Flutter. How to check that autorenewal subscription is still valid

Question

My app has a 1 month autorenewal subscription. When the user clicks on a "Buy a subscription" button I am saving date of purchase to shared preferences. Then, after 1 month, I need to check is this subscription is still valid. So how can I implement it?

Answer 1

==== UPDATE from 11.03.2020

Hi, I can see this post still reading by people who looking for a method of how to work with subscription in Flutter. During 2019 I made two apps with thousands installs where users can buy a renewable subscription on the 2 platforms. Until February 2020 I used for this package from Flutter team https://pub.dev/packages/in_app_purchase, BUT - there is no way to get info about the user to unsubscribe in iOS. This is not the plugin issue, but the iOS approach for the process. We should implement our own backend for security reasons (by the way Google also recommends to do the same, but still left the way to check state directly from the app).

So, after some researches, I found guys who made backend and plugin and it is free until you have less than 10 000 USD revenue for the month. https://www.revenuecat.com/ https://pub.dev/packages/purchases_flutter

I've implemented this plugin in my apps and it works like a charm. There is some good approaches that allow you to get a subscription state at any point in the app. I'm going to make an example and article, but not sure about the timing.

====

UPDATE from 15.07.2019. Just to save time. The answer below was given for an outdated plugin for payments. After that Flutter team made plugin https://pub.dev/packages/in_app_purchase and I recommend using it.

=====

The best way is to use a secure backend server for receiving Real-time Developer Notifications. But, it is possible to check status directly in the application. So, when user tries to get access to some paid functionality you can check whether his subscription is active or not. Below is the example:

Create somewhere the file with the class

import 'dart:io' show Platform;
import 'package:flutter/services.dart';
import 'package:flutter_inapp_purchase/flutter_inapp_purchase.dart';
import 'dart:async';

class SubcsriptionStatus {
static Future<bool> subscriptionStatus(
  String sku,
  [Duration duration = const Duration(days: 30),
  Duration grace = const Duration(days: 0)]) async {
    if (Platform.isIOS) {
      var history = await FlutterInappPurchase.getPurchaseHistory();

      for (var purchase in history) {
        Duration difference =
        DateTime.now().difference(purchase.transactionDate);
        if (difference.inMinutes <= (duration + grace).inMinutes &&
            purchase.productId == sku) return true;
      }
      return false;
    } else if (Platform.isAndroid) {
      var purchases = await FlutterInappPurchase.getAvailablePurchases();

      for (var purchase in purchases) {
        if (purchase.productId == sku) return true;
      }
      return false;
    }
    throw PlatformException(
        code: Platform.operatingSystem, message: "platform not supported");
  }
}

Import it where you need to check subscription status and use in Constructor. For example:

class _SubscriptionState extends State<Subscription> {
  bool userSubscribed;
  _SubscriptionState() {
  SubcsriptionStatus.subscriptionStatus(iapId, const Duration(days: 30), const 
  Duration(days: 0)).then((val) => setState(() {
  userSubscribed = val;
   }));
   }
}

In variable userSubscribed will be the state - true or false. (Please note you have to add flutter_inapp_purchase to your project).