StoreKit payment flows: implementation details

[vinhnx]

https://github.com/bizz84/SwiftyStoreKit#payment-flows-implementation-details

Payment flows: implementation details

In order to make a purchase, two operations are needed:

• Perform a SKProductRequest to obtain the SKProduct corresponding to the product identifier.

• Submit the payment and listen for updated transactions on the SKPaymentQueue.

The framework takes care of caching SKProducts so that future requests for the same SKProduct don't need to perform a new SKProductRequest.

Payment queue

The following list outlines how requests are processed by SwiftyStoreKit.

• SKPaymentQueue is used to queue payments or restore purchases requests.
• Payments are processed serially and in-order and require user interaction.
• Restore purchases requests don't require user interaction and can jump ahead of the queue.
• SKPaymentQueue rejects multiple restore purchases calls.
• Failed transactions only ever belong to queued payment requests.
• restoreCompletedTransactionsFailedWithError is always called when a restore purchases request fails.
• paymentQueueRestoreCompletedTransactionsFinished is always called following 0 or more update transactions when a restore purchases request succeeds.
• A complete transactions handler is require to catch any transactions that are updated when the app is not running.
• Registering a complete transactions handler when the app launches ensures that any pending transactions can be cleared.
• If a complete transactions handler is missing, pending transactions can be mis-attributed to any new incoming payments or restore purchases.

The order in which transaction updates are processed is:

1. payments (transactionState: .purchased and .failed for matching product identifiers)
2. restore purchases (transactionState: .restored, or restoreCompletedTransactionsFailedWithError, or paymentQueueRestoreCompletedTransactionsFinished)
3. complete transactions (transactionState: .purchased, .failed, .restored, .deferred)

Any transactions where state is .purchasing are ignored.

See this pull request for full details about how the payment flows have been implemented.