Right now, we will go through an exercise to build a simple Swift package and learn about the important files and folders. We will also publish this package and consume it in another Swift package to show how we can publish packages and import them as dependencies. For our exercise, we will create a simple cat command-line tool which will concatenate and print the contents of the files specify relative to the current directory.

In order for us to do so we will first build a package called FileReader which will read and return the contents of the file. To build this Swift package, we need to do the following:

Let's inspect the contents of the package. The following is the file and folder structure inside of FileReader:

~/W/FileReader $ tree .
.
├── Package.swift
├── README.md
├── Sources
│   └── FileReader
│       └── FileReader.swift
├── Tests
   ├── FileReaderTests
   │   └── FileReaderTests.swift
   └── LinuxMain.swift
4 directories, 5 files

Now that we know the basic file and folder structure, we can start writing our Swift code to read files from disk inside of the FileReader.swift file. By default, it will contain boilerplate code which we can remove and replace with this:

import Foundation
class FileReader {
 static func read(fileName: String) -> String? {
   let fileManager = FileManager.default
   let currentDirectoryURL = URL(fileURLWithPath: 
                          fileManager.currentDirectoryPath)
   let fileURL = currentDirectoryURL.appendingPathComponent(fileName)
   return try? String(contentsOf: fileURL, encoding: .utf8)
 }
}

In this file, we import Foundation, which is a standard library available in macOS and Linux and it provides us with the standard library to read from a file path using the FileManager. After that, we define the FileReader class and create one static function in it, called read, that takes a filename and this function will return the contents of the file if the file exists. The code inside the function does the following:

   let fileManager = FileManager.default

   let currentDirectoryURL = URL(fileURLWithPath: fileManager.currentDirectoryPath)

   let fileURL = currentDirectoryURL.appendingPathComponent(fileName)

   return try? String(contentsOf: fileURL, encoding: .utf8)

Now that we have the code, we can build it using Swift build. To test that our code is working, we need to write a test for it and we can do so by taking the following steps:

XCTAssertEqual(FileReader.read(fileName: "hello.txt"), "Hello World")

printf "Hello World" > hello.txt

~/W/FileReader $ swift test
Compile Swift Module 'FileReaderTests' (1 sources)
Linking ./.build/x86_64-apple-macosx10.10/debug/FileReaderPackageTests.xctest/Contents/MacOS/FileReaderPackageTests
Test Suite 'All tests' started at 2017-09-29 12:14:57.278
Test Suite 'FileReaderPackageTests.xctest' started at 2017-09-29 12:14:57.278
Test Suite 'FileReaderTests' started at 2017-09-29 12:14:57.278
Test Case '-[FileReaderTests.FileReaderTests testExample]' started.
Test Case '-[FileReaderTests.FileReaderTests testExample]' passed (0.094 seconds).
Test Suite 'FileReaderTests' passed at 2017-09-29 12:14:57.372.
Executed 1 test, with 0 failures (0 unexpected) in 0.094 (0.094) seconds
Test Suite 'FileReaderPackageTests.xctest' passed at 2017-09-29 12:14:57.372.
Executed 1 test, with 0 failures (0 unexpected) in 0.094 (0.094) seconds
Test Suite 'All tests' passed at 2017-09-29 12:14:57.372.
Executed 1 test, with 0 failures (0 unexpected) in 0.094 (0.094) seconds

Now that we have a working Swift package, we can publish it.

Publishing a Swift package is as simple as committing code, tagging it, and pushing it up to a git repository. To publish the package, perform the following steps:

git remote add origin [email protected]:<repoaccount>/<reponame>.git

 git push origin master --tags

It is that easy to make a Swift package and publish it. All you need is a git repository to push your code to and tag your code appropriately so that whoever uses your package as a dependency can point to a specific version.

Next, we will try to use this package to create an executable package called cat that concatenates and prints the contents of the files passed in as arguments to the command. This executable will work like the built-in-system cat command found in most Unix based operating systems. To do so, we need to perform the following steps:

.package(url: "https://github.com/<repoaccount>/<reponame>", from: "1.0.0"),

import PackageDescription

let package = Package(
  name: "cat",
  dependencies: [
    .package(url: "https://github.com/ankurp/FileReader", from: "1.0.0"),
  ],
  targets: [
    .target(
      name: "cat",
      dependencies: ["FileReader"]),
  ]
)

import FileReader

for argument in CommandLine.arguments {
  guard argument != "arg1" else { continue }

  if let fileContents = FileReader.read(fileName: argument) {
    print(fileContents)
  }
}

Let's try to understand what we have done in the preceding code:

import FileReader

for argument in CommandLine.arguments {

guard argument != "arg1" else { continue }

if let fileContents = FileReader.read(fileName: argument) {
  print(fileContents)
}

Now that we have understood the code, let's build and run it to see whether it works. To build and run, just type the following command in the Terminal:

$ swift run cat Package.swift Sources/cat/main.swift

You should see the contents of both the files, Package.swift and Sources/cat/main.swift, printed in the console. Great job! We have a working command line tool written in Swift using one of our published Swift packages:

How do we install the command line tool we just created? Don't worry, it's simple too. All you need to do is build it with the release configuration, so that it builds a highly optimized binary and also add flags to statically link the Swift standard library. This means that the executable can work even when Swift versions change on your operating system, or if you plan on distributing it on another platform, such as Linux. The following is the command to build the executable command with the release configuration:

$ swift build -c release -Xswiftc -static-stdlib

Once you have the binary built, you need to copy it to one of the directories where binaries are stored in your user path. One such place is /usr/local/bin. To copy it, just run the following command and call your binary file whatever you want. In my case, I chose to rename my command to swiftycat:

$ cp -f .build/release/cat /usr/local/bin/swiftycat

Now, try it out in the Terminal by running the following command:

$ swiftycat Package.swift Sources/cat/main.swift