Can I make a payment *before* creating a request?

[MantisClone]

Is it possible to build a payment app in which the payer sends a payment first and then creates a request to serve as an immutable receipt?

[MantisClone]

Yes. Payments are linked to requests via a paymentReference which is derived from the requestId which is derived from the request contents. Therefore, it is possible to calculate the paymentReference, send a payment using the paymentReference as input to the smart contract, and then create a request.

Details

• Sending a payment before creating the request can be done using functions in the payment-processor package.
• The higher-level request-client.js package doesn't support payments before the request is created.
• This paymentReference consists of the last 8 bytes of a salted hash of the requestId: last8Bytes(hash(lowercase(requestId + salt + info))):
  • requestId is the id of the request
  • salt is a random number with at least 8 bytes of randomness. It must be unique to each request
  • info is the JSON-stringified string of the paymentInfo for a payment, or refundInfo for a refund.
  • lowercase() transforms all characters to lowercase
  • hash() is a keccak256 hash function
  • last8Bytes() take the last 8 bytes

[MantisClone]

This answer was technically correct, but when it was written, the RN SDK did not support creating a Request in memory without persisting it. This means one must create the Request object manually, without using the RN SDK.

Work is being done in #1331 and #1373 to make it possible to create a Request object in memory without persisting it.