[Feature] Support for subscription

[alexstrat]

As noted in Readme, today's implementation does not support subscriptions and it is, a priori, not a big deal because as noted in Readme:

in reactive-graphql everything is treated as subscriptions

Though, I figured out some benefits of having support for proper GraphQL Subscriptions:

Support conceptual difference between live queries and subscription

There is a conceptual difference between live queries and subscription that is well summarized with:

Live Queries observe data, subscriptions observe events

I stumbled upon cases where I specifically want to react to events. Example: to trigger some animations in the front-end when something happens.

Note that GQL eco-system (example: apollo-client, graphql-code-generator..) uses this distinction between the 2 concepts, and making the distinction helps to leverage the eco-system.

Use live queries as event subscription can lead to pitfalls

If one really wants to implement an event subscription with reactive-graphql, as a workaround one can use a regular reactive query:

type Query {
  postAdded: PostAddedEvent!
}

type PostAddedEvent {
  postId: Int!
}

But this can lead to emitting much more events than expected if the Post is changing live:

type Query {
  postAdded: PostAddedEvent!
}

type PostAddedEvent {
  post: Post!
}

[DanielMSchmidt]

I think you are right and I would assume it's a doable change. We need to implement the Subscription interface, there should be tests in graphql-js that we can borrow to make sure we are spec compliant