Re: [paymentrequest] API needs equivalent of PassKit's paymentAuthorizationViewController:didAuthorizePayment:completion: callback (#23)

>Would we need some way in the request to let merchants know that calling this function is required, or would this just be something that the merchant would have to know as part of accepting apple pay for example? My hunch is the latter, but need to think more.

We have talked about the API accommodating a flag in the request that indicates to the payment instrument that there will be a second message from the Website either requesting that the payment should be completed (in the case of a credit push payment) or a notification of the status of the transaction.

Others have proposed that payment instruments [1] should deal with this out of band. i.e. The payment instrument returns a callback URL to the Website and it sends the completion message to that URL after the PSP has processed the payment.

It would be very useful if someone could capture the ApplePay flow in a flow diagram in the same way as others have been captured already [here](https://github.com/w3c/webpayments/tree/gh-pages/PaymentFlows). The WG will use these as the mechanism to ensure the API accommodates existing use cases.

[1] We're tending toward using more developer friendly language that isn't loaded with existing definitions and calling payment instruments, [payment apps](https://github.com/w3c/webpayments/wiki/A-Payments-Initiation-Architecture-for-the-Web))

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