> Markdown version of [Navigation Lifecycle](https://vaadin.com/docs/next/flow/routing/lifecycle). Section index: [llms.txt](https://vaadin.com/docs/next/flow/llms.txt)

# Navigation Lifecycle

The navigation lifecycle is made up of various events that are fired when a user navigates from one state or view to another. The events are fired to listeners added to the `UI` instance and to attached components that implement related *observer* interfaces.

## <a id="beforeleaveevent"></a>BeforeLeaveEvent

`BeforeLeaveEvent` is the first event fired during navigation.

The event allows the navigation to be postponed, canceled, or changed to a different destination.

This event is delivered to any component instance implementing `BeforeLeaveObserver` that’s attached to the UI before the navigation starts.

It’s also possible to register a standalone listener for this event using the `addBeforeLeaveListener(BeforeLeaveListener)` method in the `UI` class.

A typical use case for this event is to ask the user whether they want to save any unsaved changes before navigating to another part of the application.

> **Note: Event call order**
>
> `BeforeLeaveEvent` is first sent to manually registered `BeforeLeaveListener` instances and then to `BeforeLeaveObserver` instances connected to the UI.

> **Note: Listener order**
>
> `BeforeLeaveListener` instances are called in the order they’re registered. `BeforeLeaveObserver` instances are called in descendant order starting from the UI.

### <a id="postpone"></a>Postponing a Navigation Transition

`BeforeLeaveEvent` includes the `postpone()` method, which can be used to postpone the current navigational transition until a specific condition is met.

**Example**: The client requests the user’s confirmation before leaving the page:

```java
public class SignupForm extends Div
        implements BeforeLeaveObserver {
    @Override
    public void beforeLeave(BeforeLeaveEvent event) {
        if (hasChanges()) {
            ContinueNavigationAction action =
                    event.postpone();
            ConfirmDialog confirmDialog = new ConfirmDialog();
                 confirmDialog.setText("Your form has changes! Are you sure you want to leave?");
                 confirmDialog.setCancelable(true);
                 confirmDialog.addConfirmListener(__ -> action.proceed());
                 confirmDialog.addCancelListener(__ -> action.cancel());
                 confirmDialog.open();
        }
    }

    private boolean hasChanges() {
        // no-op implementation
        return true;
    }
}
```

Postponing interrupts the process of notifying observers and listeners. When the transition resumes, the remaining observers (those that come after the observer that initiated the postpone) are called.

**Example**:

- Assume the current page has three observers, `a`, `b` and `c`, which are notified in the same order.

- If `b` calls `postpone()`, the call to `c` (and the rest of the transition process), is deferred.

  - If the transition isn’t resumed, `c` is never notified of the event and the transition never finishes.

  - If `b` executes `ContinueNavigationAction` to resume the transition, it continues from the point of interruption: `a` and `b` aren’t called again, but `c` is notified.

> **Note: Only one navigation event may be postponed at a time.**
>
> Starting a new navigation transition while a previous one is in a postponed state makes the postponed state obsolete, and executing `ContinueNavigationAction` has no effect.

## <a id="beforeenterevent"></a>BeforeEnterEvent

`BeforeEnterEvent` is the second event fired during navigation.

The event allows you to change the navigation to go to a destination that’s different from the original.

This event is typically used to react to special situations, for example if there is no data to show, or if the user doesn’t have appropriate permissions.

This event is delivered to any component instance implementing `BeforeEnterObserver` that’s attached to the UI, beginning with the UI and moving through the child components.

> **Note:** Components in the chain are instantiated after a parent handles `BeforeEnter` and doesn’t [reroute](#rerouting) or [forward](#forwarding)

The event is fired:

- only after a [Postpone method](#postpone) (called during a `BeforeLeaveEvent`) has been continued;

- before detaching and attaching components to make the UI match the location being navigated to.

It’s also possible to register a standalone listener for this event using the `addBeforeEnterListener(BeforeEnterListener)` method in the `UI` class.

> **Note: Event call order**
>
> `BeforeEnterEvent` is first sent to the target component if it implements `HasUrlParameter`, then to manually registered `BeforeEnterListener` instances and lastly to `BeforeEnterObserver` instances connected to the UI.

> **Note: Listener order**
>
> `BeforeEnterListener` instances are called in the order they’re registered. `BeforeEnterObserver` instances are called in descendant order starting from the UI.

### <a id="rerouting"></a>Rerouting

Both [`BeforeLeaveEvent`](#beforeleaveevent) and [`BeforeEnterEvent`](#beforeenterevent) can be used to reroute dynamically.

Rerouting is typically used when there is a need to show completely different information in a particular state.

When the `reroute()` method is called:

- the event isn’t fired to any further listeners or observers;

- the method triggers a new navigation phase, based on the new navigation target, and events are fired based on this instead;

- for [BeforeEnterEvent](#beforeenterevent): child components of the rerouting component aren’t instantiated.

**Example**: Rerouting when entering a `BlogList` with no results.

```java
@Route("no-items")
public class NoItemsView extends Div {
    public NoItemsView() {
        setText("No items found.");
    }
}

@Route("blog")
public class BlogList extends Div
        implements BeforeEnterObserver {
    @Override
    public void beforeEnter(BeforeEnterEvent event) {
        // implementation omitted
        Object record = getItem();

        if (record == null) {
            event.rerouteTo(NoItemsView.class);
        }
    }

    private Object getItem() {
        // no-op implementation
        return null;
    }
}
```

> **Note:** `rerouteTo()` has overloads for different use cases.

> **Note:** `rerouteTo()` keeps the original URL in the browser’s address bar and doesn’t change it to a new URL based on the new target.

### <a id="forwarding"></a>Forwarding

The `forwardTo()` method [reroutes](#rerouting) navigation and updates the browser URL.

Forwarding can be used during [BeforeEnter](#beforeenterevent) and [BeforeLeave](#beforeleaveevent) lifecycle states to dynamically redirect to a different URL.

When the `forwardTo()` method is called:

- the event isn’t fired to any further listeners or observers;

- the method triggers a new navigation phase, based on the new navigation target, and fires new lifecycle events for the new forward navigation target;

- for [BeforeEnterEvent](#beforeenterevent): child components of the forwarding component aren’t instantiated.

**Example**: Forwarding when viewing `BlogList` without the required permissions.

```java
@Route("no-permission")
public class NoPermission extends Div {
    public NoPermission() {
        setText("No permission.");
    }
}

@Route("blog-post")
public class BlogPost extends Div
        implements BeforeEnterObserver {
    @Override
    public void beforeEnter(BeforeEnterEvent event) {
        if (!hasPermission()) {
            event.forwardTo(NoPermission.class);
        }
    }

    private boolean hasPermission() {
        // no-op implementation
        return false;
    }
}
```

> **Note:** `forwardTo()` has overloads for different use cases

> **Note:** `forwardTo()` changes the URL in the browser’s address bar to the URL of the new target. The URL of the original target isn’t kept in the browser history.

## <a id="afternavigationevent"></a>AfterNavigationEvent

`AfterNavigationEvent` is the third and **last** event fired during navigation.

This event is typically used to update various parts of the UI after the actual navigation is complete. Examples include adjusting the content of a breadcrumb component and visually marking the active menu item as active.

`AfterNavigationEvent` is fired:

- after [`BeforeEnterEvent`](#beforeenterevent), and

- after updating which components are attached to the UI.

At this point, the current navigation state is actually shown to the user, and further reroutes and similar changes are no longer possible.

The event is delivered to any component instance implementing `AfterNavigationObserver` that’s attached after completing the navigation.

It’s also possible to register a standalone listener for this event using the `addAfterNavigationListener(AfterNavigationListener)` method in the `UI` class.

**Example**: Marking the active navigation element as active.

```java
public class SideMenu extends Div
        implements AfterNavigationObserver {
    Anchor blog = new Anchor("blog", "Blog");

    @Override
    public void afterNavigation(
          AfterNavigationEvent event) {
        boolean active = event.getLocation()
                .getFirstSegment()
                .equals(blog.getHref());
        blog.getElement()
                .getClassList()
                .set("active", active);
    }
}
```

> **Note: Event call order**
>
> `AfterNavigationEvent` is first sent to manually registered `AfterNavigationListener` instances and then to `AfterNavigationObserver` instances found connected to the UI.

> **Note: Listener order**
>
> `AfterNavigationListener` instances are called in the order they were registered. `AfterNavigationObserver` instances are called in descendant order staring from the UI.

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