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As with the app purchase, any changes in product license status will trigger the licensechanged
event; your handler for that event makes a great place to update your app status and initiate any
downloads related to the purchase. The same event will be fired if a time-limited product license
expires, which is your signal to make the purchase available again. It’s likely that you might also want to
alert the user to that status, perhaps with a toast notification or with inline messages when the user tries
to access that feature.
Receipts
Both the requestAppPurchaseAsync and requestProductPurchaseAsync methods of CurrentApp have an
option, as we saw earlier, to provide a receipt string to the completed handler of the async operation. Its
getAppReceiptAsync method also provides an all-up receipt (app and products) at any time. Generally
speaking, receipts are most useful when a service needs to validate that an app is authorized to use
certain functionality. The app acquires the receipt and sends it to the service, which can then do
whatever validation it requires.
In all cases, the receipt is an XML string that contains information such as the app or product id, the
dates when the purchase was made and when the receipt was issued, and a digital signature. The details
of the XML schema can be found on the reference page linked above.
As an example, here’s the receipt string provided from requestProductPurchaseAsync when
purchasing Product 2 in Scenario 2 of the sample:
Here’s what Scenario 5 of the same sample receives from getAppReceiptAsync: