how to use conditional compiling in SPM modules

To use conditional compiling in Swift Package Manager (SPM) modules, you can leverage build settings and feature macros to control which parts of your code are compiled under different conditions. This is particularly useful when you need to support multiple configurations or environments.

Here are the steps to achieve this:

Define Conditional Compilation Flags: You can define custom flags in your Package.swift file or in Xcode build settings. These flags can be used to conditionally include or exclude code.
Use #if Directives in Code: In your Swift code, use #if, #elseif, and #else directives to conditionally compile code based on the flags you defined.

Example

Step 1: Define Flags in Package.swift

// swift-tools-version:5.7
import PackageDescription

let package = Package(
    name: "MyPackage",
    targets: [
        .target(
            name: "MyTarget",
            swiftSettings: [
                .define("FEATURE_A"),
                .define("FEATURE_B", .when(configuration: .release))
            ]
        )
    ]
)

Step 2: Use #if Directives in Your Code

public struct MyFeature {
    public init() {}

    public func performAction() {
        #if FEATURE_A
        print("Feature A is enabled")
        #elseif FEATURE_B
        print("Feature B is enabled")
        #else
        print("No feature is enabled")
        #endif
    }
}

Additional Resources

For more detailed information on how to manage and optimize your builds, you can refer to the session Demystify explicitly built modules from WWDC 2024. This session covers how to handle different module variants and optimize build settings to reduce unnecessary work.

Relevant Sessions

These sessions provide insights into module management, build optimizations, and the latest updates in Xcode that can help you with conditional compiling in SPM modules.