Send and decode CPMs from elements/sessions

[porter-stripe]

Summary

• We send CPM ids from merchants to elements/session then get them back and decode them
• A following PR will render these CPMs in MPE

Motivation

• CPMs!

Testing

• Manual
• New unit tests

Changelog

N/A

[github-actions]

🚨 New dead code detected in this PR:

STPElementsSession.swift:50 warning: Property 'customPaymentMethods' is assigned, but never used

Please remove the dead code before merging.

If this is intentional, you can bypass this check by adding the label skip dead code check to this PR.

ℹ️ If this comment appears to be left in error, double check that the flagged code is actually used and/or make sure your branch is up-to-date with master.

[porter-stripe]

Noting that all these properties (besides type) are intentionally optional, see an example response here for why: https://github.com/stripe/stripe-ios/pull/4637/files#diff-c71d9097e968f7c5fd01a4cb9425c80f53998412b3026808eca160f1ca08b35fR139

[wooj-stripe]

(Just a comment, no changes required, but something to think about for the future:)

I verified that you have modeled this correctly based on there response object, but feels like we need to add a layer of abstraction or something -- e.g. if error is nil, then i'd expect displayName, logoUrlandisPreset` not to be nil -- and if error has a value, then i'd expect those other values to be nil.

[joyceqin-stripe]

What does it mean for isPreset to be nil vs false?

[porter-stripe]

Good question, I touched on it in this comment: #4637 (comment)

For more of an explanation though, this is optional based on the behavior of the backend. There are instances where the backend may not return us "is_preset" in the response so our model client-side needs to handle that. The case where isPreset is nil, is the case where there is no value specified by the backend. The goal of CustomPaymentMethod is to match the backend definition/behavior.

[joyceqin-stripe]

Why do we need to check both response[customPaymentMethodDataKey] != nil and !(response[customPaymentMethodDataKey] is NSNull)?

[porter-stripe]

If you look higher up in this function you will see we do this for external payment methods and customer. The reason is b/c:

1. nil check: This checks if the key doesn't exist in the response dictionary at all.
2. NSNull check: This checks if the key exists but its value is explicitly set to NSNull (null)

Here is an example:

Case 1: Key doesn't exist (nil check)

{
  "other_data": "some value",
  "payment_methods": ["card", "paypal"]
  // "custom_payment_method_data" key is completely absent
}

Case 2:

{
  "other_data": "some value",
  "payment_methods": ["card", "paypal"],
  "custom_payment_method_data": null
}

We need to be able to handle both scenarios just to be safe.

[joyceqin-stripe]

Looks fine to me

[wooj-stripe]

(Just a comment, no changes required, but something to think about for the future:)

I verified that you have modeled this correctly based on there response object, but feels like we need to add a layer of abstraction or something -- e.g. if error is nil, then i'd expect displayName, logoUrlandisPreset` not to be nil -- and if error has a value, then i'd expect those other values to be nil.

[wooj-stripe]

should logo_url be a fully qualified url: etc: https:// -- i'd expect this fail given the type is of URL

[porter-stripe]

Added https!