How can you ensure interface compatibility with legacy Swift code?

1 @objc and @nonobjc attributes

One of the main sources of interface compatibility issues in Swift is the interaction with Objective-C, the language that preceded Swift in Apple platforms. Objective-C has a different syntax, runtime, and type system than Swift, and not all Swift features are available or compatible with Objective-C. To mark which Swift classes, methods, properties, or protocols can be exposed to Objective-C, you can use the @objc attribute. Conversely, to prevent a Swift declaration from being exposed to Objective-C, you can use the @nonobjc attribute. These attributes can help you avoid errors or warnings when bridging between Swift and Objective-C, or when using Objective-C libraries or frameworks in Swift.

2 #if compiler directives

Another way to ensure interface compatibility in Swift is to use #if compiler directives. These are special statements that tell the compiler to include or exclude certain blocks of code depending on some conditions. For example, you can use #if swift to check the version of the Swift compiler, or #if os to check the operating system. By using these directives, you can write different code for different scenarios, and avoid compilation errors or runtime crashes. For example, you can use #if swift(>=5.0) to write code that only works with Swift 5 or later, and #else to write code for earlier versions of Swift.

3 Availability checks

A third way to ensure interface compatibility in Swift is to use availability checks. These are expressions that check if a certain feature, API, or platform version is available at runtime. For example, you can use if #available(iOS 13, *) to check if the app is running on iOS 13 or later, and use the appropriate APIs for that version. Alternatively, you can use the @available attribute to mark which declarations are available for specific platforms or versions. By using availability checks, you can write code that adapts to different environments, and avoid using features or APIs that are not supported or deprecated.