DrawingGroup on Bezier Curve Causing Crash (Xcode 12.1 / iOS 14.1)

Hi All,

I updated to Xcode 12.1 today and encountered some unexpected behavior with Metal & curved paths in my app.

It took me a few hours to track down the source of this issue, so wanted to share in case anyone else has encountered this problem or has suggestions.

Problem

Applying .drawingGroup() to a View containing a bezier curve (e.g., path.addCurve() or path.addQuadCurve()) causes the app to crash immediately, with only cryptic output in the console.

Notes:

• doesn't happen when drawing straight lines (path.addLine()), only curves

• appears to be limited to the Simulators, as the crash doesn't occur on my iPhone XR / iOS 14.1

• removing .drawingGroup() stops app from crashing

• problem did not occur when using Xcode 12.0

Example Code

Code Block language
import SwiftUI
@main
struct TestApp: App {
  var body: some Scene {
    WindowGroup {
      TestView()
    }
  }
}
struct TestView: View {
   
  @State private var redFill: Bool = true
   
  var body: some View {
     
    TestPath()
    .fill(redFill ? Color.red : Color.blue)
    // Drawing group here causes app to crash
    .drawingGroup()
     
    Button(action: {
      redFill.toggle()
    }) {
      Text("Change Color")
    }
     
  }
}
struct TestPath: Shape {
   
  func path(in rect: CGRect) -> Path {
    let width: CGFloat = rect.size.width
    let height: CGFloat = rect.size.height
     
    let pathWidth: CGFloat = 200
     
    let startPoint = CGPoint(
      x: 0.5*(width - pathWidth),
      y: 0.5*(height)
    )
    let endPoint = CGPoint(
      x: 0.5*(width + pathWidth),
      y: 0.5*(height)
    )
     
    let control1 = CGPoint(
      x: 0.5*(width),
      y: 0.5*(height + pathWidth)
    )
    let control2 = CGPoint(
      x: 0.5*(width),
      y: 0.5*(height - pathWidth)
    )
     
    let path = Path { path in
      path.move(to: startPoint)
      path.addCurve(
        to: endPoint,
        control1: control1,
        control2: control2
      )
      path.closeSubpath()
    }
     
    return path
     
  }
}

Console Output

Code Block language
... Metal GPU Frame Capture Enabled
... Metal API Validation Enabled
... [] nw_protocol_get_quic_image_block_invoke dlopen libquic failed
... Connection 1: received failure notification
... Connection 1: failed to connect 3:-9816, reason -1
... Connection 1: encountered error(3:-9816)
... Task <...>.<1> HTTP load failed, 0/0 bytes (error code: -1200 [3:-9816])
... [error] precondition failure: pipeline error: accumulator_color-13f4500040004f: Compiler encountered an internal error
... CoreSimulator 732.17 - Device: iPhone 11 (...) - Runtime: iOS 14.1 (18A8394) - DeviceType: iPhone 11
... RenderBox precondition failure: pipeline error: accumulator_color-13f4500040004f: Compiler encountered an internal error.