Conditionally including a system library (FFmpeg) in Swift Package Manager builds?

I’m working on a Swift project and would like to create two builds of my package:

• A basic version without the external C library (FFmpeg).
• And a version that includes an optional dependency on FFmpeg.

Here’s what I have so far is two separate Package.swift files:

    ...
    targets: [
        .executableTarget(
            name: "macSubtitleOCR",
            dependencies: [
                .product(name: "ArgumentParser", package: "swift-argument-parser")
            ]),
    ...

and:

    ...
    targets: [
        .systemLibrary(
            name: "CFFmpeg",
            pkgConfig: "libavformat libavcodec libavutil",
            providers: [
                .brew(["ffmpeg"])
            ]),
        .executableTarget(
            name: "macSubtitleOCR",
            dependencies: [
                .product(name: "ArgumentParser", package: "swift-argument-parser"),
                "CFFmpeg"
            ]),
     ...

I've tried adding compile flags like #if FFMPEG and then running swift build -Xswiftc -DFFMPEG which works for the source files, but compile flags don't work properly, and understandably so, in Package.swift.

My question is:

1. How can I conditionally include a system library like FFmpeg in Package.swift based on build flags or any other approach?
2. Is using multiple Package.swift files (one with and one without FFmpeg) a reasonable approach, or is there a more correct way to handle optional dependencies in SwiftPM?

I’m relatively new to Swift development, so any guidance or alternative approaches would be greatly appreciated!

Swift Package Manager (SwiftPM) doesn’t directly support conditional dependencies, but you can manage optional dependencies using environment variables. This approach enables conditional inclusion of a dependency in a single Package.swift file.

Solution

Define an environment variable and use conditional logic in Package.swift to include the CFFmpeg target only when needed:

import PackageDescription

let hasFFmpeg = ProcessInfo.processInfo.environment["USE_FFMPEG"] == "1"

let package = Package(
    name: "macSubtitleOCR",
    dependencies: [
        .package(url: "https://github.com/apple/swift-argument-parser", from: "1.0.0")
    ],
    targets: [
        .executableTarget(
            name: "macSubtitleOCR",
            dependencies: [
                .product(name: "ArgumentParser", package: "swift-argument-parser")
            ] + (hasFFmpeg ? ["CFFmpeg"] : [])
        )
    ] + (hasFFmpeg ? [
        .systemLibrary(
            name: "CFFmpeg",
            pkgConfig: "libavformat libavcodec libavutil",
            providers: [
                .brew(["ffmpeg"])
            ]
        )
    ] : [])
)