Supporting SwiftUI with Core Graphics

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

Part 1: Supporting SwiftUI with Core Graphics

05. Drawing with Pencil

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Previous episode: 04. Build a UIKit Drawing Pad Next episode: 06. Shading with Pencil

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To test the code in this video and the next one, you’ll need an Apple Pencil and a recent iPad that supports it. You can skip this video if you don’t have them, and just use the touch drawing pad for the rest of the course.

private let defaultLineWidth: CGFloat = 6
private let minLineWidth: CGFloat = 5
private let forceSensitivity: CGFloat = 4
var lineWidth = defaultLineWidth
if touch.force > 0 {
  lineWidth = touch.force * forceSensitivity

Coalesced Touches

In the SwiftUI drawing pad, we replaced the stroke after drawing it with a Bezier spline which smoothed out the curve, but Pencil has a clever way of capturing touches that are lost when you drag fast.

var touches: [UITouch] = []
if let coalescedTouches = event?.coalescedTouches(for: touch) {
  touches = coalescedTouches
if let coalescedTouches = event?.coalescedTouches(for: touch) {
  touches = coalescedTouches
} 🟩else {
print("Touch count: ", touches.count)
for touch in touches {
  drawStroke(context: context.cgContext, touch: touch)