Is there any API to show the route is off?

[LoveofRedMoon]

What problem does this feature solve?

http://jsfiddle.net/b7f4dpkg/10

1. click 'foo'
2. click 'back'
   you will see how computed value testParam changes by watch

when go to foo, testParam.value === 'queryParam'
when go back to the first page, testParam.value === null

I know why this value becomes null --- it shows the next route query value.
But this is not in common sense.
Usually, the query value use in this view page only for current scene.

For example. the edit page will show edit when query.id !== null, will show create when query.id === null
This scene will turn this page to create when leave this page

What does the proposed API look like?

Should not load new query value when the previous view haven't be deactivated (instance.isDeactivated)

[posva]

Duplicate of vuejs/core#2291