Mostly taken from https://github.com/flutter/plugins/blob/master/packages/in_app_purchase/example/lib/main.dart, i just change class name

gistfile1.txt

import 'package:flutter/material.dart';
import 'package:pigment/pigment.dart';
import 'package:prj035apps/src/blocs/auth/forgot_password_bloc.dart';
import 'package:prj035apps/src/resources/index.dart';
import 'package:prj035apps/src/translations.dart';
import 'package:prj035apps/src/ui/widgets/custom_textfield.dart';
import 'package:prj035apps/src/ui/widgets/loading.dart';
import 'package:prj035apps/src/blocs/analytics/analytics_firebase.dart';
import 'package:font_awesome_flutter/font_awesome_flutter.dart';
import 'package:superellipse_shape/superellipse_shape.dart';
import 'package:prj035apps/src/blocs/admob/admob_bloc.dart';
import 'package:firebase_admob/firebase_admob.dart';
import 'dart:async';
import 'dart:io';
import 'package:in_app_purchase/in_app_purchase.dart';
import 'consumable_store.dart';


const bool kAutoConsume = true;

const String _kConsumableId = 'consumable';
const List<String> _kProductIds = <String>[
  _kConsumableId,
  'upgrade',
  'subscription'
];

class PermanentSubscriberScreen extends StatefulWidget {

  @override
  _PermanentSubscriberScreenState createState() => _PermanentSubscriberScreenState();

}

class _PermanentSubscriberScreenState extends State<PermanentSubscriberScreen>{

  final InAppPurchaseConnection _connection = InAppPurchaseConnection.instance;
  StreamSubscription<List<PurchaseDetails>> _subscription;
  List<String> _notFoundIds = [];
  List<ProductDetails> _products = [];
  List<PurchaseDetails> _purchases = [];
  List<String> _consumables = [];
  bool _isAvailable = false;
  bool _purchasePending = false;
  bool _loading = true;
  String _queryProductError = null;

  @override
  void initState() {
    Stream purchaseUpdated =
        InAppPurchaseConnection.instance.purchaseUpdatedStream;
    _subscription = purchaseUpdated.listen((purchaseDetailsList) {
      _listenToPurchaseUpdated(purchaseDetailsList);
    }, onDone: () {
      _subscription.cancel();
    }, onError: (error) {
      // handle error here.
    });
    initStoreInfo();
    super.initState();
  }

  Future<void> initStoreInfo() async {
    final bool isAvailable = await _connection.isAvailable();
    if (!isAvailable) {
      setState(() {
        _isAvailable = isAvailable;
        _products = [];
        _purchases = [];
        _notFoundIds = [];
        _consumables = [];
        _purchasePending = false;
        _loading = false;
      });
      return;
    }

    ProductDetailsResponse productDetailResponse =
    await _connection.queryProductDetails(_kProductIds.toSet());
    if (productDetailResponse.error != null) {
      setState(() {
        _queryProductError = productDetailResponse.error.message;
        _isAvailable = isAvailable;
        _products = productDetailResponse.productDetails;
        _purchases = [];
        _notFoundIds = productDetailResponse.notFoundIDs;
        _consumables = [];
        _purchasePending = false;
        _loading = false;
      });
      return;
    }

    if (productDetailResponse.productDetails.isEmpty) {
      setState(() {
        _queryProductError = null;
        _isAvailable = isAvailable;
        _products = productDetailResponse.productDetails;
        _purchases = [];
        _notFoundIds = productDetailResponse.notFoundIDs;
        _consumables = [];
        _purchasePending = false;
        _loading = false;
      });
      return;
    }

    final QueryPurchaseDetailsResponse purchaseResponse =
    await _connection.queryPastPurchases();
    if (purchaseResponse.error != null) {
      // handle query past purchase error..
    }
    final List<PurchaseDetails> verifiedPurchases = [];
    for (PurchaseDetails purchase in purchaseResponse.pastPurchases) {
      if (await _verifyPurchase(purchase)) {
        verifiedPurchases.add(purchase);
      }
    }
    List<String> consumables = await ConsumableStore.load();
    setState(() {
      _isAvailable = isAvailable;
      _products = productDetailResponse.productDetails;
      _purchases = verifiedPurchases;
      _notFoundIds = productDetailResponse.notFoundIDs;
      _consumables = consumables;
      _purchasePending = false;
      _loading = false;
    });
  }

  @override
  void dispose() {
    _subscription.cancel();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    List<Widget> stack = [];
    if (_queryProductError == null) {
      stack.add(
        ListView(
          children: [
            _buildConnectionCheckTile(),
            _buildProductList(),
            _buildConsumableBox(),
          ],
        ),
      );
    } else {
      stack.add(Center(
        child: Text(_queryProductError),
      ));
    }
    if (_purchasePending) {
      stack.add(
        Stack(
          children: [
            new Opacity(
              opacity: 0.3,
              child: const ModalBarrier(dismissible: false, color: Colors.grey),
            ),
            new Center(
              child: new CircularProgressIndicator(),
            ),
          ],
        ),
      );
    }

    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('IAP Example'),
        ),
        body: Stack(
          children: stack,
        ),
      ),
    );
  }

  Card _buildConnectionCheckTile() {
    if (_loading) {
      return Card(child: ListTile(title: const Text('Trying to connect...')));
    }
    final Widget storeHeader = ListTile(
      leading: Icon(_isAvailable ? Icons.check : Icons.block,
          color: _isAvailable ? Colors.green : ThemeData.light().errorColor),
      title: Text(
          'The store is ' + (_isAvailable ? 'available' : 'unavailable') + '.'),
    );
    final List<Widget> children = <Widget>[storeHeader];

    if (!_isAvailable) {
      children.addAll([
        Divider(),
        ListTile(
          title: Text('Not connected',
              style: TextStyle(color: ThemeData.light().errorColor)),
          subtitle: const Text(
              'Unable to connect to the payments processor. Has this app been configured correctly? See the example README for instructions.'),
        ),
      ]);
    }
    return Card(child: Column(children: children));
  }

  Card _buildProductList() {
    if (_loading) {
      return Card(
          child: (ListTile(
              leading: CircularProgressIndicator(),
              title: Text('Fetching products...'))));
    }
    if (!_isAvailable) {
      return Card();
    }
    final ListTile productHeader = ListTile(
        title: Text('Products for Sale',
            style: Theme.of(context).textTheme.headline));
    List<ListTile> productList = <ListTile>[];
    if (!_notFoundIds.isEmpty) {
      productList.add(ListTile(
          title: Text('[${_notFoundIds.join(", ")}] not found',
              style: TextStyle(color: ThemeData.light().errorColor)),
          subtitle: Text(
              'This app needs special configuration to run. Please see example/README.md for instructions.')));
    }

    // This loading previous purchases code is just a demo. Please do not use this as it is.
    // In your app you should always verify the purchase data using the `verificationData` inside the [PurchaseDetails] object before trusting it.
    // We recommend that you use your own server to verity the purchase data.
    Map<String, PurchaseDetails> purchases =
    Map.fromEntries(_purchases.map((PurchaseDetails purchase) {
      if (Platform.isIOS) {
        InAppPurchaseConnection.instance.completePurchase(purchase);
      }
      return MapEntry<String, PurchaseDetails>(purchase.productID, purchase);
    }));
    productList.addAll(_products.map(
          (ProductDetails productDetails) {
        PurchaseDetails previousPurchase = purchases[productDetails.id];
        return ListTile(
            title: Text(
              productDetails.title,
            ),
            subtitle: Text(
              productDetails.description,
            ),
            trailing: previousPurchase != null
                ? Icon(Icons.check)
                : FlatButton(
              child: Text(productDetails.price),
              color: Colors.green[800],
              textColor: Colors.white,
              onPressed: () {
                PurchaseParam purchaseParam = PurchaseParam(
                    productDetails: productDetails,
                    applicationUserName: null,
                    sandboxTesting: true);
                if (productDetails.id == _kConsumableId) {
                  _connection.buyConsumable(
                      purchaseParam: purchaseParam,
                      autoConsume: kAutoConsume || Platform.isIOS);
                } else {
                  _connection.buyNonConsumable(
                      purchaseParam: purchaseParam);
                }
              },
            ));
      },
    ));

    return Card(
        child:
        Column(children: <Widget>[productHeader, Divider()] + productList));
  }

  Card _buildConsumableBox() {
    if (_loading) {
      return Card(
          child: (ListTile(
              leading: CircularProgressIndicator(),
              title: Text('Fetching consumables...'))));
    }
    if (!_isAvailable || _notFoundIds.contains(_kConsumableId)) {
      return Card();
    }
    final ListTile consumableHeader = ListTile(
        title: Text('Purchased consumables',
            style: Theme.of(context).textTheme.headline));
    final List<Widget> tokens = _consumables.map((String id) {
      return GridTile(
        child: IconButton(
          icon: Icon(
            Icons.stars,
            size: 42.0,
            color: Colors.orange,
          ),
          splashColor: Colors.yellowAccent,
          onPressed: () => consume(id),
        ),
      );
    }).toList();
    return Card(
        child: Column(children: <Widget>[
          consumableHeader,
          Divider(),
          GridView.count(
            crossAxisCount: 5,
            children: tokens,
            shrinkWrap: true,
            padding: EdgeInsets.all(16.0),
          )
        ]));
  }

  Future<void> consume(String id) async {
    await ConsumableStore.consume(id);
    final List<String> consumables = await ConsumableStore.load();
    setState(() {
      _consumables = consumables;
    });
  }

  void showPendingUI() {
    setState(() {
      _purchasePending = true;
    });
  }

  void deliverProduct(PurchaseDetails purchaseDetails) async {
    // IMPORTANT!! Always verify a purchase purchase details before delivering the product.
    if (purchaseDetails.productID == _kConsumableId) {
      await ConsumableStore.save(purchaseDetails.purchaseID);
      List<String> consumables = await ConsumableStore.load();
      setState(() {
        _purchasePending = false;
        _consumables = consumables;
      });
    } else {
      setState(() {
        _purchases.add(purchaseDetails);
        _purchasePending = false;
      });
    }
  }

  void handleError(IAPError error) {
    setState(() {
      _purchasePending = false;
    });
  }

  Future<bool> _verifyPurchase(PurchaseDetails purchaseDetails) {
    // IMPORTANT!! Always verify a purchase before delivering the product.
    // For the purpose of an example, we directly return true.
    return Future<bool>.value(true);
  }

  void _handleInvalidPurchase(PurchaseDetails purchaseDetails) {
    // handle invalid purchase here if  _verifyPurchase` failed.
  }

  static ListTile buildListCard(ListTile innerTile) =>
      ListTile(title: Card(child: innerTile));

  void _listenToPurchaseUpdated(List<PurchaseDetails> purchaseDetailsList) {
    purchaseDetailsList.forEach((PurchaseDetails purchaseDetails) async {
      if (purchaseDetails.status == PurchaseStatus.pending) {
        showPendingUI();
      } else {
        if (purchaseDetails.status == PurchaseStatus.error) {
          handleError(purchaseDetails.error);
        } else if (purchaseDetails.status == PurchaseStatus.purchased) {
          bool valid = await _verifyPurchase(purchaseDetails);
          if (valid) {
            deliverProduct(purchaseDetails);
          } else {
            _handleInvalidPurchase(purchaseDetails);
          }
        }
        if (Platform.isIOS) {
          InAppPurchaseConnection.instance.completePurchase(purchaseDetails);
        } else if (Platform.isAndroid) {
          if (!kAutoConsume && purchaseDetails.productID == _kConsumableId) {
            InAppPurchaseConnection.instance.consumePurchase(purchaseDetails);
          }
        }
      }
    });
  }
}