RxSwift-using-extension.swift

struct DisposableWrapper: Disposable {
    private let disposable: Disposable
    init(disposable: Disposable) {
        self.disposable = disposable
    }
    func dispose() {
        disposable.dispose()
    }
}

public extension Observable {
    public func `do`(onDisposed: @escaping () -> ()) -> Observable {
        return Observable.using({
            return DisposableWrapper(disposable: Disposables.create(with: onDisposed))
            }, observableFactory: { _ -> Observable in
                return self
        })
    }
}

DisposableWrapper でくるまないと、以下のコンパイルエラー(´・︵・｀)

Cannot convert value of type '() -> Disposable' to expected argument type '() -> _'

Observable.using does not accept resourceFactory of type () -> Disposable · Issue #925 · ReactiveX/RxSwiftかな？