iap_init.dart

// Subscribe to any incoming purchases at app initialization. These can
// propagate from either storefront so it's important to listen as soon as
// possible to avoid losing events.
class _MyAppState extends State<MyApp> {
  StreamSubscription<List<PurchaseDetails>> _subscription;

  @override
  void initState() {
    final Stream purchaseUpdates =
        InAppPurchaseConnection.instance.purchaseUpdatedStream;
    _subscription = purchaseUpdates.listen((purchases) {
      _handlePurchaseUpdates(purchases);
    });
    super.initState();
  }

  @override
  void dispose() {
    _subscription.cancel();
    super.dispose();
  }
Connecting to the Storefront 
final bool available = await InAppPurchaseConnection.instance.isAvailable();
if (!available) {
  // The store cannot be reached or accessed. Update the UI accordingly.
}
Loading products for sale 
// Set literals require Dart 2.2. Alternatively, use `Set<String> _kIds = <String>['product1', 'product2'].toSet()`.
const Set<String> _kIds = {'product1', 'product2'};
final ProductDetailsResponse response = await InAppPurchaseConnection.instance.queryProductDetails(_kIds);
if (!response.notFoundIDs.isEmpty) {
    // Handle the error.
}
List<ProductDetails> products = response.productDetails;
Loading previous purchases 
final QueryPurchaseDetailsResponse response = await InAppPurchaseConnection.instance.queryPastPurchases();
if (response.error != null) {
    // Handle the error.
}
for (PurchaseDetails purchase in response.pastPurchases) {
    _verifyPurchase(purchase);  // Verify the purchase following the best practices for each storefront.
    _deliverPurchase(purchase); // Deliver the purchase to the user in your app.
    if (Platform.isIOS) {
        // Mark that you've delivered the purchase. Only the App Store requires
        // this final confirmation.
        InAppPurchaseConnection.instance.completePurchase(purchase);
    }
}