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Navigating in an Application · Vaadin
Vaadin Framework - Advanced Topics - Navigating in an Application
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Navigating in an Application

Plain Vaadin applications do not have normal web page navigation as they usually run on a single page, as all Ajax applications do. Quite commonly, however, applications have different views between which the user should be able to navigate. The Navigator in Vaadin can be used for most cases of navigation. Views managed by the navigator automatically get a distinct URI fragment, which can be used to be able to bookmark the views and their states and to go back and forward in the browser history.

Setting Up for Navigation

The Navigator class manages a collection of views that implement the View interface. The views can be either registered beforehand or acquired from a view provider. When registering, the views must have a name identifier and be added to a navigator with addView(). You can register new views at any point. Once registered, you can navigate to them with navigateTo().

Navigator manages navigation in a component container, which can be either a ComponentContainer (most layouts) or a SingleComponentContainer ( UI, Panel, or Window). The component container is managed through a ViewDisplay. Two view displays are defined: ComponentContainerViewDisplay and SingleComponentContainerViewDisplay, for the respective component container types. Normally, you can let the navigator create the view display internally, as we do in the example below, but you can also create it yourself to customize it.

Let us consider the following UI with two views: start and main. Here, we define their names with enums to be typesafe. We manage the navigation with the UI class itself, which is a SingleComponentContainer.

public class NavigatorUI 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());
    }
}

The Navigator automatically sets the URI fragment of the application URL. It also registers a URIFragmentChangedListener in the page

(see "Managing URI Fragments") to show the view identified by the URI fragment if entered or navigated to in the browser. This also enables browser navigation history in the application.

View Providers

You can create new views dynamically using a view provider that implements the ViewProvider interface. A provider is registered in Navigator with addProvider().

The ClassBasedViewProvider is a view provider that can dynamically create new instances of a specified view class based on the view name.

The StaticViewProvider returns an existing view instance based on the view name. The addView() in Navigator is actually just a shorthand for creating a static view provider for each registered view.

View Change Listeners

You can handle view changes also by implementing a ViewChangeListener and adding it to a Navigator. When a view change occurs, a listener receives a ViewChangeEvent object, which has references to the old and the activated view, the name of the activated view, as well as the fragment parameters.

Implementing a View

Views can be any objects that implement the View interface. When the navigateTo() is called for the navigator, or the application is opened with the URI fragment associated with the view, the navigator switches to the view and calls its enter() method.

To continue with the example, consider the following simple start view that just lets the user to navigate to the main view. It only pops up a notification when the user navigates to it and displays the navigation button.

/** A start view for navigating to the main view */
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");
    }
}

You can initialize the view content in the constructor, as was done in the example above, or in the enter() method. The advantage with the latter method is that the view is attached to the view container as well as to the UI at that time, which is not the case in the constructor.

Handling URI Fragment Path

URI fragment part of a URL is the part after a hash # character. Is used for within-UI URLs, because it is the only part of the URL that can be changed with JavaScript from within a page without reloading the page. The URLs with URI fragments can be used for hyperlinking and bookmarking, as well as browser history, just like any other URLs. In addition, an exclamation mark #! after the hash marks that the page is a stateful AJAX page, which can be crawled by search engines. Crawling requires that the application also responds to special URLs to get the searchable content. URI fragments are managed by Page, which provides a low-level API.

URI fragments can be used with Navigator in two ways: for navigating to a view and to a state within a view. The URI fragment accepted by navigateTo() can have the view name at the root, followed by fragment parameters after a slash (" /"). These parameters are passed to the enter() method in the View.

In the following example, we implement within-view navigation. Here we use the following declarative design for the view:

<vaadin-vertical-layout size-full>
  <vaadin-horizontal-layout size-full :expand>
    <vaadin-panel caption="List of Equals" height-full width-auto>
      <vaadin-vertical-layout _id="menuContent" width-auto margin/>
    </vaadin-panel>

    <vaadin-panel _id="equalPanel" caption="An Equal" size-full :expand/>
  </vaadin-horizontal-layout>

  <vaadin-button _id="logout">Logout</vaadin-button>
</vaadin-vertical-layout>

The view’s logic code would be as follows:

/** Main view with a menu (with declarative layout design) */
@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()));
    }
}

The animal sub-view would have the following declarative design:

<vaadin-vertical-layout size-full>
  <vaadin-label _id="watching" size-auto :middle :center/>
  <vaadin-embedded _id="pic" :middle :center :expand/>
  <vaadin-label _id="back" size-auto :middle :center/>
</vaadin-vertical-layout>

The main view is shown in Navigator Main View. At this point, the URL would be http://localhost:8080/myapp#!main/reindeer.

navigator mainview
Figure 1. Navigator Main View