Expose a configurable beforeunload api from the router.

[samccone]

When building an application that has page state (such as uploads), it can be powerful to be able to prompt the user before they leave the page. In traditional web apps we are able to use beforeunload in order to warn / prompt the user.

In SPA land however we need to implement something similar for in app navigation. Currently the router https://github.com/sveltejs/kit/blob/master/packages/kit/src/runtime/internal/router/index.js does not expose hooks like this

Looking at prior art react-router has a concept of a Prompt
https://github.com/ReactTraining/react-router/blob/master/packages/react-router/docs/api/Prompt.md

Ember has a concept of a route willTransition
https://guides.emberjs.com/release/routing/preventing-and-retrying-transitions/

Backbone has the ability to return false from execute
https://backbonejs.org/#Router-route

[benmccann]

Yeah, this seems like a good idea. @antony had a request for this as well in Sapper: https://github.com/sveltejs/sapper/issues/1278

[benmccann]

I transferred the other issue to this repo so that we don't lose the conversation history. Closing this one as a duplicate of the other: #552