Multi-platform image resizing
In this post, let’s take a quick look at how to resize images in UIKit and AppKit. The result will work on iOS, macOS, tvOS and watchOS and lets us resize images with the same APIs regardless of platform.
Image resizing is pretty straightforward in Swift, so let’s look at how to implement it in UIKit and AppKit, then add more functionality that applies to all platforms.
In UIKit, we can add a UIImage extension that resizes images using the image context:
#if canImport(UIKit)
import UIKit
public extension UIImage {
func resized(to size: CGSize) -> UIImage? {
UIGraphicsBeginImageContextWithOptions(size, false, scale)
draw(in: CGRect(origin: CGPoint.zero, size: size))
let result = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
return result
}
}
#endif
In AppKit, we can add an NSImage extension that resizes images in a different way:
#if canImport(AppKit)
import AppKit
public extension NSImage {
func resized(to newSize: CGSize) -> NSImage? {
let newImage = NSImage(size: newSize)
newImage.lockFocus()
let sourceRect = NSMakeRect(0, 0, size.width, size.height)
let destRect = NSMakeRect(0, 0, newSize.width, newSize.height)
draw(in: destRect, from: sourceRect, operation: .sourceOver, fraction: CGFloat(1))
newImage.unlockFocus()
return newImage
}
}
#endif
Since both UIImage and NSImage now have the same API for resizing images to a certain size, we can extend both types with more resizing functionality that use this underlying functionality.
Let’s first define a platform agnostic image typealias, which you may have seen in previous posts:
#if canImport(AppKit)
import class AppKit.NSImage
public typealias ImageRepresentable = NSImage
#endif
#if canImport(UIKit)
import class UIKit.UIImage
public typealias ImageRepresentable = UIImage
#endif
This let’s us use ImageRepresentable to handle both UIImage and NSImage. Let’s use this to add ways to resize images to a certain width and height, using the resizing function we defined earlier:
public extension ImageRepresentable {
func resized(toHeight points: CGFloat) -> ImageRepresentable? {
let ratio = points / size.height
let width = size.width * ratio
let newSize = CGSize(width: width, height: points)
return resized(to: newSize)
}
func resized(toWidth points: CGFloat) -> ImageRepresentable? {
let ratio = points / size.width
let height = size.height * ratio
let newSize = CGSize(width: points, height: height)
return resized(to: newSize)
}
}
That’s it - you can now use resized(to:), resized(toWidth:) and resized(toHeight:) with UIImage and NSImage, to resize images in iOS, macOS, tvOS and watchOS.
I have added these extension to SwiftUIKit. Feel free to try them out and let me know what you think.
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