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. Also users often need to have direct links to specific views. The [classname]#Navigator# in Vaadin can be used for most cases of
navigation. Views managed by the navigator automatically get a distinct URI, 
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 parses the URI to identify and show the [interfacename]#View#. The browser navigation back and forward are also handled by it.

Starting from [literal]#++8.2.0++# there is also PushState based navigation. This new way for the Navigator to manage URLs is described here as well. To enable this feature, add the [classname]#PushStateNavigation# annotation to your UI.

[[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 parameters (the part of of URI after the viewname).



[[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 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.pathparam]]
== Handling Path Parameters

By default the URLs managed through the [classname]#Navigator# have a URI fragment
that contains the identifier of the [interfacename]#View#. The URI fragment is 
separated from the rest of the URL by a [literal]#++#++# character.

If the [classname]#PushStateNavigation# annotation is present on the [classname]#UI# 
the HTML5 History API is used. When using the PushState, the identifier is separated 
from the root path by a [literal]#++/++# like a real URL.

In addition to the View identifier, URI can contain additional parameters to be 
passed to views. The parameters are the part of the URI after the longest matching view identifier, separated by [literal]#++/++#. These parameters together with the identifier
form the __navigation state__.

The navigation state can be used with [classname]#Navigator# in two ways: for
navigating to a view and to a state within a view. The navigation state 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 ->
            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[]