setTransactionObserver method
Sets an observer to listen to all incoming transaction events.
This should be called and set as soon as the app launches in order to
avoid missing any purchase updates from the App Store. See the
documentation on StoreKit's -[SKPaymentQueue addTransactionObserver:]
.
Implementation
void setTransactionObserver(SKTransactionObserverWrapper observer) {
_observer = observer;
channel.setMethodCallHandler(handleObserverCallbacks);
}