ResourceObserver.swift

import Siesta

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        AwesomeAPI.ping().addObserver(self)
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

        AwesomeAPI.ping().loadIfNeeded()
    }
}

extension ViewController: ResourceObserver {
    func resourceChanged(_ resource: Resource, event: ResourceEvent) {
        if let text = resource.latestData?.text {
            print(text)
        }
    }
}