTokenCollection();
    var offerings = await Purchases.getOfferings();
    defaultOffering = offerings.getOffering("default");
}

Future<void> initPlatformState(String userId){
    await Purchases.logIn(userId);
 final customerInfo = await Purchases.getCustomerInfo();
 this.customerInfo = customerInfo;
  if(isPremium){
//Perform some tasks if user is premium
  }else{
//Perform some tasks if user is not premium 
  }

}

Future<void> restorePurchase(){
 final customerInfo = await Purchases.restorePurchases();
 this.customerInfo = customerInfo;
  if(isPremium){

//Perform some tasks if user is premium
  }else{
//Perform some tasks if user is not premium 
  }
}
}

Step 3: Make a purchase function in revenuecat service

class RevenueCatService {
//....
Future<void> purchasePackage(Package package){
   try{
   final customerInfo = await Purchases.purchasePackage(package);
   this.customerInfo = customerInfo;
   if(isPremium){

//Perform some tasks if user is premium
   }else{
//Perform some tasks if user is not premium 
   }
  }on PlatformException catch (e) {
      final errorCode = PurchasesErrorHelper.getErrorCode(e);
      if (errorCode == PurchasesErrorCode.purchaseCancelledError) {
    
        throw Exception("$errorCode : User Cancelled");
      } else if (errorCode == PurchasesErrorCode.purchaseNotAllowedError) {

        throw Exception("$errorCode : User not allowed to purchase");
      } else if (errorCode == PurchasesErrorCode.paymentPendingError) {
     
        throw Exception("$errorCode : Payment is pending");
      }  else if (errorCode == PurchasesErrorCode.productAlreadyPurchasedError) {
        try {
          await restorePurchase();
        } catch (e) {
          logger.e(e);
        }
       
         throw Exception("$errorCode : Subscription used by other user. please try to restore membership");
      
      } 
      throw Exception("$errorCode");
    }
}
}

Purchase function will be called when the user makes a purchase.

Step 3: Initialization of RevenueCat service


void main() async {
  WidgetsFlutterBinding.ensureInitialized();

  await RevenueCatService().init();

  runApp(const MyApp());
}

Step 4: Call the login method when the user is logged in.

await RevenueCatService().initPlatformState(userId);

Need to pass user's id

Step 5: Create a purchase page that has a list of packages to buy

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Purchase Page"),
      ),
      body: ListView.separated(
        padding: EdgeInsets.symmetric(horizontal: 20, vertical: 20),
        itemBuilder: (context, index) => ListTile(
          onTap: () {
            RevenueCatService().purchasePackage(
                RevenueCatService().offering!.availablePackages[index]);
          },
          title: Text(
              "${RevenueCatService().offering!.availablePackages[index].storeProduct.title}"),
          subtitle: Text(
              "${RevenueCatService().offering!.availablePackages[index].storeProduct.priceString}"),
        ),
        itemCount: RevenueCatService().offering!.availablePackages.length,
        separatorBuilder: (BuildContext context, int index) {
          return SizedBox(
            height: 10,
          );
        },
      ),
    );
  }
}

On this page we are calling to make a purchase
note this page must be open after the initialization of revenuecat service

Use the below API to verify whether the user is premium or not:

RevenueCatService().isPremium();//returns true or false