Usage.swift

// Example data set to URLRequest httpBody
// {
//  "query": "query HeroNameAndFriends($episode: Episode) { hero(episode: $episode) { name friends { name } } }",
//  "operationName": "",
//  "variables": { "episode": "JEDI" }
// }
struct Payload: Decodable {
  let query: String
}
let outputData = try Data(inputStream: httpBodyStream)
let payload = try JSONDecoder().decode(Payload.self, from: outputData)
print(payload.query) 
// Prints: query HeroNameAndFriends($episode: Episode) { hero(episode: $episode) { name friends { name } } }