We are developing Shopify custom storefront app for Android platform using Storefront API.
For completing checkout, we have implemented steps provided for web checkout at below link for Android.
As mentioned in the accompanying note for web checkout that polling should be called to observe state of checkout completion, as web view doesn't provide any callbacks for various checkout states.
Polling for checkout completion: https://github.com/Shopify/mobile-buy-sdk-android#polling-for-checkout-completion-
The example uses onPayment graph query as node for paymentId to observe if checkout order is placed successfully.
However, paymentId is not available when creating web checkout.
How to implement polling for Android mobile web checkout completion when loading checkout URL within Android app web view component?
As stated in the docs, you can poll based on Storefront.Payment value - It is a boolean so true/false will signal a completed vs. non completed request.