[[view-change-confirmations]]
View change confirmations
-------------------------

The `Navigator` API provides ways to prevent the user from navigating away
from a view in some cases, usually when the view has some unsaved
changes. We'll make a simple example that does just that (and only
that).

We'll create our simple `SettingsView` later, because it has the actual
meat of this example. Let's set up the basic stuff first, our UI and our
`MainView`.

UI:

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

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

    // no fragment for main view
    navigator.addView(MainView.NAME, new MainView(navigator));

    // #settings
    navigator.addView(SettingsView.NAME, new SettingsView(navigator));
  }
}
....

Minimalistic. The only thing to notice is that we pass the `Navigator` to
the `SettingsView`, so that it can attach a listener and trigger
navigation. More on that when we actually create the `SettingsView`.

Let's do the `MainView`:

[source,java]
....
import com.vaadin.navigator.View;
import com.vaadin.server.ExternalResource;
import com.vaadin.ui.Link;
import com.vaadin.ui.Panel;

public class MainView extends Panel implements View {

  public static final String NAME = "";

  public MainView(final Navigator navigator) {
    Link lnk = new Link("Settings", new ExternalResource("#!"
        + SettingsView.NAME));
    setContent(lnk);
  }

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

Yeah, really nothing to see here - we just create this so we can
navigate back and forth when trying it out.

Now let's do the SettingsView, which has some more things going on in
order to make it fairly complete:

[source,java]
....
import java.util.Date;
import com.vaadin.data.Property.ValueChangeEvent;
import com.vaadin.data.Property.ValueChangeListener;
import com.vaadin.data.util.ObjectProperty;
import com.vaadin.navigator.Navigator;
import com.vaadin.navigator.View;
import com.vaadin.navigator.ViewChangeListener;
import com.vaadin.navigator.ViewChangeListener.ViewChangeEvent;
import com.vaadin.ui.Button;
import com.vaadin.ui.Button.ClickEvent;
import com.vaadin.ui.DateField;
import com.vaadin.ui.InlineDateField;
import com.vaadin.ui.Layout;
import com.vaadin.ui.Notification;
import com.vaadin.ui.Notification.Type;
import com.vaadin.ui.Panel;
import com.vaadin.ui.VerticalLayout;
import com.vaadin.ui.themes.Reindeer;

public class SettingsView extends Panel implements View {

  public static String NAME = "settings";

  Navigator navigator;
  DateField date;
  Button apply;
  Button cancel;

  String pendingViewAndParameters = null;

  public SettingsView(final Navigator navigator) {
    this.navigator = navigator;
    Layout layout = new VerticalLayout();

    date = new InlineDateField("Birth date");
    date.setImmediate(true);
    layout.addComponent(date);
    // pretend we have a datasource:
    date.setPropertyDataSource(new ObjectProperty<Date>(new Date()));
    date.setBuffered(true);
    // show buttons when date is changed
    date.addValueChangeListener(new ValueChangeListener() {
      public void valueChange(ValueChangeEvent event) {
        hideOrShowButtons();
        pendingViewAndParameters = null;
      }
    });

    // commit the TextField changes when "Save" is clicked
    apply = new Button("Apply", new Button.ClickListener() {
      public void buttonClick(ClickEvent event) {
        date.commit();
        hideOrShowButtons();
        processPendingView();
      }
    });
    layout.addComponent(apply);

    // Discard the TextField changes when "Cancel" is clicked
    cancel = new Button("Cancel", new Button.ClickListener() {
      public void buttonClick(ClickEvent event) {
        date.discard();
        hideOrShowButtons();
        processPendingView();
      }
    });
    cancel.setStyleName(Reindeer.BUTTON_LINK);
    layout.addComponent(cancel);

    // attach a listener so that we'll get asked isViewChangeAllowed?
    navigator.addViewChangeListener(new ViewChangeListener() {
      public boolean beforeViewChange(ViewChangeEvent event) {
        if (event.getOldView() == SettingsView.this
            && date.isModified()) {

          // save the View where the user intended to go
          pendingViewAndParameters = event.getViewName();
          if (event.getParameters() != null) {
            pendingViewAndParameters += "/";
            pendingViewAndParameters += event
                .getParameters();
          }

          // Prompt the user to save or cancel if the name is changed
          Notification.show("Please apply or cancel your changes",
              Type.WARNING_MESSAGE);

          return false;
        } else {
          return true;
        }
      }

      public void afterViewChange(ViewChangeEvent event) {
        pendingViewAndParameters = null;
      }
    });

    setContent(layout);
  }

  // Hide or show buttons depending on whether date is modified or not
  private void hideOrShowButtons() {
    apply.setVisible(date.isModified());
    cancel.setVisible(date.isModified());
  }

  // if there is a pending view change, do it now
  private void processPendingView() {
    if (pendingViewAndParameters != null) {
      navigator.navigateTo(pendingViewAndParameters);
      pendingViewAndParameters = null;
    }
  }

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

First we set up a `DateField` with buffering and a (dummy) datasource to
make this work more as a real application would. With buffering on, the
value (date in this case) can be changed, but it will not be written to
the datasource before we `commit()`, which is what the Save -button does.
The Cancel -button does `discard()` on the DateField, which returns the
field to its unmodified state.

The buttons do not need to be shown if nothing has changed, so we add a
`ValueChangeListener` to the `DateField` for that purpose.

But the main thing that we're trying to demonstrate here happens in the
`ViewChangeListener` that we attach to the `Navigator`. There, if we're
about to change _away_ from our settings _and_ the date is changed but
_not_ saved, we'll make note of where the user wanted to go, but cancel
that navigation and prompt the user to save or cancel the changes.

When the user saves or cancels changes, we also check if the user
previously tried to navigate away form the page, and sends him on his
way if that is the case.

That is basically all there is to this. You'll notice we try to
carefully clear or set the 'pending view' and hide/show the buttons at
the right places to make the user happy, other than that this is pretty
straightforward.
