mvvmFX example implementing a registration form. Dependencies needed: `de.saxsys:mvvmfx` and `eu.lestard:advanced-bindings`

MvvmfxExampleApp.java

import javafx.application.Application;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

import de.saxsys.mvvmfx.FluentViewLoader;

public class MvvmfxExampleApp extends Application {

	public static void main(String... args) {
		launch(args);
	}
	
	@Override
	public void start(Stage stage) throws Exception {
		final Parent root = FluentViewLoader.fxmlView(RegisterView.class).load().getView();
		
		stage.setScene(new Scene(root));
		stage.show();
	}
}

RegisterView.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.geometry.*?>
<?import javafx.scene.control.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>


<AnchorPane fx:controller="RegisterView" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" xmlns="http://javafx.com/javafx/8.0.40" xmlns:fx="http://javafx.com/fxml/1">
   <children>
      <VBox spacing="5.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
         <children>
            <Label style="-fx-font-size: 2em; -fx-text-fill: #555;" text="Registrierungsformular" />
            <Separator prefWidth="200.0" />
            <GridPane hgap="5.0" vgap="5.0" VBox.vgrow="ALWAYS">
              <columnConstraints>
                <ColumnConstraints hgrow="SOMETIMES" maxWidth="185.0" minWidth="10.0" prefWidth="133.0" />
                <ColumnConstraints hgrow="SOMETIMES" maxWidth="145.0" minWidth="10.0" prefWidth="145.0" />
              </columnConstraints>
              <rowConstraints>
                <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
                  <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
              </rowConstraints>
               <children>
                  <Label text="Nutzername:" />
                  <Label text="Passwort:" GridPane.rowIndex="1" />
                  <Label text="Passwort bestätigen:" GridPane.rowIndex="2" />
                  <Button fx:id="registerButton" defaultButton="true" mnemonicParsing="false" text="Registrieren" GridPane.columnIndex="1" GridPane.rowIndex="3" />
                  <TextField fx:id="usernameInput" GridPane.columnIndex="1" />
                  <PasswordField fx:id="passwordInput" GridPane.columnIndex="1" GridPane.rowIndex="1" />
                  <PasswordField fx:id="confirmInput" GridPane.columnIndex="1" GridPane.rowIndex="2" />
               </children>
            </GridPane>
         </children>
         <padding>
            <Insets bottom="5.0" left="5.0" right="5.0" top="5.0" />
         </padding>
      </VBox>
   </children>
</AnchorPane>

RegisterView.java

import de.saxsys.mvvmfx.FxmlView;
import de.saxsys.mvvmfx.InjectViewModel;
import javafx.fxml.FXML;
import javafx.scene.control.Button;
import javafx.scene.control.PasswordField;
import javafx.scene.control.TextField;

public class RegisterView implements FxmlView<RegisterViewModel> {

	@FXML
	public TextField usernameInput;
	@FXML
	public PasswordField passwordInput;
	@FXML
	public PasswordField confirmInput;
	@FXML
	public Button registerButton;

	@InjectViewModel
	private RegisterViewModel viewModel;
	
	public void initialize() {
		usernameInput.textProperty().bindBidirectional(viewModel.usernameProperty());
		passwordInput.textProperty().bindBidirectional(viewModel.passwordProperty());
		confirmInput.textProperty().bindBidirectional(viewModel.confirmPasswordProperty());
		
		registerButton.disableProperty().bind(viewModel.registerButtonEnabledProperty().not());
	}
}

RegisterViewModel.java

import de.saxsys.mvvmfx.ViewModel;
import eu.lestard.advanced_bindings.api.LogicBindings;
import javafx.beans.binding.Bindings;
import javafx.beans.binding.BooleanBinding;
import javafx.beans.property.ReadOnlyBooleanProperty;
import javafx.beans.property.ReadOnlyBooleanWrapper;
import javafx.beans.property.SimpleStringProperty;
import javafx.beans.property.StringProperty;

public class RegisterViewModel implements ViewModel{
	
	private StringProperty username = new SimpleStringProperty("");
	private StringProperty password = new SimpleStringProperty("");
	private StringProperty confirmPassword = new SimpleStringProperty("");
	private ReadOnlyBooleanWrapper registerButtonEnabled = new ReadOnlyBooleanWrapper();
	
	public RegisterViewModel() {
		
		final BooleanBinding passwordsEqual = Bindings.createBooleanBinding(() -> {
			final String pw = password.get() == null ? "" : password.get();
			final String confirm = confirmPassword.get() == null ? "" : confirmPassword.get();
			
			return pw.equals(confirm);
		}, password, confirmPassword);


		registerButtonEnabled.bind(
				LogicBindings.and(
						username.isNotEmpty(),
						password.isNotEmpty(),
						confirmPassword.isNotEmpty(),
						passwordsEqual));
	}
	
	public StringProperty usernameProperty() {
		return username;
	}

	public StringProperty passwordProperty() {
		return password;
	}

	public StringProperty confirmPasswordProperty() {
		return confirmPassword;
	}

	public ReadOnlyBooleanProperty registerButtonEnabledProperty() {
		return registerButtonEnabled.getReadOnlyProperty();
	}
}

RegisterViewModelTest.java

import org.junit.Before;
import org.junit.Test;

import static eu.lestard.assertj.javafx.api.Assertions.assertThat;

public class RegisterViewModelTest {
	
	RegisterViewModel viewModel;
	
	@Before
	public void setup() {
		viewModel = new RegisterViewModel();
	}
	
	@Test
	public void test() {
		// initial values
		assertThat(viewModel.usernameProperty()).hasValue("");
		assertThat(viewModel.passwordProperty()).hasValue("");
		assertThat(viewModel.confirmPasswordProperty()).hasValue("");
		assertThat(viewModel.registerButtonEnabledProperty()).isFalse();
		
		
		// entering username
		viewModel.usernameProperty().setValue("manuel");
		
		// still disabled
		assertThat(viewModel.registerButtonEnabledProperty()).isFalse();
		
		// enter password
		viewModel.passwordProperty().set("geheim"); // no, that's not really my password ;-)
		assertThat(viewModel.registerButtonEnabledProperty()).isFalse();
		
		// confirm password
		viewModel.confirmPasswordProperty().set("geheim");
		
		// now register button is enabled
		assertThat(viewModel.registerButtonEnabledProperty()).isTrue();
		
		// change confirm password
		viewModel.confirmPasswordProperty().set("secret");
		
		// disabled again
		assertThat(viewModel.registerButtonEnabledProperty()).isFalse();
		
		viewModel.passwordProperty().set("secret");
		
		// now both passwords are equal again
		assertThat(viewModel.registerButtonEnabledProperty()).isTrue();
		
		
		// when username is removed ...
		viewModel.usernameProperty().setValue("");
		
		// ... the button is disabled again
		assertThat(viewModel.registerButtonEnabledProperty()).isFalse();
	}
}