add paymentmethodchange event

[marcoscaceres]

closes #662

The following tasks have been completed:

• [x] Confirmed there are no ReSpec errors/warnings.
• [x] Added Web platform tests
• [x] added MDN Docs
• [ ] added MDN compat data

Implementation commitment:

• [ ] Safari (link to issue)
• [ ] Chrome
• [x] Firefox
• [ ] Edge (public signal)

---

Preview | Diff

[marcoscaceres]

Only indirectly and in a backwards compatibile way. I hope to have a draft ready for review tomorrow.

[rsolomakhin]

No rush! Just checking.

[marcoscaceres]

Basic card integration https://github.com/w3c/payment-method-basic-card/pull/53

[marcoscaceres]

Sent tests for review https://github.com/w3c/web-platform-tests/pull/10912

[marcoscaceres]

Realized that when other events fire (e.g., onshippingaddresschange) or generally things happen on the user interaction task source (e.g., user aborts, request.abort()), the methodDetails needs to be reset to null... same with methodName.

[marcoscaceres]

Hmmm... having spec'ed this out, now I don't know if I like this design anymore 😓 Doesn't feel right that the only thing that changes .methodName and .methodData is this event.

Maybe this should switch back to having special event for this, as per https://github.com/w3c/payment-request/issues/662#issuecomment-359038775

The name paymentmethodchange is also super ambiguous tho, because it's really a "Instrument change" of a payment method. So, I I'm kind thinking "instrumentchange" is more appropriate as an event name.

I think I'd like to change this to:

PaymentMethodChangeEvent : PaymentRequestUpdateEvent {
   readonly attribute DOMString methodName;
   readonly attribute object? methodDetails;
}

So that:

request.oninstrumentchange = ev => {
  const { type: cardType } = ev.methodDetails;
  if (ev.methodName === "https://apple.com/apple-pay") {
    switch (cardType) {
      case "store":
        // do Apple Pay specific handling for store card...
        break;
    }
  }
  // finally...
  ev.updateWith(newStuff);
};

@aestes wdyt?

[marcoscaceres]

Updated this to match https://github.com/w3c/payment-request/pull/695#issuecomment-387978220

[marcoscaceres]

Updated WPTests to match updated spec.

[marcoscaceres]

@domenic, this one too should now be good to go (🤞).

[marcoscaceres]

@domenic, I think I've addressed all the feedback 🤞

[marcoscaceres]

Added link to Firefox tracking bug.

[marcoscaceres]

Added developer documentation. Please review. https://developer.mozilla.org/en-US/docs/Web/API/PaymentRequest/onpaymentmethodchange

[marcoscaceres]

PaymentMethodChangeEvent is now in Firefox Nightly behind a pref. We don't currently have any plans to fire it for Basic Card, but other payment handlers could theoretically make use of it.