promisekit-ui.swift

class ViewController: UIViewController {
    private let (promise, seal) = Guarantee<...>.pending()

    func show(in: UIViewController) -> Promise<…> {
        in.show(self, sender: in)
        return promise
    }

    func done() {
        dismiss(animated: true)
        seal.fulfill(…)
    }
}

// use:

ViewController().show(in: self).done {
    ...
}.catch { error in
    ...
}