Define a serializer for PushSubscription

[beverloo]

Given this snippet:

swRegistration.pushManager.subscribe().then(function (subscription) {
  var x = JSON.stringify(subscription);
});

What is x meant to be? Today it's "{}" because attributes on an interface don't get serialized by default. This makes sense for DOM objects, but much less so for POD-like structures like this.

Since it's likely that developers will want to send all data to their application server, we should define a serializer for the interface.

I believe we can do this as follows:

interface PushSubscription {
  ...
  serializer = { endpoint, subscriptionId };
}

[martinthomson]

SGTM, minus the actual fields, over which I think we need to continue discussion.

[beverloo]

This is of course orthogonal to the actual fields, which is tracked in #56.

[bzbarsky]

@beverloo Why not just use serializer = { attributes };?

[beverloo]

@bzbarsky Even better, thanks for the suggestion!

[martinthomson]

As I've proposed in #130, one of the attributes would be an ArrayBuffer, which is not supported in the declarative syntax for serializers. That means we'd need prose for the serializer behaviour (sadly).

[owencm]

FWIW, I strongly support the proposal to define sensible JSON serialization for subscription - the fact that we no longer do this broke a very reasonable flow, causing me to do:

subscription = {endpoint: subscription.endpoint, subscriptionId: subscription.subscriptionId}

which is pretty gross imho.