This is a major refactor of how the router previously worked. There are a couple major advantages of this refactor, and future work will be built on top of it.
First, we will no longer have multiple navigations running at the same time. Previously, a new navigation wouldn't cause the old navigation to be cancelled and cleaned up. Instead, multiple navigations could be going at once, and we imperatively checked that we were operating on the most current `router.navigationId` as we progressed through the Observable streams. This had some major faults, the biggest of which was async races where an ongoing async action could result in a redirect once the async action completed, but there was no way to guarantee there weren't also other redirects that would be queued up by other async actions. After this refactor, there's a single Observable stream that will get cleaned up each time a new navigation is requested.
Additionally, the individual pieces of routing have been pulled out into their own operators. While this was needed in order to create one continuous stream, it also will allow future improvements to the testing APIs as things such as Guards or Resolvers should now be able to be tested in much more isolation.
* Add the new `router.transitions` observable of the new `NavigationTransition` type to contain the transition information
* Update `router.navigations` to pipe off of `router.transitions`
* Re-write navigation Observable flow to a single configured stream
* Refactor `switchMap` instead of the previous `mergeMap` to ensure new navigations cause a cancellation and cleanup of already running navigations
* Wire in existing error and cancellation logic so cancellation matches previous behavior
PR Close#25740
b7baf632c0