How to prevent navigation?

Flow
1

I understand that I can implement a BeforeLeaveObserver but how can I cancel the navigation?

Or is there a way to have something like a BeforeNavigationObserver to prevent the navigation?

2

Hi Simon,

Could you come up a valid use case for preventing the navigation? if that cannot be achieved with the current flow implementation, we could open an enhancement ticket for the product.

BR.

3

Hi Simon

Yes this is possible with a BeforeLeaveObserver. You can call the postpone() method on the BeforeLeaveEvent to achieve this.

Please note that the beforeLeave method is not triggered if you close the browser or the browser-tab, or when you navigate by entering a new URL manually. If you need to catch those types of leaving too, have a look at [this thread]
(https://vaadin.com/forum/thread/17523194/unsaved-changes-detect-page-exit-or-reload)

@Override
public void beforeLeave(BeforeLeaveEvent event) {
	// only prevent if certain condition is met. In my example I prevent navigation if binder has changes.
	if(binder.hasChanges()){
	
		// prevents navigation
		BeforeLeaveEvent.ContinueNavigationAction action = event.postpone();

		// after you prevented the navigation, you are still able to proceed with the navigation, by using action.proceed();
		// it is good practice IMO to give the user the choice to navigate away anyway, if they wish so. 
		ConfirmDialog dialog = new ConfirmDialog(
				"Unsaved Changes",
				"You have unsaved changes. Are you sure you want to leave this page anyway?",
				"yes", confirmEvent -> {
					// do the navigation anyway
					action.proceed();
				},
				"no", cancelEvent -> {
					// navigation was already prevented with event.postpone() so nothing has to be done here
				}
		);
		dialog.open();
	}
}
4

Ok this works.

But the URL is wrong because it is already the URL of the target route.
Is there a way to reset that?

5

That is an known issue currently, https://github.com/vaadin/flow/issues/3619 and you can find a workaround in the ticket https://github.com/vaadin/flow/issues/3619#issuecomment-472851575