private fun sendLoginRequest() {

        val progressDialog = ProgressDialog.show(
                context,
                getString(R.string.sign_in_progress_title),
                getString(R.string.sign_in_progress_message)
        )

        val username = viewModel?.signInId?.get()
        val password = viewModel?.password?.get()

        val loginRequest = LoginRequest(username, password)

        val loginDisposable = Single.fromCallable { viewModel?.doLoginRequest(loginRequest) }
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .doFinally (progressDialog::dismiss)
                .subscribe (this@LogInFragment::handleLoginResponse)

        //compositeDisposable is a member variable. compositeDisposable.dispose() is called in onDestroy()
        compositeDisposable.add(loginDisposable)
    }