Creating and publishing a Swift package

In the previous post we looked into how to separate code with local Swift packages within a project. This time let’s create a Swift package, publish it on GitHub, and add it to a separate project. We’ll create a package which extends UIImage and enables calculating color contrast ratios. Color contrast is important factor for keeping text readable in apps.

Creating a Swift package

Open Xcode and select “Swift Package” item from the File > New menu. We’ll set the name of the package to “ColorContrastRatio”.

Xcode’s template of new Swift packages is configured to have a basic hello world example with an unit-test. Before we change the package’s implementation, we’ll add minimum platform versions to the Package.swift file, in other words, minimum deployment target.

	// swift-tools-version:5.3
	// The swift-tools-version declares the minimum version of Swift required to build this package.

	import PackageDescription

	let package = Package(
	name: "ColorContrastRatio",
	platforms: [
	.iOS(.v13), .macOS(.v10_15)
	],
	products: [
	// Products define the executables and libraries a package produces, and make them visible to other packages.
	.library(
	name: "ColorContrastRatio",
	targets: ["ColorContrastRatio"]),
	],
	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 this package depends on.
	.target(
	name: "ColorContrastRatio",
	dependencies: []),
	.testTarget(
	name: "ColorContrastRatioTests",
	dependencies: ["ColorContrastRatio"]),
	]
	)

view raw Package.swift hosted with ❤ by GitHub

Next step is to implement functionality of the package. We’ll keep the package simple and make it UIKit only which can be built for iOS and macCatalyst apps. Package is adding an UIColor extension and providing functionality for calculating relative luminance and contrast ratio. Functionality is set, next step is to publish it.

	import UIKit

	/*
	Contrast ratio is calculated using the proceedure here:
	https://www.w3.org/TR/WCAG20-TECHS/G17.html#G17-procedure
	*/

	public extension UIColor {
	/// Relative luminance of the color.
	var relativeLuminance: CGFloat {
	var red: CGFloat = 0
	var green: CGFloat = 0
	var blue: CGFloat = 0
	guard getRed(&red, green: &green, blue: &blue, alpha: nil) else { return 1.0 }

	let convert: (CGFloat) -> CGFloat = { component in
	guard component > 0.03928 else { return component / 12.92 }
	return pow(((component + 0.055) / 1.055), 2.4)
	}
	return 0.2126 * convert(red) + 0.7152 * convert(green) + 0.0722 * convert(blue)
	}

	/// Returns contrast ratio with other color.
	/// – Parameter otherColor: UIColor in RGB color space.
	/// – Returns: Contrast ratio of two colors.
	func contrastRatio(_ otherColor: UIColor) -> CGFloat {
	let luminance1 = relativeLuminance
	let luminance2 = otherColor.relativeLuminance
	return (min(luminance1, luminance2) + 0.05) / (max(luminance1, luminance2) + 0.05)
	}
	}

view raw ColorContrastRatio.swift hosted with ❤ by GitHub

Publishing a Swift package

First step is to go to GitHub and adding a new repository. I chose to include an automatically created license file, cloned the repository on my mac and then moved the code we added in the previous step to that checkout, then committed and pushed. Alternative is to add a remote to your local git repository. Either way is fine as long as our project ends up on GitHub. In addition, will add a tag which will mark the first release 0.1.0. Tags can be added by running those commands in Terminal:

git tag -a 0.1.0 -m “0.1 release of the package”

git push origin 0.1.0

Another option is creating a tag in the GitHub’s web interface: releasing projects on GitHub.

Adding the published Swift package to another project

We published our package on GitHub and it requires only a few steps for adding it to an existing project. In an existing project, open the target settings and click on the plus button in the “Frameworks, Libraries, and Embedded Content”. In the opened view, click on the “Add Other” and select “Add Package Dependency”. Then we can paste the package’s GitHub url (https://github.com/laevandus/ColorContrastRatio) to the search field and complete the flow. From then on, it is just a matter of importing the new package and using it in the main project. Described steps are shown below:

Selecting the target where to the package is added.

Selecting “Add Package Dependency” which allows adding Swift package.

Finished example application using the Swift package’s color contrast ratio function.

	import ColorContrastRatio
	import SwiftUI

	struct ContentView: View {
	var body: some View {
	VStack {
	Text("Luminance for red color").padding()
	Rectangle()
	.frame(width: 50, height: 50)
	.fixedSize()
	.foregroundColor(.red)
	Text("\(UIColor.red.relativeLuminance)")
	}
	}
	}

view raw ContentView.swift hosted with ❤ by GitHub

Summary

We created a Swift package and a repository on GitHub. Then we proceed with making the first release by adding a git tag. After that, we went ahead and added the package to another example project.

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