购买后，IAP收据为零

Question

根据这份文档Complete Transaction，一旦成功购买StoreKit交易，我应该将收据发送到我的后端。但是经过一些测试，收据在第一时间为零，有时必须经过3分钟才能使收据停止为零。

public func fetchReceipt() -> String? {
    if let receiptURL = Bundle.main.appStoreReceiptURL,
        let data = try? Data(contentsOf: receiptURL) {
        let data64 = data.base64EncodedString()
        return data64
    } else {
        return nil
    }
}

private func processTransactions(_ queue: SKPaymentQueue,
                                 updatedTransactions transactions: [SKPaymentTransaction]) {
    for transaction in transactions {
        switch transaction.transactionState {
        case .purchased:
            complete(transaction: transaction)
        case .failed:
            fail(transaction: transaction)
        case .restored:
            restore(transaction: transaction)
        case .deferred:
            print("IAPHelper: deferred...")
        case .purchasing:
            print("IAPHelper: purchasing...")
        default:
            break
        }
    }
}


private func complete(transaction: SKPaymentTransaction) {        
    SKPaymentQueue.default().finishTransaction(transaction)
    if let receipt = fetchReceipt() {
        deliverPurchaseNotificationFor(identifier: transaction.payment.productIdentifier)
    } else {
        print("receipt is nil")
    }
}