Supporting SwiftUI with Core Graphics

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

Part 1: Supporting SwiftUI with Core Graphics

07. Create a Thumbnail

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We’ve got one important piece missing from our drawing pad implementation. We aren’t storing our drawings, and we can’t see a thumbnail version in the mindmap cell. We’ll fix that up in this episode, with some more help from UIGraphicsImageRenderer.


We’ll start with updating the cell’s model, in CellModel.swift

var drawing: 🟩UIImage?
mutating func update(drawing: 🟩UIImage) {
  self.drawing = drawing
  func updateDrawing(cell: Cell, drawing: 🟩UIImage) {


We’ll need to update the actual cell view, as well. Down in the body, find the if statement checking for a drawing. And replace this drawing view, with an Image made from our drawing UIImage.

Image(uiImage: drawing)


Back in DrawingPadView.swift, we can uncomment the code in the save button, and because we updated the underlying types and CellStore method, this will just work!

if let cell = cellStore.selectedCell,
 let drawing = drawing {
  cellStore.updateDrawing(cell: cell, drawing: drawing)


One more thing, in ContentView.swift, we need to pass the selected cell’s drawing into the drawing pad again.

DrawingPadView(drawing: cellStore.selectedCell?.drawing)


We’ll start in the model, again. Create a new computed property on Cell for the thumbnail. We’ll return an Image that we’ll create from the resized UIImage. Make sure that there is a drawing image. If there isn’t then just return nil.

var thumbnail: Image? {
  guard let drawing = self.drawing else {
    return nil
let thumbnailSize = CGSize(width: drawing.size.width / 6,
                           height: drawing.size.height / 6)
let thumbnail = UIGraphicsImageRenderer(size: thumbnailSize).image { context in
return Image(uiImage: thumbnail)
drawing.draw(in: CGRect(origin: .zero, size: thumbnailSize))


So back in CellView.swift. Just under CellView, open a new extension. Create a new View structure for the Cell thumbnail. Give it size and image properties to define the thumbnail.

extension CellView {
    struct Thumbnail: View {
      let size: CGSize
      let drawing: Image
      var body: some View {
  ZStack {
    Color(uiColor: .systemBackground)
      .aspectRatio(contentMode: .fit)
      .frame(width: size.width, height: size.height)
Thumbnail(size: cell.size, drawing: Image(uiImage: drawing))