ApplePurchaseService - Buy

ApplePurchaseService.cs

//NOTE: should be a singleton for your entire app
public class ApplePurchaseService : PurchaseService
{
    private readonly ObserverDelegate _observer = new ObserverDelegate();

    public ApplePurchaseService()
    {
        SKPaymentQueue.DefaultQueue.AddTransactionObserver(_observer);
    }
    
    protected async override Task<Receipt> BuyNative(Purchase purchase)
    {
        var product = (SKProduct)purchase.NativeObject;
        var payment = SKPayment.CreateFrom(product);

        var source = new TaskCompletionSource<Receipt>();
        _observer.Sources[purchase.Id] = source;
        SKPaymentQueue.DefaultQueue.AddPayment(payment);
        return await source.Task;
    }
    
    private class ObserverDelegate : SKPaymentTransactionObserver
    {
        public readonly Dictionary<string, TaskCompletionSource<Receipt>> Sources = new Dictionary<string, TaskCompletionSource<Receipt>>();

        public override void UpdatedTransactions(SKPaymentQueue queue, SKPaymentTransaction[] transactions)
        {
            foreach (var transaction in transactions)
            {
                //NOTE: you have to call "Finish" on every state
                if (transaction.TransactionState != SKPaymentTransactionState.Purchasing)
                {
                    SKPaymentQueue.DefaultQueue.FinishTransaction(transaction);
                }

                TaskCompletionSource<Receipt> source;
                if (Sources.TryGetValue(transaction.Payment.ProductIdentifier, out source))
                {
                    switch (transaction.TransactionState)
                    {
                        case SKPaymentTransactionState.Failed:
                            source.TrySetException(new Exception(transaction.Error?.LocalizedDescription ?? "Unknown Error"));
                            break;
                        case SKPaymentTransactionState.Purchased:
                        case SKPaymentTransactionState.Restored:

                            //First make sure the receipt exists
                            var url = NSBundle.MainBundle.AppStoreReceiptUrl;
                            if (!NSFileManager.DefaultManager.FileExists(url.Path))
                            {
                                source.TrySetException(new Exception("No app store receipt file found!"));
                                return;
                            }

                            //If the NSData is null
                            var data = NSData.FromUrl(url);
                            if (data == null)
                            {
                                source.TrySetException(new Exception("Could not load app store receipt!"));
                                return;
                            }

                            var receipt = new AppleReceipt
                            {
                                BundleId = NSBundle.MainBundle.BundleIdentifier,
                                Id = transaction.Payment.ProductIdentifier,
                                TransactionId = transaction.TransactionIdentifier,
                                Data = data.ToArray(),
                            };
                            source.TrySetResult(receipt);
                            break;
                        default:
                            break;
                        }
                    }
                }
            }
        }
    }
}