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In Vaadin, to change the components displayed on the screen, there are a few options. The most straightforward way is to use the setContent() method on the UI. The most widespread way is to use the https://vaadin.com/docs/framework/advanced/advanced-navigator.html[navigator^].

[quote, Vaadin documentation] 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.

This is the main asset for apps managing catalogs, such as e-commerce shops, or item management apps. Every item gets assigned a specific URL, through https://en.wikipedia.org/wiki/Fragment_identifier[fragment identifiers^].

However, the view per URL trick is also a double-edged blade. It lets the user bypass the server-side navigation and directly type the URL of the view he wants to display in the browser. Also, the View interface requires to implement the enter() method. Yet, this method is generally empty and thus is just boiler plate.

Let’s see how we can overcome those issues.

== A solution to the empty method

With Java 8, it’s possible for an interface to define a default implementation. Vaadin takes advantage of that by providing a default empty method implementation:

[source,java]

public interface View extends Serializable {

public default void enter(ViewChangeEvent event) {
}

... } ----

Even better, all methods of the View interface are default. That lets you implement the interface without writing any boiler-plate code and rely on default behaviour.

== A solution to the view per URI

=== The default implementation

The navigator API consists of the following classes:

image::navigator.svg[Navigator API class diagram]

As can be seen, the handling of the view by the URI occurs in the UriFragmentManager class. This implementation relies on the fragment identifier in the URL. But this is only the default implementation. The topmost abstraction is NavigationStageManager and it has no constraint where to store the state.

=== An alternative

To prevent users from jumping to a view by typing a specific URL, one should store the state in a place unaccessible by them. For example, it’s feasible to store the state in cookies. A simple implementation could look like:

[source,kotlin]

private const val STATE_KEY = “state”

class CookieManager : NavigationStateManager {

private var navigator: Navigator? = null

override fun setNavigator(navigator: Navigator) {
    this.navigator = navigator
}

override fun getState(): String {
    val cookies = VaadinService.getCurrentRequest().cookies?.asList()
    val value = cookies?.find { it.name == STATE_KEY }?.value
    return when(value) {
        null -> ""
        else -> value
    }
}

override fun setState(state: String) {
    val cookie = Cookie(STATE_KEY, state)
    VaadinService.getCurrentResponse().addCookie(cookie)
} } ----

Creating a navigator using the previous state manager is very straightforward:

[source,kotlin]

class NavigatorUI : ViewDisplay, UI() {

override fun init(vaadinRequest: VaadinRequest) {
    navigator = Navigator(this, CookieManager(), this)
}

override fun showView(view: View) {
    content = view as Component
} } ----

At this point, it’s possible to place components which change the view on the UI and spy upon the request and response. Here are the HTTP request and response of an AJAX call made by Vaadin to change the view:

[source]

POST /UIDL/?v-uiId=0 HTTP/1.1 Host: 127.0.0.1:8080 Connection: keep-alive Content-Length: 297 Origin: http://127.0.0.1:8080 User-Agent: … Content-Type: application/json; charset=UTF-8 Accept: / DNT: 1 Referer: http://127.0.0.1:8080/ Accept-Encoding: gzip, deflate, br Accept-Language: fr-FR,fr;q=0.8,en-US;q=0.6,en;q=0.4 Cookie: state=Another; JSESSIONID=B584B759D78A0CDBA43496EF4AEB3F25

HTTP/1.1 200 Set-Cookie: state=Third Cache-Control: no-cache Content-Type: application/json;charset=UTF-8 Content-Length: 850 Date: Fri, 27 Oct 2017 09:19:42 GMT —-

== Conclusion

Before Vaadin 7, I personally didn’t use the Navigator API because of the requirement to implement the enter() method on the view. Java 8 provide default methods, Vaadin 8 provide the implementation. There’s now no reason not to use it.

If the default fragment identifier way of navigating doesn’t suit your own use-case, it’s also quite easy to rely on another way to store the current view.

More generally, in its latest versions, Vaadin makes it easier to plug-in your implementation if the default behaviour doesn’t suit you.