Creating hex-based colors in UIKit, AppKit and SwiftUI
In this post, let’s take a quick look at how to we can extend UIKit, AppKit & SwiftUI colors with hex-based initializers that accept strings ("#abcdef") and numeric values (0xabcdef).
Although you can use asset catalogs to define colors, there may come a time when you need to create colors from hex codes, for instance when parsing colors from an API.
You may find it strange that UIKit, AppKit & SwiftUI have no initializers for creating colors with hex codes, and you’d be right to think so. It’s a bit strange.
I first thought that this was to encourage developers to use asset catalogs, since color assets can support dark and high contrast mode, but then why have the rgba initializers?
However, let’s take a look at how you can add your own extensions to provide a way to use hex codes to create colors. It’s pretty straightforward.
Multi-platform bridging
To simplify working with colors on multiple platforms, let’s define a convenience typealias that we can use on all platforms, to make the rest of the code cleaner:
#if os(macOS)
import AppKit.NSColor
typealias ColorRepresentable = NSColor
#endif
#if os(iOS) || os(tvOS) || os(watchOS)
import UIKit.UIColor
typealias ColorRepresentable = UIColor
#endif
ColorRepresentable resolves to NSColor on macOS, and UIColor on all other platforms. The rest of our code doesn’t have to know which type we really use.
Adding hex support to ColorRepresentable
With ColorRepresentable in place, we can extend it with a few initializers that let us create platform-specific colors with hex-based strings and integers.
Let’s start with the integer-based initializer:
extension ColorRepresentable {
convenience init(hex: UInt64, alpha: CGFloat = 1) {
let r = CGFloat((hex >> 16) & 0xff) / 255
let g = CGFloat((hex >> 08) & 0xff) / 255
let b = CGFloat((hex >> 00) & 0xff) / 255
self.init(red: r, green: g, blue: b, alpha: alpha)
}
}
This initializer lets us provide numeric values like 0xabcdef, which lets you express hex colors without the # that you often see when working with colors.
For string-based hex codes, we need a few string extensions, which we can keep private:
private extension String {
func cleanedForHex() -> String {
if hasPrefix("0x") {
return String(dropFirst(2))
}
if hasPrefix("#") {
return String(dropFirst(1))
}
return self
}
func conforms(to pattern: String) -> Bool {
let pattern = NSPredicate(format:"SELF MATCHES %@", pattern)
return pattern.evaluate(with: self)
}
}
cleanedForHex() removes any hex-specific prefixes, like # and 0x while conforms(to:) can be used to evaluate if a string conforms to a certain regular expression.
We can now implement the string-based initializer:
public extension ColorRepresentable {
convenience init?(hex: String, alpha: CGFloat = 1) {
let hex = hex.cleanedForHex()
guard hex.conforms(to: "[a-fA-F0-9]+") else { return nil }
let scanner = Scanner(string: hex)
var hexNumber: UInt64 = 0
guard scanner.scanHexInt64(&hexNumber) else { return nil }
self.init(hex: hexNumber, alpha: alpha)
}
}
The initializer cleans the string, then ensures that it contains valid characters, then uses a Scanner to generate a UInt64 which it passes to the integer-based initializer from earlier.
With this, we now have a set of initializers that apply to both NSColor and UIColor, which means that they can be used in both AppKit and UIKit.
Let’s move on to SwiftUI.
Adding hex support to SwiftUI’s Color
Implementing the same functionality for the SwiftUI Color type is super-simple, since we can just build upon what we’ve already implemented.
We basically just have to add these two initializers:
public extension Color {
init(hex: UInt64, alpha: CGFloat = 1) {
let color = ColorRepresentable(hex: hex, alpha: alpha)
self.init(color)
}
init?(hex: String, alpha: CGFloat = 1) {
guard
let color = ColorRepresentable(hex: hex, alpha: alpha)
else { return nil }
self.init(color)
}
}
The initializers just creates a ColorRepresentable value, then return a Color that wraps that value. Everything else is defined in the initializers that we created earlier.
Conclusion
Creating colors from hex codes is nice to have in place. Just keep in mind that light & dark mode with programatically created colors can be messy. For most cases, use color assets.
If you’re interested in the source code, you can find it in my SwiftUIKit library. Don’t hesitate to comment or reach out with any thoughts you may have.
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