How to write CLI apps for macOS using swift, which packages to use, common challenges to overcome, how to structure your CLI app code.

1. Let's build macOS CLI Utilities
...using Swift!
Diego Freniche | Realm Developer Advocate | @dfreniche

2. Why CLI tools in Swift?
● small scripts to scratch an itch
● to automate things
● because they look cool

3. Demo Time!
● based on Cowsay
● brew install cowsay
● for extra dramatism try cowsay
Hello, World && say "Hello,
World"

4. Demo Time!

5. The quick & dirty Just-One-File approach
#!/usr/bin/swift
func fibonacci(_ n: Int) -> Int {
if n <= 2 {
return 1
} else {
return fibonacci(n - 1) + fibonacci(n - 2)
}
}
print(fibonacci(10))

6. $ swift fibonacci.swift
lib/swift -module-name fibonacci -target-sdk-version 12.0.0
55
Running it!
$ swift -v fibonacci.swift
Apple Swift version 5.5 (swiftlang-1300.0.27.6 clang-1300.0.27.2)
Target: x86_64-apple-macosx11.0
/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchai
n/usr/bin/swift-frontend -frontend -interpret fibonacci.swift -enable-objc-interop
-stack-check -sdk
/Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Develope
r/SDKs/MacOSX12.0.sdk -color-diagnostics -new-driver-path
/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchai
n/usr/bin/swift-driver -resource-dir
/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchai
n/usr/lib/swift -module-name fibonacci -target-sdk-version 12.0.0

7. ● only one file per script
● no include other .swift file
● no easy way to add 3rd party code
● We need to launch it using a bash script most likely *
* Hint: we don’t want bash, that’s the whole point of this talk!
⚠ Limitations of Swift Scripts

8. Command line tools 🛠 are software. Structuring their code in
massive files that read like bash scripts is missing an opportunity to
leverage the abstractions of the programming language and
platform to have a codebase that is easier to reason about. *
* https://twitter.com/pedropbuendia/status/1230414793191890950?s=20
🙏Always write good code

9. Create a Xcode project!
● New Project
● Mac > Command Line Tool

10. ● put code in main.swift
● hit Cmd + R
● watch for output in Xcode's
Terminal
● Starting point:
main.swift
CLI tool new project: Apple Template

11. Program Starting point, a better way
// We can call this file whatever we want, just not main.swift
// also, no code can be in main.swift
import Foundation
@main
struct MainChiquitoSay {
static func main() {
print("Tea duck queen?")
}
}

12. 🙈 No tests!
● Yes, we can create a Test Target
● Problem: no way to reach our code (no module defined)

13. 🙈 No tests!
● “Solution”: add each file to the Test target’s (🙈 x 2)

14. We need a better way!
Modularize the code!

15. How to add tests? Modules!
● Create a Framework and use it from the CLI app.
● BUT
○ We need to install that Framework in our system
(dynamically linked)
● We can do the same with a Static Library

16. The Static Library route
● Add a Library target
● Static library
● Move all code there
● Add a test target for your library

17. 🥇 Best way to modularize?
● SPM!
○ For 3rd party dependencies
○ For our own code (local packages)

18. Managing 3rd party dependencies
Go to Project > Swift Packages
1. Add new packages
2. See list of packages
3. Change versions, etc.
File > Packages: bunch of interesting options

19. My default Pack Of Packages 🎁
● swift-argument-parser:
○ CLI arguments parsing, help, etc.
○ https://github.com/apple/swift-argument-parser
● ANSITerminal:
○ Prints ANSI escape sequences, easy
○ https://github.com/pakLebah/ANSITerminal
● Files:
○ Makes using Files and Folders easy
○ https://github.com/johnsundell/files
● SwiftFigletKit:
○ https://github.com/dfreniche/SwiftFiglet

20. Main pieces of the CLI puzzle 🧩
🖥 Printing/reading to/from console
🎨 Nice ANSI colors
🔍 Argument Parsing
🗂 Working with Files and Folders
🚀 Launching other Processes
󰝊 Background Jobs

21. 🖥 Printing to Console
● just use good old print
● use print(message, separator: "", terminator: "") if you don't
want CR + LF before your message
● 🎨 Nice ANSI colors if possible
○ 🎁 https://github.com/pakLebah/ANSITerminal
○ print("ChiquitoSay starting".green)

22. 🖥 Reading from Console
● readLine(), readLine(strippingNewline: true)
● returns String?, so better do readLine() && ""
● readLine does not work in Playgrounds 🙈
https://developer.apple.com/documentation/swift/1641199-readline

23. 🔍 Argument parsing
ARGUMENTS:
<image-url> URL with the image to show
<message> The message to print
OPTIONS:
--print-banner Print a text banner instead of image
--say-text Say text aloud
-h, --help Show help information.

24. 🔍 Argument parsing
● 🎁 https://github.com/apple/swift-argument-parser
● make our main struct conform to ParsableCommand
import ArgumentParser
@main
struct MainChiquitoSay: ParsableCommand {
...

25. 🔍 Argument parsing
import ArgumentParser
@main
struct MainChiquitoSay: ParsableCommand {
@Flag(help: "Print a text banner instead of image")
var printBanner = false
@Argument(help: "URL with the image to show")
var imageURL: String

26. Passing arguments from Xcode

27. 🗂 Working with Files and Folders
SPM: https://github.com/JohnSundell/Files
extension Folder {
public static func cd(path: String) -> Folder {
do {
let parent = try Folder.init(path: path)
return parent
} catch {
return Folder.current
}
}
}

28. 🚀 Launching other processes
/// - command: full path to the program we want to launch
/// - arguments: Array of arguments passed to that program
static func launch(command: String, arguments:[String] = []) {
let url = URL(fileURLWithPath: command)
do {
try Process.run(url, arguments: arguments) { (process) in
print("ndidFinish: (!process.isRunning)")
}
} catch {
print("Error opening file (error)")
}
}

29. 󰝊Background Jobs

30. 󰝊Background Jobs
● we can't use GCD here
● If we launch a background process it will be killed: our app will continue and finish
before the BG process finishes
● So we need a way to wait
● we'll use Semaphores
more info on GCD + CLI here:
https://stackoverflow.com/questions/8366195/using-grand-central-dispatch-outsi
de-of-an-application-or-runloop

31. 󰝊Background Jobs
let sema = DispatchSemaphore( value: 0)
// define a background task
let task = URLSession.shared.dataTask(with: imageURL) { (data, response, error) in
sema.wait() // decrement semaphore, no more operations at the time
// do something that takes time
sema.signal()
end = true // signals the process to continue
};
task.resume() // launch the task in a new thread

32. “Continuous Integration” LOL

33. If you want more...
https://github.com/migueldeicaza/TermKit
https://github.com/migueldeicaza/TurboSwift

34. ...and more
● fdir:
○ Lists directory contents adding comments to files
○ https://github.com/dfreniche/fdir
● ChiquitoSay:
○ A meme creator
○ https://github.com/dfreniche/chiquitosay
● Memo:
○ A really minimalistic memo app that stores messages in a local
Realm DB
○ https://github.com/dfreniche/memo

35. Thank you!
That’s all folks
Diego Freniche | Realm Mobile Developer Advocate | @dfreniche