This tutorial is a nice guide/post-mortem from Skyscanner explaining how they were able to migrate their apps from Swift to Objective-C.
Specifically the tutorial covers how to work with your objective-c code from Swift, and your Swift code in Objective-C. Also covered is how to test your code that utilizes both languages.
You can find the tutorial over on the Code Voyagers blog.
Also be sure to check out the earlier guide on transitioning from objective-c to Swift.
A nice guide for those looking to use Swift with a legacy objective-c app.
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