object SessionController {

...

    fun signIn(username: String, password: String, signInCallBack: SignInCallBack) {
        sessionApi.attemptSignIn(username,password,object: SignInCallBack{
            override fun onSignInComplete() {
                signInCallBack.onSignInComplete()
            }

            override fun onErrorOccurred(errorCode: Int, errorMessage: String) {
                signInCallBack.onErrorOccurred(errorCode)
            }

        })
    }
    ....
}