What is a Swift file?

Swift files contain source code in Swift, a modern, compiled programming language developed by Apple and introduced at WWDC 2014. Swift was designed by Chris Lattner (also the creator of LLVM and Clang) to be safer, faster, and more expressive than Objective-C while maintaining full interoperability with Objective-C code. Apple open-sourced Swift in December 2015 under the Apache 2.0 license, enabling its use beyond Apple platforms.

Swift enforces safety at the type level: variables must be initialized before use, arrays are bounds-checked, integer overflow is trapped, and optionals (?) make null-handling explicit. The compiler catches entire categories of bugs before code ever runs.

How to open Swift files

• Xcode (macOS only) — Apple’s official IDE with full Swift support, SwiftUI previews, and simulator
• VS Code (Windows, macOS, Linux) — With the Swift extension (sourcekit-lsp)
• Swift Playgrounds (macOS, iPad) — Interactive learning environment
• Any text editor — Swift files are plain UTF-8 text

Technical specifications

Common use cases

• iOS apps: iPhone and iPad applications on the App Store
• macOS apps: Native desktop applications for Mac
• watchOS / tvOS / visionOS: Apple platform apps across all devices
• Server-side: Vapor and Hummingbird web frameworks on Linux
• System scripting: Swift scripts replacing shell scripts with type safety
• Cross-platform: Swift on Linux and Windows (still maturing)

Swift code example

import Foundation

struct User: Codable {
    let id: Int
    let name: String
    let email: String
}

func fetchUsers() async throws -> [User] {
    let url = URL(string: "https://api.example.com/users")!
    let (data, _) = try await URLSession.shared.data(from: url)
    return try JSONDecoder().decode([User].self, from: data)
}

// Usage in async context
Task {
    do {
        let users = try await fetchUsers()
        users.forEach { print("\($0.name): \($0.email)") }
    } catch {
        print("Error: \(error)")
    }
}

SwiftUI

SwiftUI (introduced 2019) is Apple’s declarative UI framework written in Swift. It uses Swift’s result builders to describe UI in code:

struct ContentView: View {
    @State private var count = 0

    var body: some View {
        VStack {
            Text("Count: \(count)")
                .font(.title)
            Button("Increment") {
                count += 1
            }
            .buttonStyle(.borderedProminent)
        }
        .padding()
    }
}

SwiftUI previews in Xcode update live as you type, without running the simulator.

Optionals and safety

Swift’s optional type makes the possibility of a missing value explicit:

var name: String? = nil   // might be nil
var required: String = "hello"  // can never be nil

// Safe unwrapping
if let name = name {
    print("Hello, \(name)")
}

// Nil coalescing
let display = name ?? "Anonymous"

This eliminates NullPointerException-style crashes. Swift’s compiler forces you to handle the nil case explicitly, making runtime crashes due to unexpected nil far less common than in Objective-C or many other languages.

Swift Package Manager

swift package init --type executable   # Create package
swift build                            # Compile
swift run                              # Run
swift test                             # Test
swift package add-dependency           # Add dependency

SPM is integrated into Xcode and supports libraries, executables, and plugins. It handles versioning via semantic versioning in Package.swift.