为什么ReactiveSwift Action只运行一次？

Question

我想使用ReactiveSwift实现一个简单的登录功能。

这是我的ViewController代码：

func setupRac() {
    viewmodel = SignInViewModel()

    self.viewmodel.username <~ self.usernameTextField.reactive.continuousTextValues
    self.viewmodel.password <~ self.passwordTextField.reactive.continuousTextValues

    self.loginButton.reactive.pressed = CocoaAction(viewmodel.submit) { sender in
        return (self.usernameTextField.text, self.passwordTextField.text)
    }

ViewModel：

class SignInViewModel {
let username = MutableProperty<String?>(nil)
let password = MutableProperty<String?>(nil)

let usernameValid: Property<Bool>
let passwordValid: Property<Bool>
let valid: Property<Bool>
let submit: Action<(String?, String?), String, NetworkError>

init() {
    self.usernameValid = username.map {
        return ($0 ?? "").characters.count > 0
    }

    self.passwordValid = password.map {
        return ($0 ?? "").characters.count > 0
    }

    self.valid = Property.combineLatest(self.usernameValid, self.passwordValid).map({ (usernameValid, passwordValid) in
        return usernameValid && passwordValid
    })

    self.submit = Action(enabledIf: self.valid, { username, password in
        return MembershipManager.sharedInstance.signin(username: username!, password: password!)
    })
}

当我将断点放到return MembershipManager.sharedInstance.signin(...行时，只按一次登录按钮时，它只会打一次。之后，当我再次按下按钮时，没有任何反应。

有谁知道原因？我刚刚开始使用ReactiveSwift。