RxSwift/Moya-如果通过函数返回,则序列不会启动

    let provider = MoyaProvider<User>()

    provider.rx.request(.Auth(username: username, password: password))
        .filterSuccessfulStatusAndRedirectCodes()
        .debug()
        .mapOptional(to: Authentication.self)
        .observeOn(MainScheduler.instance)
        .subscribe {
            event in
            print(event)
        }.disposed(by: disposeBag)

当我用上面的方法做的时候,一切都很好。

但如果我这样做:

func logIn(username: username, password: password) -> Single<Authentication?> {

    let provider = MoyaProvider<User>()

    return provider.rx.request(.Auth(username: username, password: password))
        .filterSuccessfulStatusAndRedirectCodes()
        .debug()
        .mapOptional(to: Authentication.self) 
}

然后调用函数:

self.loginIn(username: username, password: password)
        .observeOn(MainScheduler.instance)
        .subscribe {
            event in
            print(event)
        }
        .disposed(by: disposeBag)

调试输出表明它已订阅,但如果我通过函数返回它,它将不会启动序列。