Spike: Disabling card as a payment method

[FangedParakeet]

Description

A long-requested feature has been to allow store owners to disable the card payment method, exclusively presenting EPMs or APMs at checkout instead. With the current state of our plugin, this should be technically feasible and is very achievable in our V2 plugin.

However, in WooPayments the card payment method is still closely tied to the main plugin gateway, with other payment methods relying on hooks, settings, and asset enqueuing functionality provided by this main gateway. In order disable the card payment method, we will need to refactor our plugin to separate the card payment method from the core payment gateway. We will also need to refactor the main plugin so that it can be initialised without registering the card payment method as a gateway and to allow this to be controlled by the enabled flag, as is the case with all of our other payment methods.

Acceptance criteria

These are problems that we will need to solve in order to accomplish this goal. We don't need to actually solve them within this issue, but this spike is to explore their feasibility and plot out a pathway towards doing so, preferably in the form of a set of new issues.

• Card payment method can be separated from core plugin.
• Card payment method present as its own separate payment gateway with configuration similar to other APMs.
• Existing hooks, settings, and asset functionality still works as expected when card payment method is disabled.
• EPMs still work as expected when card payment method is disabled.
  • I presume that neither Link nor WooPay would be supported in this case, so we should explore this and confirm that this would be satisfactory.

[danielmx-dev]

Please add your planning poker estimate with Zenhub @FangedParakeet

[danielmx-dev]

I was able to put together a functioning proof of concept in the following branch: https://github.com/Automattic/woocommerce-payments/pull/9844/files

I've tested several flows using these changes and here are some of my most relevant findings:

• The WC_Payment_Gateway_WCPay class is used to instantiate all the Gateways for every Payment Method that exists in WooPayments. Currently, the Credit Card Gateway and the "main" Gateway use the same instance, so they are deeply intertwined. In order to be able to disable Credit Cards without affecting other functionality, we need to separate the main gateway from the credit card gateway (if not the implementation, at least create separate instances).
• The WC_Payment_Gateway_WCPay class has a strict dependency on the UPE_Payment_Method subclasses. If we want to minimize the amount of changes we need to implement a "dummy" UPE_Payment_Method subclass for the main gateway.
  • We could create a separate Gateway class that doesn't rely on UPE Payment Method, but since it's not strictly necessary to achieve the goal of disabling the credit card payment method, I didn't look into it.
• Since the Credit Card gateway will no longer be able to use the same gateway ID as the main gateway, we need to make sure that all the required references are properly updated to the new gateway ID (woocommerce_payments_card, if we follow the convention used by other payment methods). A migration will also be required to make sure this gateway is enabled in all the existing installations after updating the plugin. We'll also need to make sure that in all fresh installations the credit card gateway is enabled by default when enabling WooPayments.
• The "main" gateway is not an actual payment method, this means that it should not be displayed as an option during checkout; however, the gateway is currently used "behind the scenes" to process payments using Express Checkout, and requires the gateway to be both enabled and available. If we created separate gateways for each one of these methods then we could make the main gateway much leaner.
• We need to add new notices that prevent from enabling Link and WooPay in the Settings if the Credit Card payment method is disabled.
• As a "cleanup" task, we should use Constants or Methods as much as possible to retrieve a certain gateway instead of relying on hardcoded strings. (e.g. Use Payment_Method::CARD instead of 'card').

At this point I'm still unsure if there's a way to implement this gradually (maybe with a feature flag?) or if we require to use a main feature branch that we can merge later on. I'll start outlining the required tasks and while I think more about the specifics of the implementation.