promisekit-delegate.swift

extension Foo {
    static func promise() -> Promise<FooResult> {
        return PromiseWithDelegate().promise
    }
}

class PromiseWithDelegate: FooDelegate {
    let (promise, seal) = Promise<FooResult>.pending()
    private let foo = Foo()

    init() {
        super.init()
        retainCycle = self
        foo.delegate = self // does not retain hence the `retainCycle` property

        promise.ensure {
            // ensure we break the retain cycle
            self.retainCycle = nil
        }
    }

    func fooSuccess(data: FooResult) {
        seal.fulfill(data)
    }
  
    func fooError(error: FooError) {
        seal.reject(error)
    }
}