Establishes a step in the network execution chain.

• operation A map representing the operation to be executed when the end of the chain is reached. Contains a :graphql key holding the information to construct a request and an :unpack key, which is a function that allows you to unpack the response.
• context A map that can contain arbitrary context to be passed through the execution chain.

The final step in the chain should return a core.async channel. Prior-steps should return the result of calling artemis.core/exec on an enclosed step.

For example, if you wanted to log the operation before making a request, you could add a step to the execution chain by doing the following:

(defn post-chan [url params]
  ;; do http post here and return a core.async channel when ready
  ;; make sure to unpack the response before putting it onto the channel
  )

(def http
  (reify
    GQLNetworkStep
    (-exec [_ operation context]
      (post-chan "/api/graphql"
                 (operation->http-params operation)))))

(defn logger [next-step]
  (reify
    GQLNetworkStep
    (-exec [_ operation context]
      (logger/log (:graphql operation))
      (artemis.core/exec next-step operation context))))

(def network-chain (-> http logger))

Any number of additional steps can be added to the chain by enclosing the execution of earlier steps.