hi, is it good use swift inside a objc application without rewrite all (the app is very big and complex)
using swift inside a old objc application
Depends on the Swift code you want to use. Some swift codes cannot be exposed to ObjC.
If you are going to write a new Swift class from now on, it is not so difficult to write an ObjC compatible Swift class.
If you are going to write a new Swift class from now on, it is not so difficult to write an ObjC compatible Swift class.