When do you use Network.framework (NWConnection, NWListener, NWPathMonitor) instead of URLSession, and how does a connection move through its states?

Predict first

A loopback echo server and client. What happens?

import Foundation
import Network

let listener = try NWListener(using: .tcp, on: .any)
let ready = DispatchSemaphore(value: 0)
listener.newConnectionHandler = { conn in
    conn.start(queue: .global())
    conn.receive(minimumIncompleteLength: 1, maximumLength: 100) { data, _, _, _ in
        let msg = String(decoding: data ?? Data(), as: UTF8.self)
        conn.send(content: Data("echo:\(msg)".utf8), completion: .contentProcessed { _ in })
    }
}
listener.stateUpdateHandler = { if case .ready = $0 { ready.signal() } }
listener.start(queue: .global())
ready.wait()

let conn = NWConnection(host: "127.0.0.1", port: listener.port!, using: .tcp)
let done = DispatchSemaphore(value: 0)
nonisolated(unsafe) var states: [String] = []
conn.stateUpdateHandler = { s in
    switch s {
    case .preparing: states.append("preparing")
    case .ready:
        states.append("ready")
        conn.send(content: Data("hello".utf8), completion: .contentProcessed { _ in })
        conn.receive(minimumIncompleteLength: 1, maximumLength: 100) { data, _, _, _ in
            print("received:", String(decoding: data ?? Data(), as: UTF8.self))
            done.signal()
        }
    default: break
    }
}
conn.start(queue: .global())
done.wait()
print("states:", states)
conn.cancel(); listener.cancel()

• received: echo:hello / states: ["preparing", "ready"]
• It fails with connection refused
• received: hello / states: ["ready"]