TreeRotations.swift

import SwiftUI

struct TriangleView: View {
  
  var body: some View {
    GeometryReader { proxy in
      
      return ZStack {
        
        Path { path in
          let fromCenter =  min(proxy.size.width, proxy.size.height) * 0.67
          let center = CGPoint(x: proxy.size.width / 2,
                               y: proxy.size.width / 2)
          let top = CGPoint(x: center.x, y: center.y - fromCenter / 2.0)
          let angle: CGFloat = 120.0
          let bottomRight = top.rotate(around: center, with: angle)
          let bottomLeft = top.rotate(around: center, with: -angle)
          path.addLines([
            top,
            bottomRight,
            bottomLeft,
            top,
          ])
        }.stroke().fill(Color.white)
      }
    }
  }
}

// SO: https://stackoverflow.com/a/40752728/283460
extension CGPoint {
  func rotate(around center: CGPoint, with degrees: CGFloat) -> CGPoint {
    let dx = self.x - center.x
    let dy = self.y - center.y
    let radius = sqrt(dx * dx + dy * dy)
    let azimuth = atan2(dy, dx) // in radians
    let newAzimuth = azimuth + degrees * CGFloat(.pi / 180.0) // convert it to radians
    let x = center.x + radius * cos(newAzimuth)
    let y = center.y + radius * sin(newAzimuth)
    return CGPoint(x: x, y: y)
  }
}

struct TriangleView_Previews: PreviewProvider {
  static var previews: some View {
    TriangleView()
  }
}