vaadin-framework/documentation/advanced/advanced-navigator.asciidoc

---
title: Navigating in an Application
order: 9
layout: page
---

[[advanced.navigator]]
= 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 [classname]#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.

[[advanced.navigator.navigating]]
== Setting Up for Navigation

The [classname]#Navigator# class manages a collection of __views__ that
implement the [interfacename]#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
[methodname]#addView()#. You can register new views at any point. Once
registered, you can navigate to them with [methodname]#navigateTo()#.

[classname]#Navigator# manages navigation in a component container, which can be
either a [interfacename]#ComponentContainer# (most layouts) or a
[interfacename]#SingleComponentContainer# ( [classname]#UI#, [classname]#Panel#,
or [classname]#Window#). The component container is managed through a
[interfacename]#ViewDisplay#. Two view displays are defined:
[classname]#ComponentContainerViewDisplay# and
[classname]#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 [interfacename]#SingleComponentContainer#.


[source, java]
----
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 [classname]#Navigator# automatically sets the URI fragment of the
application URL. It also registers a [interfacename]#URIFragmentChangedListener#
in the page

ifdef::web[]
(see <<dummy/../../../framework/advanced/advanced-urifu#advanced.urifu,"Managing
URI
Fragments">>)
endif::web[]
 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.

[[advanced.navigator.navigating.viewprovider]]
=== View Providers

You can create new views dynamically using a __view provider__ that implements
the [interfacename]#ViewProvider# interface. A provider is registered in
[classname]#Navigator# with [methodname]#addProvider()#.

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

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


[[advanced.navigator.navigating.viewchangelistener]]
=== View Change Listeners

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



[[advanced.navigator.view]]
== Implementing a View

Views can be any objects that implement the [interfacename]#View# interface.
When the [methodname]#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 [methodname]#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.


[source, java]
----
/** 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 [methodname]#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.


[[advanced.navigator.urifragment]]
== Handling URI Fragment Path

URI fragment part of a URL is the part after a hash [literal]#++#++# 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
[literal]#++#!++# 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 [classname]#Page#, which provides a low-level API.

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

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


[source, html]
----
<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:


[source, java]
----
/** 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:


[source, html]
----
<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 <<figure.advanced.navigator.mainview>>. At this point,
the URL would be [literal]#++http://localhost:8080/myapp#!main/reindeer++#.

[[figure.advanced.navigator.mainview]]
.Navigator Main View
image::img/navigator-mainview.png[]