[13.x] Preview upcoming invoice

[benjamindoe]

Noticed this issue and thought I would suggest a PR since I've done something similar in my current project. Nothing fancy. Just a very basic method that has the same signature as the swap method

$upcomingInvoice = $subscription->previewInvoice('price_to_swap')

Closes #1142

[driesvints]

Cool stuff, thanks @benjamindoe! I think we can improve this further. Can we re-use the upcomingInvoice method from the billable? I also think we need a separate general upcomingInvoice method on the subscription itself that previewUpcomingInvoice calls.

We can add an argument to upcomingInvoice here so we can pass options and don't need to re-use the stripeOptions call etc.

Also: I believe we need to re-use swapOptions as well, no?

[benjamindoe]

For sure, I kind of just carved this out of my current app without much refactoring.

I'll look into reusing swapOptions. There must have been a reason why I didn't do it. Probably the payload data is slightly different when interacting with invoices directly?

I'll look at factoring out upcomingInvoice and previewUpcomingInvoice

[benjamindoe]

So unfortunately, getSwapOptions won't work in its current state with previewing upcoming invoices. The payload is incompatible due to calling Stripe invoices directly instead of via a subscription. A good example of this is subscription_items for invoices vs. items for subscription payloads.

[driesvints]

Okay. Thanks for working on this @benjamindoe. I'll take it from here.

[driesvints]

Thanks @benjamindoe. I've finished this PR. Managed to re-use swapOption in a different way.

[benjamindoe]

Nice one... Interesting method