iOS IAP, how to judge multiple user at the same device - ios

Question

For example: There are two different game characters in a single iPhone device,
we call it A and B.
First, app user login as A, he perform an IAP action, the A logout without the purchase completed.
Then, app user login as B, then purchase finish event arrival, this is the problem, how to judge the receipt belongs to A, not B.
I googled, found SKMutablePayment.requestData may be used for solve this problem, but Apple Document told me this is a reserved property, and must be nil, or else then payment will be reject.
requestData
Reserved for future use. (read-only)
#property(nonatomic, copy, readonly) NSData *requestData Discussion
The default value is nil. If requestData is not nil, your payment
request will be rejected.
Availability Available in iOS 3.0 and later. Declared In SKPayment.h

Answer 1

thanks for everyone's reply
finally, I found SKMutablePayment.applicationUsername can save my private account username,
but It only available on iOS7.0.
save:
SKMutablePayment* payment = [SKMutablePayment paymentWithProduct:product];
payment.applicationUsername = #"my_username_1001";
read:
-(void)paymentQueue:(SKPaymentQueue *)queue updatedTransactions:(NSArray *)transactions
{
for (SKPaymentTransaction* trans in transactions) {
switch (trans.transactionState) {
case SKPaymentTransactionStateFailed:
[[SKPaymentQueue defaultQueue] finishTransaction:trans];
break;
case SKPaymentTransactionStatePurchased:
// got private user account here
NSString* applicationUsername = [[trans payment] applicationUsername];
break;
case SKPaymentTransactionStatePurchasing:
break;
case SKPaymentTransactionStateRestored:
break;
}
}
}