CGPatternClosure.swift

extension CGPattern {

  static func make(bounds: CGRect,
                   matrix: CGAffineTransform,
                   step: CGSize,
                   tiling: CGPatternTiling,
                   isColored: Bool,
                   draw: @escaping (CGContext) -> Void) -> CGPattern {

    let drawPattern: CGPatternDrawPatternCallback = { info, ctx in
      let box = Unmanaged<Box>.fromOpaque(info!).takeUnretainedValue()
      box.closure(ctx)
    }

    let releaseInfo: CGPatternReleaseInfoCallback = { info in
      Unmanaged<Box>.fromOpaque(info!).release()
    }

    var callbacks = CGPatternCallbacks(version: 0,
                                       drawPattern: drawPattern,
                                       releaseInfo: releaseInfo)

    return CGPattern(info: Unmanaged.passRetained(Box(draw)).toOpaque(),
                     bounds: bounds,
                     matrix: matrix,
                     xStep: step.width,
                     yStep: step.height,
                     tiling: tiling,
                     isColored: isColored,
                     callbacks: &callbacks)!
  }

  private final class Box {
    let closure: (CGContext) -> Void
    init(_ closure: @escaping (CGContext) -> Void) {
      self.closure = closure
    }
  }
}