Swift
Theos is capable of compiling Swift files, and supports using them in conjunction with files written in other languages, such as Objective-C. These capabilities are supported on macOS, iOS, and Linux.
Swift Runtime
In order to run Swift binaries built with Theos, you must have the Swift runtime installed on your iOS device. The runtime is bundled with iOS as of iOS 12.2. If your device is on an older version, however, you’ll have to install the runtime. It can be found on BigBoss under the org.swift.libswift package (Swift 5+) or the com.modmyi.libswift4 package (Swift 4). It is recommended that you add libswift as a dependency in your control file with Depends: firmware (>= 12.2) | ${LIBSWIFT} if your project targets earlier iOS versions as this will automatically select the correct package based on your Swift version. This is not necessary if your project only targets iOS versions >= iOS 12.2.
Interoperability with Objective-C
Swift to Objective-C
In order to use Swift classes in Objective-C, you must import the auto-generated swiftmodule header by adding #import <[instance]-Swift.h> to your Objective-C file. Swift declarations that you wish to expose to Objective-C must be marked with the public or open modifier.
Objective-C to Swift
Create a file named <instance>-Bridging-Header.h in your project directory and import any Objective-C headers into it which you wish to expose to Swift. This header file is automatically imported into any Swift files you create.
Variables
<instance>_SWIFTFLAGSor<file>_SWIFTFLAGSstring. Default: empty. Custom flags to pass to the Swift compiler for all files or a specific file (respectively).<instance>_SWIFT_BRIDGING_HEADERfilename. Default:<instance>-Bridging-Header.h. The path to the Objective-C bridging header to be imported into all Swift files during compilation. For more information, see Objective-C to Swift Interoperability.
Tweaks
You can write tweaks in Swift using Orion. Do note that there is no solution for hooking Swift code at the moment.