Supporting SwiftUI with Core Graphics

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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.

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private let defaultLineWidth: CGFloat = 6
private let minLineWidth: CGFloat = 5
private let forceSensitivity: CGFloat = 4

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var lineWidth = defaultLineWidth
if touch.force > 0 {
  lineWidth = touch.force * forceSensitivity
}

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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.

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var touches: [UITouch] = []

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if let coalescedTouches = event?.coalescedTouches(for: touch) {
  touches = coalescedTouches
}

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if let coalescedTouches = event?.coalescedTouches(for: touch) {
  touches = coalescedTouches
} 🟩else {
  touches.append(touch)
}
print("Touch count: ", touches.count)

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for touch in touches {
  drawStroke(context: context.cgContext, touch: touch)
}

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