In iOS in-app purchase flow, what's the best way to handle interrupted transactions for unrenewing-subscription?

Question

I want to know in iOS in-app purchase flow, what's the best way to handle interrupted transactions?

Let's see the following interrupted puchasing scenario:

1.Make purchase with AppStore

SKPaymentQueue.defaultQueue().addPayment(payment)

2.On response is received

public func paymentQueue(queue: SKPaymentQueue, updatedTransactions)

We'll try to persist the purchase in our own server

3. Try to communicate with our server to persist the purchase

4. The communication failed due to unstable network

Therefore, we do not call the finishTransaction

5. The user closes the app, and relaunch

6. We should now try to resume step 3 before we can finish the transaction

However, I really don't know how to handle the following situation...

>>Problem:

It seems that if there exists an unfinished transaction, a login dialog pops up every time my app is launched even if I didn't register the observer:

SKPaymentQueue.defaultQueue().addTransactionObserver(self)

I've done the following experiment:

1. Make a purchase of nonrenewable-subscription Do not finish the transaction on purpose

2. Relaunch the app. The login dialog pops up as expected

3. Modify the code to remove the observer in my

   func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool

4. Relaunch the app. The login dialog still pops up -> not good

>>What I want:

I want to control when the login dialog pops up...as well as when the paymentQueue get called so our app can do things that actually finish the transaction (persist the purchase data to our server or download the content purchased by the user)

The reason is that we want the user to understand the login dialog is for us to finish the transaction for him, and we'll try to persist the transaction in our own server.

Therefore I need a way to trigger the login dialog as well as the following transaction update callback

public func paymentQueue(queue: SKPaymentQueue, updatedTransactions transactions: [SKPaymentTransaction])

Pls help! Thank you so much for reading this