throw when a transition starts while another is still in progress

[mjackson]

Using the high-level transitionTo, replaceWith and goBack methods are a footgun when used inside willTransitionTo, so we should warn users if they try and kick off a transition while another is still in progress. The easiest way to do this is probably inside Routes#dispatch. Just have some flag that indicates whether or not a transition is currently in progress, and throw if a new one comes in.

See #198 (comment)

[machty]

Why not do the Ember thing and treat this situation as an implicit abort of the first transition attempt?

[ryanflorence]

I agree with @machty. If a transition hook returns a slow promise (which we wait for) the user could have clicked another link. I think we should allow that, and just abort the current transition.

[mjackson]

👍 Yeah, that totally makes sense.

[ryanflorence]

@mjackson is this a part of your big refactor?

[mjackson]

I think we can include this in the work I'm doing now.

[mjackson]

This was fixed by #480