TornadoFX Login App - https://www.youtube.com/watch?v=Jyuf4xn0oy4

LoginApp.kt

package loginapp.app

import loginapp.views.LoginScreen
import tornadofx.App

class LoginApp : App(LoginScreen::class)

LoginController.kt

package loginapp.controllers

import javafx.beans.property.SimpleStringProperty
import loginapp.models.UserModel
import loginapp.views.LoginScreen
import loginapp.views.WelcomeScreen
import tornadofx.*

class LoginController : Controller() {
    val statusProperty = SimpleStringProperty("")
    var status by statusProperty

    val api: Rest by inject()
    val user: UserModel by inject()

    init {
        api.baseURI = "https://api.github.com/"
    }

    fun login(username: String, password: String) {
        runLater { status = "" }
        api.setBasicAuth(username, password)
        val response = api.get("user")
        val json = response.one()
        runLater {
            if (response.ok()) {
                user.item = json.toModel()
                find(LoginScreen::class).replaceWith(WelcomeScreen::class, sizeToScene = true, centerOnScreen = true)
            } else {
                status = json.string("message") ?: "Login failed"
            }
        }
    }

    fun logout() {
        user.item = null
        primaryStage.uiComponent<UIComponent>()?.replaceWith(LoginScreen::class, sizeToScene = true, centerOnScreen = true)
    }

}

LoginScreen.kt

package loginapp.views

import javafx.beans.property.SimpleStringProperty
import javafx.geometry.Orientation
import javafx.scene.paint.Color
import javafx.scene.text.FontWeight
import loginapp.controllers.LoginController
import tornadofx.*

class LoginScreen : View("Login") {
    val model = ViewModel()
    val username = model.bind { SimpleStringProperty() }
    val password = model.bind { SimpleStringProperty() }
    val loginController: LoginController by inject()

    override val root = form {
        fieldset(labelPosition = Orientation.VERTICAL) {
            field("Username") {
                textfield(username).required()
            }
            field("Password") {
                passwordfield(password).required()
            }
            button("Log in") {
                enableWhen(model.valid)
                isDefaultButton = true
                useMaxWidth = true
                action {
                    runAsyncWithProgress {
                        loginController.login(username.value, password.value)
                    }
                }
            }
        }
        label(loginController.statusProperty) {
            style {
                paddingTop = 10
                textFill = Color.RED
                fontWeight = FontWeight.BOLD
            }
        }
    }

    override fun onDock() {
        username.value = ""
        password.value = ""
        model.clearDecorators()
    }
}

User.kt

package loginapp.models

import javafx.beans.property.SimpleStringProperty
import tornadofx.*
import javax.json.JsonObject

class User : JsonModel {
    val nameProperty = SimpleStringProperty()
    var name by nameProperty

    override fun updateModel(json: JsonObject) {
        with (json) {
            name = string("name")
        }
    }
}

class UserModel : ItemViewModel<User>() {
    val name = bind(User::nameProperty)
}

WelcomeScreen.kt

package loginapp.views

import javafx.geometry.Pos
import javafx.scene.text.FontWeight
import loginapp.controllers.LoginController
import loginapp.models.UserModel
import tornadofx.*

class WelcomeScreen : View("Welcome") {
    val user: UserModel by inject()
    val loginController: LoginController by inject()

    override val root = vbox(10) {
        setPrefSize(800.0, 600.0)
        alignment = Pos.CENTER

        label(user.name) {
            style {
                fontWeight = FontWeight.BOLD
                fontSize = 24.px
            }
        }

        button("Logout").action(loginController::logout)

    }
}