There's been a few React animation experiments (including my own: https://github.com/chenglou/react-tween-state), but I think there are two main problems here and this library, albeit nice, is mingling the two:

- Actual transition API (e.g. fade in over a period of 1s while moving in from left). Interruptible and composable. My lib and most libs I've seen are geared toward this.

- API for transitioning in/out and moving around. This is the actual hard part that TransitionGroup is trying to tackle. How do you express an "animating out" API (that's also interruptible?) when the item doesn't exist anymore at the next render? Can you stop the component from unmounting midway, and potentially bring it back through an "un-unmounting" API? I haven't seen a library that actually tackles this problem. We can debate about a good API all we want for animation but as long as this problem isn't solved, the whole animation problem isn't imo.

These two are separate issues. Ideally I think you should pull out the velocity part and create a wrapper API around it, and solve transitioning in/out independently.

(I'm actively searching for solutions, so please ping me in #reactjs if you have good ideas. Here are a few messy ones to begin with: https://gist.github.com/chenglou/40b75d820123a9ed53d8).