The key enabler of Swift conversions are the Xcode bridging headers, of which there are two. In addition the @objc attribute is used to expose Swift classes, protocols, and enums to Objective-C code. This is essential as you will be converting your code file-by-file.

Firstly, beware of cross dependencies. You are going to create them. For example, say you just finished converting Protocol.h into Protocol.swift. Now:

• Xxx-Bridging-Header.h contains DependsOnProtocol.h
• Xxx-Swift.h contains Protocol.swift (as Objective-C) which is inherited by DependsOnProtocol.m

You'll get a build error and a headache. In this case the solution is to keep going and convert DependsOnProtocol.h/.m as well.

Secondly, beware of rebuild issues. In Xcode 7.1 changing the Xxx-Bridging-Header.h does not always correctly build all dependencies. Until this is fix you may need to Clean and Build.

The Xxx-Swift.h Header

This header is auto-generated by the Swift compiler and exposes Swift code to Objective-C.

The @objc Attribute

Any Swift class that needs to be accessible from Swift needs the @objc attribute. Adding this attribute causes the class/protocol/enum to be added to the Xxx-Swift.h bridging header (see above).

The Xxxx-Bridging-Header.h Header

This header is manually maintained by you and needs to include any Objective-C classes that need to be made available to the Swift compiler. Put as little as possible in here and remove things as you convert.

If you use Objective-C Cocoapods, this is where they will live, probably forever or until Swift versions of the libraries become available.