“Package.swft”是一个常规的快速文件，您可以为其中的逻辑和条件编写任何代码。例如，您可以使用ProcessInfo检查环境变量并组装所需的依赖项数组：

import PackageDescription
import Foundation

let useRealm = ProcessInfo.processInfo.environment["realm"] == "1"

let packageDependencies: [Package.Dependency] = useRealm
    ? [.package(url: "https://github.com/realm/realm-cocoa.git", from: "10.15.1")]
    : []

let targetDependencies: [Target.Dependency] = useRealm
    ? [.product(name: "Realm", package: "realm-cocoa")]
    : []

let package = Package(
    name: "MyPackage",
    platforms: [
        .iOS(.v12),
        .macOS(.v10_14)
    ],
    products: [
        .library(name: "MyPackage", targets: ["MyPackage"]),
    ],
    dependencies: packageDependencies,
    targets: [
        .target(name: "MyPackage", dependencies: targetDependencies),
        .testTarget(name: "MyPackageTests", dependencies: ["MyPackage"]),
    ]
)

现在，您可以构建没有依赖项的包：

$ xcrun --sdk macosx swift build
Building for debugging...
[2/2] Emitting module MyPackage
Build complete! (0.77s)

并通过在环境变量中设置realm=1来实现领域依赖关系：

$ export realm=1
$ xcrun --sdk macosx swift build
Fetching https://github.com/realm/realm-cocoa.git from cache
Fetched https://github.com/realm/realm-cocoa.git (2.12s)
Computing version for https://github.com/realm/realm-cocoa.git
Computed https://github.com/realm/realm-cocoa.git at 10.32.0 (0.02s)
Fetching https://github.com/realm/realm-core from cache
Fetched https://github.com/realm/realm-core (1.37s)
Computing version for https://github.com/realm/realm-core
Computed https://github.com/realm/realm-core at 12.9.0 (0.02s)
Creating working copy for https://github.com/realm/realm-cocoa.git
Working copy of https://github.com/realm/realm-cocoa.git resolved at 10.32.0
Creating working copy for https://github.com/realm/realm-core
Working copy of https://github.com/realm/realm-core resolved at 12.9.0
Building for debugging...
[63/63] Compiling MyPackage MyPackage.swift
Build complete! (41.64s)