One very common challenge that occurs when migrating from Swing to Vaadin is that blocking dialog is very common pattern in Swing. That is possible there, because Swing application is single threaded and non-distributed. Vaadin application has UI running in browser making it distributed system. Hence blocking dialog is not a natural concept. The…

BlockingDialogView.java

package com.example.application.views;

import com.vaadin.flow.component.AttachEvent;
import com.vaadin.flow.component.DetachEvent;
import com.vaadin.flow.component.Text;
import com.vaadin.flow.component.UI;
import com.vaadin.flow.component.UIDetachedException;
import com.vaadin.flow.component.button.Button;
import com.vaadin.flow.component.dialog.Dialog;
import com.vaadin.flow.component.notification.Notification;
import com.vaadin.flow.component.notification.Notification.Position;
import com.vaadin.flow.component.orderedlayout.VerticalLayout;
import com.vaadin.flow.component.progressbar.ProgressBar;
import com.vaadin.flow.router.PageTitle;
import com.vaadin.flow.router.Route;
import com.vaadin.flow.server.Command;

import java.util.Optional;
import java.util.concurrent.*;

@PageTitle("Blocking Dialog")
@Route(value = "blockingdialog", layout = MainLayout.class)
public class BlockingDialogView extends VerticalLayout {

    private ExecutorService executor = Executors.newSingleThreadExecutor();

    private CompletableFuture<Optional<String>> mainFuture;
    private CompletableFuture<String> dialogFuture;

    private Button button;
    private UI ui;

    public BlockingDialogView() {
        button = new Button("Process", buttonClickEvent -> startTheTask());
        button.setDisableOnClick(true);
        add(button);
    }

    private void accessUI(Command command) {
        try {
            ui.access(() -> {
                if (isAttached()) {
                    command.execute();
                }
            });
        } catch (UIDetachedException e) {
        }
    }

    private void startTheTask() {
        var progress = new ProgressBar();
        progress.setIndeterminate(true);
        progress.setWidth("50px");
        button.setText("");
        button.setIcon(progress);
        mainFuture = CompletableFuture.supplyAsync(this::longRunningTask,
                executor);
        mainFuture.thenAccept(optResult -> optResult.ifPresentOrElse(result -> {
            accessUI(() -> {
                Notification.show("Final: <" + result + ">", 2000,
                        Position.MIDDLE);
                reEnableButton();
            });
        }, () -> {
            accessUI(() -> {
                Notification.show("Cancelled", 2000, Position.MIDDLE);
                reEnableButton();
            });

        }));
    }

    private void reEnableButton() {
        button.setEnabled(true);
        button.setIcon(null);
        button.setText("Process");
    }

    private Optional<String> longRunningTask() {
        dialogFuture = new CompletableFuture<>();
        String result = null;

        accessUI(() -> {
            Notification.show("Processing ...", 2000, Position.MIDDLE);
        });

        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
        }

        // need user input for further processing
        launchDialog(dialogFuture);

        var answer = dialogFuture.join();

        accessUI(() -> {
            Notification.show("Got input [" + answer + "]", 2000,
                    Position.MIDDLE);
        });

        if (answer.equals("Closed")) {
            return Optional.ofNullable(result);
        }

        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
        }

        if (answer.equals("Yes")) {
            result = "Complete Result";
        } else if (answer.equals("No")) {
            result = "Partial Result";
        }
        return Optional.ofNullable(result);
    }

    private void launchDialog(CompletableFuture<String> future) {
        accessUI(() -> {
            Dialog dialog = new Dialog();
            dialog.setHeaderTitle("Choose");
            dialog.add(new Text("Esc to cancel."));
            dialog.getFooter().add(new Button("Yes", buttonClickEvent -> {
                future.complete("Yes");
                dialog.close();
            }));
            dialog.getFooter().add(new Button("No", buttonClickEvent -> {
                future.complete("No");
                dialog.close();
            }));
            dialog.open();
            dialog.setCloseOnOutsideClick(true);
            dialog.addOpenedChangeListener(e -> {
                if (!e.isOpened()) {
                    future.complete("Closed");
                }
            });
        });

    }

    @Override
    public void onAttach(AttachEvent event) {
        super.onAttach(event);
        ui = event.getUI();
    }

    @Override
    public void onDetach(DetachEvent event) {
        ui = null;
        if (dialogFuture != null) {
            dialogFuture.cancel(true);
            dialogFuture = null;
        }
        if (mainFuture != null) {
            mainFuture.cancel(true);
            mainFuture = null;
        }
        super.onDetach(event);
    }
}