Supporting SwiftUI with Core Graphics

Nov 22 2022 · Swift 5.5, iOS 15, Xcode 13

Part 1: Supporting SwiftUI with Core Graphics

08. Color Picking from a Bitmap Image

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We’re going to do one more cool thing with our DrawingPad. Drawing programs often allow you to long press on a color and choose it as your drawing color.

DrawingColorView.swift

The starter project for this video has a pre-built UIView class in DrawingColorView.swift. This will be the round view that magnifies the pixel color at the long press location.

DrawingPadExtensions.swift

Create a new Swift file called DrawingPadExtensions.swift. Import UIKit

import UIKit

extension UIImage {
  func getColor(at location: CGPoint) -> UIColor? {
    
  }
}

guard let cgImage = cgImage,

let dataProvider = cgImage.dataProvider,

  let pixelData = dataProvider.data
  else {
    return nil
}

let scale = UIScreen.main.scale
let pixelLocation = CGPoint(x: location.x * scale,
                            y: location.y * scale)

let pixel = cgImage.bytesPerRow * Int(pixelLocation.y) + 
            cgImage.bitsPerPixel / 8 * Int(pixelLocation.x)
guard pixel < CFDataGetLength(pixelData) else {
  print("WARNING: mismatch of pixel data")
  return nil
}

guard let pointer = CFDataGetBytePtr(pixelData) else {
  return nil
}

func convert(_ color: UInt8) -> CGFloat {
  CGFloat(color) / 255.0
}

let red = convert(pointer[pixel])
let green = convert(pointer[pixel + 1])
let blue = convert(pointer[pixel + 2])
let alpha = convert(pointer[pixel + 3])

return UIColor(red: red, green: green, blue: blue, alpha: alpha)

CanvasView.swift

Open CanvasView.swift. Create a new property to hold a DrawingColorView.

private let drawingColorView = DrawingColorView()

@objc func handleLongPress(gesture: UILongPressGestureRecognizer) {
  guard let drawingImage = drawingImage else { return }
}

let location = gesture.location(in: self)
guard let color = drawingImage.getColor(at: location) else {
  return
}

switch gesture.state {
case .began:
  drawingColorView.show(color: color, location: location)
case .ended:
  drawingColorView.hide()
  self.color = color
default: break
}

let longPress = UILongPressGestureRecognizer(target: self,
                                             action: #selector(handleLongPress))
addGestureRecognizer(longPress)

addSubview(drawingColorView)