swift – Is it doable to create a null CGPath on iOS?

The issue you are dealing with is that CGPath’s == doesn’t imply “comprises the identical factors.” It means “comprises the identical drawing directions in the identical order.” And union(utilizing:) doesn’t promise to protect the order of the drawing directions. It will probably’t, because it has a fill rule. It is not simply appending two paths; it is merging them.

You’ll be able to see this by printing out the contents:

func printPath(ptr: UnsafePointer<CGPathElement>) {
    let factor = ptr.pointee
    swap factor.sort {
    case .moveToPoint:
        print("MOVE: (factor.factors[0])")
    case .addCurveToPoint:
        print("CURVE")  // Do not care; have no
    case .addLineToPoint:
        print("LINE: (factor.factors[0])")
    case .closeSubpath:
        print("CLOSE")
    case .addQuadCurveToPoint:
        print("QUAD") // Do not care; have no
    }
}

let rect1 = CGPath(rect: CGRect(origin: .zero, dimension: CGSize(width: 100, top: 100)), 
                   rework: nil)

rect1.applyWithBlock(printPath)

=>
MOVE: (0.0, 0.0)
LINE: (100.0, 0.0)
LINE: (100.0, 100.0)
LINE: (0.0, 100.0)
CLOSE


CGMutablePath().union(rect1, utilizing: .evenOdd).applyWithBlock(printPath)

=>
MOVE: (100.0, 100.0)
LINE: (0.0, 100.0)
LINE: (0.0, 0.0)
LINE: (100.0, 0.0)
CLOSE

These will not be “equal.” The device you need right here is .addPath.

let added = CGMutablePath()
added.addPath(rect1)
added.applyWithBlock(printPath)

=>
MOVE: (0.0, 0.0)
LINE: (100.0, 0.0)
LINE: (100.0, 100.0)
LINE: (0.0, 100.0)
CLOSE

added == rect1    // true

Now this can be a bit annoying on your use case, however you possibly can clear it up:

extension CGPath {
    static var zero: CGPath { CGMutablePath() }
    func appending(_ path: CGPath) -> CGPath {
        var copy = self.mutableCopy()!
        copy.addPath(path)
        return copy
    }
}

let bunchOfPaths = [rect1]
let unionPath = bunchOfPaths.cut back(CGPath.zero) { $0.appending($1) }
unionPath == rect1    // true