Swift version: 5.1
If you want to use Objective-C code in your Swift app – and let's face it, that's going to happen quite a lot! – then you need to create a bridging header that allows your Swift code to work with your Objective-C code.
To create an Objective-C bridging header file, all you need to do is drag some Objective-C code into your Swift project – Xcode should prompt you with the message "Would you like to configure an Objective-C bridging header?" Click "Creating Bridging Header" and you'll see a file called YourProjectName-Bridging-Header.h appear in your project.
But that's only half the problem: Xcode has created the bridging header and modified your build settings so that it gets used, but it hasn't actually put anything into it. If you want to start using your Objective-C code in Swift, you need to add import lines to that bridging header file, like this:
You can add as many of these as you want, and indeed you'll want to import all the Objective-C code you want to use in Swift.
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Available from iOS 7.0
This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.