And I thought 2017 is the year when I start a new app in swift... guess will be sticking to objc and react native depending on what makes sense where.
ps: Well, maybe pure objc is not the best choice, since it's on its way out, but it's a long way to its end.
There are none. Don't transition an existing, stable ObjC project. I don't understand why this is a thought anyone would entertain, especially since ObjC itself isn't going anywhere.
If you are starting a new project in Swift and it will be pure Swift, that's fine. Go for it. Do not attempt to migrate existing ObjC projects or use Swift in them.