Vaadin 视图 - 无法解析导航器

Question

I'm new to Vaadin. I'm currently writing a simple server side app. I'm learning from Vaadin site docs and a current example of navigation doesn't work. I'm not able to compile and run/debug the app.

This is my main UI class:

@Theme("mytheme")
public class MyUI extends UI {

    Navigator navigator;
    protected static final String MAINVIEW = "main";

    @Override
    protected void init(VaadinRequest request) {
        getPage().setTitle("Navigation Example");

        // Create a navigator to control the views
        navigator = new Navigator(this, this);

        // Create and register the views
        navigator.addView("", new StartView());
        navigator.addView(MAINVIEW, new MainView());
    }

    @WebServlet(urlPatterns = "/*", name = "MyUIServlet", asyncSupported = true)
    @VaadinServletConfiguration(ui = MyUI.class, productionMode = false)
    public static class MyUIServlet extends VaadinServlet {
    }
}

I dont have any problems here. I have implemented StartView class and that is where I get an error. At navigator.navigateTo(MAINVIEW); There are 2 errors

navigator can't be resolved

and

MAINVIEW cannot be resolved to variable

I read everything many times and tried to found the solution, but nothing really seems to work.

public class StartView extends VerticalLayout implements View {
    public StartView() {
        setSizeFull();

        Button button = new Button("Go to Main View",
                new Button.ClickListener() {
            @Override
            public void buttonClick(ClickEvent event) {
                navigator.navigateTo(MAINVIEW);
            }
        });
        addComponent(button);
        setComponentAlignment(button, Alignment.MIDDLE_CENTER);
    }

    @Override
    public void enter(ViewChangeEvent event) {
        Notification.show("Welcome to the Animal Farm");
    }
}

Exactly the same thing happens in MainView class.

@DesignRoot
public class MainView extends VerticalLayout implements View {
    // Menu navigation button listener
    class ButtonListener implements Button.ClickListener {
        String menuitem;
        public ButtonListener(String menuitem) {
            this.menuitem = menuitem;
        }

        @Override
        public void buttonClick(ClickEvent event) {
            // Navigate to a specific state
            navigator.navigateTo(MAINVIEW + "/" + menuitem);
        }
    }

    VerticalLayout menuContent;
    Panel equalPanel;
    Button logout;

    public MainView() {
        Design.read(this);

        menuContent.addComponent(new Button("Pig",
                  new ButtonListener("pig")));
        menuContent.addComponent(new Button("Cat",
                  new ButtonListener("cat")));
        menuContent.addComponent(new Button("Dog",
                  new ButtonListener("dog")));
        menuContent.addComponent(new Button("Reindeer",
                  new ButtonListener("reindeer")));
        menuContent.addComponent(new Button("Penguin",
                  new ButtonListener("penguin")));
        menuContent.addComponent(new Button("Sheep",
                  new ButtonListener("sheep")));

        // Allow going back to the start
        logout.addClickListener(event -> // Java 8
            navigator.navigateTo(""));
    }

    @DesignRoot
    class AnimalViewer extends VerticalLayout {
        Label watching;
        Embedded pic;
        Label back;

        public AnimalViewer(String animal) {
            Design.read(this);

            watching.setValue("You are currently watching a " +
                              animal);
            pic.setSource(new ThemeResource(
                "img/" + animal + "-128px.png"));
            back.setValue("and " + animal +
                " is watching you back");
        }
    }

    @Override
    public void enter(ViewChangeEvent event) {
        if (event.getParameters() == null
            || event.getParameters().isEmpty()) {
            equalPanel.setContent(
                new Label("Nothing to see here, " +
                          "just pass along."));
            return;
        } else
            equalPanel.setContent(new AnimalViewer(
                event.getParameters()));
    }
}

Could you tell me what am I doing wrong? I can't understand why it can't go through.

Answer 1

Problems you are facing are actually basics of Java.

First error:

navigator can't be resolved

StartView and MainView don't get navigator instance. You can pass it by setter or constructor or you can get to navigator this way:

UI.getCurrent().getNavigator()

Secondly, there is actually no need to create instance of Navigator , because you can call line above in your MyUI , so it will look like this:

    protected void init(VaadinRequest request) {
        getPage().setTitle("Navigation Example");

        // Create and register the views
        UI.getCurrent().getNavigator().addView("", new StartView());
        UI.getCurrent().getNavigator().addView(MAINVIEW, new MainView());
    }

@Edit

If its about next error:

MAINVIEW cannot be resolved to variable

If you are using static fields, you should access to them this way:

ClassName.FIELD

So in your example (as long as you will pass navigator to your Main and Start View classes:

navigator.navigateTo(MyUI.MAINVIEW);