How to conditionally compile code in a Flutter plugin using Swift Package Manager

Sometimes it's useful to have "optional" code in a plugin that the app can opt-out.

For example, camera access requires user consent. Apps that don't need this optional camera feature might want to remove this feature to reduce the app's permissions.

Here's how a plugin might do this:

#if MY_CAMERA_FEATURE_ENABLED
public func useCamera() {
  // ...
}
#endif

In this example, the useCamera function is included in the compilation only if MY_CAMERA_FEATURE_ENABLED is defined.

In CocoaPods, the MY_CAMERA_FEATURE_ENABLED can be defined by customizing targets' build settings.

Unfortunately, Swift Package Manager doesn't support conditional compilation yet. A proposal to add package traits is currently under review, but it is unclear when this feature will be available.

Hacky workaround

Here's a hack to add conditional compilation to your Swift package:

1. Update your Package.swift file to define MY_CAMERA_FEATURE_ENABLED if the MY_CAMERA_FEATURE_ENABLED environment variable is set to 1:

    // swift-tools-version: 5.9
    // The swift-tools-version declares the minimum version of Swift required to build this package.
   
    import PackageDescription
   +import Foundation
   +
   +var swiftSettings: [SwiftSetting] = []
   +
   +if ProcessInfo.processInfo.environment["MY_CAMERA_FEATURE_ENABLED"] == "1" {
   +    swiftSettings.append(.define("MY_CAMERA_FEATURE_ENABLED"))
   +}
    
    let package = Package(
       name: "my_plugin",
       platforms: [],
       products: [],
       dependencies: [],
       targets: [
           .target(
               name: "my_plugin",
               dependencies: [],
   -            resources: []
   +            resources: [],
   +            swiftSettings: swiftSettings
           )
       ]
   )

2. Clean your previous Xcode compilation so that your Swift package is rebuilt.

   flutter clean

3. Run your app with the MY_CAMERA_FEATURE_ENABLED environment variable.

   Using the command line:

   MY_CAMERA_FEATURE_ENABLED=1 flutter run
   

   Or, using VS Code launch configurations:

   {
     "configurations": [
       {
         "name": "Flutter",
         "request": "launch",
         "type": "dart",
         "env": {
           "MY_CAMERA_FEATURE_ENABLED": "1"
         }
      }
    ]
   }

Warning

Swift Package Manager does not automatically rebuild your Swift package when the value of MY_CAMERA_FEATURE_ENABLED changes.

You must run flutter clean whenever the value of MY_CAMERA_FEATURE_ENABLED changes!

You must also run flutter clean if you switch between different projects that use your plugin with different MY_CAMERA_FEATURE_ENABLED values.