I'm a big fan of React (and one of the developers working on App Designer).
So for these reasons there is a large impedence mismatch between App Designer and React. There may be ways to extend App Designer to support React, but it is an open question about whether users would find that valuable. There may be other ways the XDK could better expose React, rather than the App Designer.
If you have ideas, or opinions, we would love to hear them. Now is definitely the time to weigh in.
Thank you so much for your great works in App designer and XDK.
Hope to see React in XDK soon.
May be just create a new tool named React Designer will be much clear and easier.