[[access-control-for-views]]
Access control for views
------------------------

The Navigator API provides a simple mechanism to allow or disallow
navigating to a View. Before a View is shown, each ViewChangeListener
that is registered with the Navigator is given the opportunity to veto
the View change.

One can also make the View itself trigger a navigation to another View
in navigateTo(), but let's take a look at the more flexible
beforeViewChange() and afterViewChange(), that exists specifically for
this purpose.

First, let's continue from previous examples and create a MessageView
for secret messages:

[source,java]
....
import com.vaadin.navigator.View;
import com.vaadin.ui.Label;

public class SecretView extends MessageView implements View {
  public static final String NAME = "secret";

  public SecretView() {
    setCaption("Private messages");
    ((Layout) getContent()).addComponent(new Label("Some private stuff."));
  }
}
....

As you can see, there is absolutely nothing special going on here, we
just customize the View enough to be able to distinguish from the
regular MessageView.

Next, we'll register this new View with the Navigator, exactly as
before. At this point our SecretView is not secret at all, but let's fix
that by adding a ViewChangeListener to the Navigator:

[source,java]
....
navigator.addViewChangeListener(new ViewChangeListener() {

  @Override
  public boolean beforeViewChange(ViewChangeEvent event) {
    if (event.getNewView() instanceof SecretView &&
    ((NavigationtestUI)UI.getCurrent()).getLoggedInUser() == null) {
      Notification.show("Permission denied", Type.ERROR_MESSAGE);
      return false;
    } else {
      return true;
    }
  }

  @Override
  public void afterViewChange(ViewChangeEvent event) {
  }

});
....

So if we're on our way to the SecretView, but not logged in
(getLoggedInUser() == null), the View change is cancelled. Quite simple
rules in our case, but you could check anything - most probably you'll
want to call a helper method that checks the user for permission.

Let's go ahead and add some links to the MainView again, so that we
don't have to muck with the address-bar to try it out:

[source,java]
....
import com.vaadin.navigator.View;
import com.vaadin.navigator.ViewChangeListener.ViewChangeEvent;
import com.vaadin.server.ExternalResource;
import com.vaadin.ui.Button;
import com.vaadin.ui.Button.ClickEvent;
import com.vaadin.ui.Link;
import com.vaadin.ui.Panel;
import com.vaadin.ui.UI;
import com.vaadin.ui.VerticalLayout;

public class MainView extends Panel implements View {

    public static final String NAME = "";

    public MainView() {

        VerticalLayout layout = new VerticalLayout();

        Link lnk = new Link("Count", new ExternalResource("#!" + CountView.NAME));
        layout.addComponent(lnk);

        lnk = new Link("Message: Hello", new ExternalResource("#!"
                + MessageView.NAME + "/Hello"));
        layout.addComponent(lnk);

        lnk = new Link("Message: Bye", new ExternalResource("#!"
                + MessageView.NAME + "/Bye/Goodbye"));
        layout.addComponent(lnk);

        lnk = new Link("Private message: Secret", new ExternalResource("#!"
                + SecretView.NAME + "/Secret"));
        layout.addComponent(lnk);

        lnk = new Link("Private message: Topsecret", new ExternalResource("#!"
                + SecretView.NAME + "/Topsecret"));
        layout.addComponent(lnk);

        // login/logout toggle so we can test this
        Button logInOut = new Button("Toggle login",
                new Button.ClickListener() {
                    public void buttonClick(ClickEvent event) {
                        Object user = ((NavigationtestUI)UI.getCurrent()).getLoggedInUser();
                        ((NavigationtestUI)UI.getCurrent()).setLoggedInUser(
                                user == null ? "Smee" : null);
                    }
                });
        layout.addComponent(logInOut);
        setContent(layout);
    }

    @Override
    public void enter(ViewChangeEvent event) {
    }
}
....

Instead of just showing a notification and leaving the user wondering,
we should obviously allow the user to log in and continue. We'll do just
that in the separate tutorial about Handling login, but for now we just
add a button that toggles our logged in/out state.

Meanwhile, here is the the full source for the UI so far:

[source,java]
....
import com.vaadin.navigator.Navigator;
import com.vaadin.navigator.ViewChangeListener;
import com.vaadin.server.VaadinRequest;
import com.vaadin.ui.Notification;
import com.vaadin.ui.Notification.Type;
import com.vaadin.ui.UI;

public class NavigationtestUI extends UI {

    Navigator navigator;

    String loggedInUser;

    @Override
    public void init(VaadinRequest request) {
        // Create Navigator, make it control the ViewDisplay
        navigator = new Navigator(this, this);

        // Add some Views
        navigator.addView(MainView.NAME, new MainView()); // no fragment

        // #count will be a new instance each time we navigate to it, counts:
        navigator.addView(CountView.NAME, CountView.class);

        // #message adds a label with whatever it receives as a parameter
        navigator.addView(MessageView.NAME, new MessageView());

        // #secret works as #message, but you need to be logged in
        navigator.addView(SecretView.NAME, new SecretView());

        // we'll handle permissions with a listener here, you could also do
        // that in the View itself.

        navigator.addViewChangeListener(new ViewChangeListener() {

            @Override
            public boolean beforeViewChange(ViewChangeEvent event) {
                if (event.getNewView() instanceof SecretView
                        && ((NavigationtestUI)UI.getCurrent()).getLoggedInUser() == null) {
                    Notification.show("Permission denied", Type.ERROR_MESSAGE);
                    return false;
                } else {
                    return true;
                }
            }

            @Override
            public void afterViewChange(ViewChangeEvent event) {
                System.out.println("After view change");
            }

        });
    }

    public String getLoggedInUser(){
         return loggedInUser;
    }

    public void setLoggedInUser(String user){
         loggedInUser = user;
   }
}
....