[FEATURE ember-routing-will-change-hooks] finer-grain transition hooks

[machty]

Brought to you by @ef4, and demonstrated at his EmberConf talk:

http://confreaks.com/videos/3302-emberconf2014-animations-and-transitions-in-an-ember-app

See this router.js PR for details:

tildeio/router.js#89

TL;DR adds willLeave and willChangeModel hooks (not that Ember renames the router.js willChangeContext hook to willChangeModel)

[tomdale]

Allegedly, this is no longer used in Liquid Fire. We want to wait to see if there is an existing primitive that can be used before adding more API surface area. @machty will talk to @ef4.

[stefanpenner]

@ef4 just confirming, you found a better way to do this?

[ef4]

liquid-fire doesn't need this.

But this is not an animation-specific feature, and I think it still fills a gap in the API. In my own app I have use cases for it, for things like a "do you really want to leave this screen without saving?" alert when there are nested child routes in play.