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The beauty of Swift is that you can mix both languages. Therefore in an existing Objective-C project, you can continue to work and update your app with new Swift code.

Apple doc explain how to mix-and-match, but isn’t clear.

Steps to Using Swift in Objective-C

Add your swift file.

(Optional) When prompted to add bridging header, select yes if you want to use Objective-C in Swift (the other way round).

In your project/target build settings, ensure:

1. Defines Module set to Yes
2. Module Name does not include spaces

In your Objective-C file, to use the Swift code,

#import "YourModuleName-Swift.h"

NOTE: The header YourModuleName-Swift.h is NOT visible in your project. It is autogenerated by Xcode.

Your module name is the one found in your build settings.

Steps to Using Objective-C in Swift

The other way round is similar, except:

1. The header name is YourModuleName-Bridging-Header.h
2. The header is visible in Xcode
3. You don’t have to import the header in Swift

In this bridging header file, you have to manually add statements to import Objective-C header which you want to expose to Swift.