LinuxでもSwiftを使ってGUI開発をしたい!【SwiftGtk】 ref: https://qiita.com/YutoMizutani/items/a550b6fa5767a7c55eea

main.swift

import SwiftGtk

let app = Application(applicationId: "com.example.application")
app.run { window in
    window.title = "Hello World"
    window.defaultSize = Size(width: 400, height: 400)
    window.resizable = true

    let button = Button(label: "Press Me")
    button.clicked = { _ in
        let newWindow = Window(windowType: .topLevel)
        newWindow.title = "Just a window"
        newWindow.defaultSize = Size(width: 200, height: 200)
        let labelPressed = Label(text: "Oh, you pressed the button.")
        newWindow.add(labelPressed)

        newWindow.showAll()
    }

    window.add(button)
}

Package.swift

// swift-tools-version:4.2
// The swift-tools-version declares the minimum version of Swift required to build this package.

import PackageDescription

let package = Package(
    name: "test",
    dependencies: [
        // Dependencies declare other packages that this package depends on.
        // .package(url: /* package url */, from: "1.0.0"),
        .package(url: "https://github.com/TomasLinhart/SwiftGtk", "0.3.1" ..< "0.4.0"),
    ],
    targets: [
        // Targets are the basic building blocks of a package. A target can define a module or a test suite.
        // Targets can depend on other targets in this package, and on products in packages which this package depends on.
        .target(
            name: "test",
            dependencies: ["SwiftGtk"]),
        .testTarget(
            name: "testTests",
            dependencies: ["test"]),
    ]
)

Terminal

$ mkdir test && cd $_
$ swift package init --type executable

$ vi Package.swift