Difference of designing an development of iOS app?

What is the role and responsibilities of the iOS app designer? The designer of iOS app should also have the advanced knowledge of coding or only the basic knowledge of coding is enough for the designer? I heard that it is not mandatory that the iOS app developer should also have the knowledge of designing but it is better that the iOS developer also gain the knowledge of designing. is it true? Also tell me that it is better if the designer of iOS app will also gain the knowledge of coding?