Feature request: ability to validate change on (or before) change.

[jasonbnewell]

I've tried returning false from the onChange event I've provided to the Tree, but even though I'm not updating the underlying data structure (this.state.tree), the UI still updates. It would be really nice to be able to run some checks and possibly prevent this from happening.

[edgesoft]

In the lazy loading exampel issue you pretty much have it all there. Tiny changes to make this happen.