How to get the CGPoint(s) of a CGPath

Question

How can I get an array with all the CGPoint(s) contained in a given CGPath (CGMutablePathRef)?

Answer 1

You can use CGPathApply() to iterate over every segment in the path and run a custom function with that segment. That will give you all the information the path has.

However, if by "all the CGPoint(s)", you meant every point that has a pixel rendered to it, that's an infinitely-sized set. Although you could certainly use the apply function to get each segment, and then for every non-move segment, evaluate your own math with the segment's control points to get a list of points at whatever density you want.

Answer 2

Using Swift 2.x (for Swift 3.x, Swift 4.x and Swift 5.x read here below..) , i've found this fantastic article about C Callbacks in Swift.

Trying to obtain "all the CGPoint(s)", as explained by Lily Ballard, can be a bad idea as she said.

So, I think maybe the best way is to get the path elements points used to create a particular CGPath:

//MARK: - CGPath extensions
extension CGPath {
    func forEach(@noescape body: @convention(block) (CGPathElement) -> Void) {
        typealias Body = @convention(block) (CGPathElement) -> Void
        func callback(info: UnsafeMutablePointer<Void>, element: UnsafePointer<CGPathElement>) {
            let body = unsafeBitCast(info, Body.self)
            body(element.memory)
        }
        print(sizeofValue(body))
        let unsafeBody = unsafeBitCast(body, UnsafeMutablePointer<Void>.self)
        CGPathApply(self, unsafeBody, callback)
    }

    func getPathElementsPoints() -> [CGPoint] {
        var arrayPoints : [CGPoint]! = [CGPoint]()
        self.forEach { element in
            switch (element.type) {
            case CGPathElementType.MoveToPoint:
                arrayPoints.append(element.points[0])
            case .AddLineToPoint:
                arrayPoints.append(element.points[0])
            case .AddQuadCurveToPoint:
                arrayPoints.append(element.points[0])
                arrayPoints.append(element.points[1])
            case .AddCurveToPoint:
                arrayPoints.append(element.points[0])
                arrayPoints.append(element.points[1])
                arrayPoints.append(element.points[2])
            default: break
            }
        }
        return arrayPoints
    }
}

With this extension you can do for example:

var bezier = UIBezierPath(ovalInRect: CGRectMake(0, 0, 400, 300))
let myOval = bezier.CGPath
let junctionPoints = myOval.getPathElementsPoints()
print("junction points are: \(junctionPoints)")

Swift 3.x and Swift 4.1 (look below for Swift 4.2 or major..)

(there are some corrections due to syntax re-introduction of @convention(c)):

extension CGPath {

    func forEach( body: @convention(block) (CGPathElement) -> Void) {
        typealias Body = @convention(block) (CGPathElement) -> Void
        let callback: @convention(c) (UnsafeMutableRawPointer, UnsafePointer<CGPathElement>) -> Void = { (info, element) in
            let body = unsafeBitCast(info, to: Body.self)
            body(element.pointee)
        }
        print(MemoryLayout.size(ofValue: body))
        let unsafeBody = unsafeBitCast(body, to: UnsafeMutableRawPointer.self)
        self.apply(info: unsafeBody, function: unsafeBitCast(callback, to: CGPathApplierFunction.self))
    }


    func getPathElementsPoints() -> [CGPoint] {
        var arrayPoints : [CGPoint]! = [CGPoint]()
        self.forEach { element in
            switch (element.type) {
            case CGPathElementType.moveToPoint:
                arrayPoints.append(element.points[0])
            case .addLineToPoint:
                arrayPoints.append(element.points[0])
            case .addQuadCurveToPoint:
                arrayPoints.append(element.points[0])
                arrayPoints.append(element.points[1])
            case .addCurveToPoint:
                arrayPoints.append(element.points[0])
                arrayPoints.append(element.points[1])
                arrayPoints.append(element.points[2])
            default: break
            }
        }
        return arrayPoints
    }

    func getPathElementsPointsAndTypes() -> ([CGPoint],[CGPathElementType]) {
        var arrayPoints : [CGPoint]! = [CGPoint]()
        var arrayTypes : [CGPathElementType]! = [CGPathElementType]()
        self.forEach { element in
            switch (element.type) {
            case CGPathElementType.moveToPoint:
                arrayPoints.append(element.points[0])
                arrayTypes.append(element.type)
            case .addLineToPoint:
                arrayPoints.append(element.points[0])
                arrayTypes.append(element.type)
            case .addQuadCurveToPoint:
                arrayPoints.append(element.points[0])
                arrayPoints.append(element.points[1])
                arrayTypes.append(element.type)
                arrayTypes.append(element.type)
            case .addCurveToPoint:
                arrayPoints.append(element.points[0])
                arrayPoints.append(element.points[1])
                arrayPoints.append(element.points[2])
                arrayTypes.append(element.type)
                arrayTypes.append(element.type)
                arrayTypes.append(element.type)
            default: break
            }
        }
        return (arrayPoints,arrayTypes)
    }
}

Swift > 4.1 (also Swift 5.x) and iOS 9.x and > compatible

extension CGPath {
    func forEach( body: @escaping @convention(block) (CGPathElement) -> Void) {
        typealias Body = @convention(block) (CGPathElement) -> Void
        let callback: @convention(c) (UnsafeMutableRawPointer, UnsafePointer<CGPathElement>) -> Void = { (info, element) in
            let body = unsafeBitCast(info, to: Body.self)
            body(element.pointee)
        }
        //print(MemoryLayout.size(ofValue: body))
        let unsafeBody = unsafeBitCast(body, to: UnsafeMutableRawPointer.self)
        self.apply(info: unsafeBody, function: unsafeBitCast(callback, to: CGPathApplierFunction.self))
    }
    func getPathElementsPoints() -> [CGPoint] {
        var arrayPoints : [CGPoint]! = [CGPoint]()
        self.forEach { element in
            switch (element.type) {
            case CGPathElementType.moveToPoint:
                arrayPoints.append(element.points[0])
            case .addLineToPoint:
                arrayPoints.append(element.points[0])
            case .addQuadCurveToPoint:
                arrayPoints.append(element.points[0])
                arrayPoints.append(element.points[1])
            case .addCurveToPoint:
                arrayPoints.append(element.points[0])
                arrayPoints.append(element.points[1])
                arrayPoints.append(element.points[2])
            default: break
            }
        }
        return arrayPoints
    }
    func getPathElementsPointsAndTypes() -> ([CGPoint],[CGPathElementType]) {
        var arrayPoints : [CGPoint]! = [CGPoint]()
        var arrayTypes : [CGPathElementType]! = [CGPathElementType]()
        self.forEach { element in
            switch (element.type) {
            case CGPathElementType.moveToPoint:
                arrayPoints.append(element.points[0])
                arrayTypes.append(element.type)
            case .addLineToPoint:
                arrayPoints.append(element.points[0])
                arrayTypes.append(element.type)
            case .addQuadCurveToPoint:
                arrayPoints.append(element.points[0])
                arrayPoints.append(element.points[1])
                arrayTypes.append(element.type)
                arrayTypes.append(element.type)
            case .addCurveToPoint:
                arrayPoints.append(element.points[0])
                arrayPoints.append(element.points[1])
                arrayPoints.append(element.points[2])
                arrayTypes.append(element.type)
                arrayTypes.append(element.type)
                arrayTypes.append(element.type)
            default: break
            }
        }
        return (arrayPoints,arrayTypes)
    }
}

Answer 3

Apple added instance method CGPath.applyWithBlock, available for iOS11.0+ and macOS10.13+

You still can examine each element of a path with CGPath.apply as explained in previous answers. But if you want to avoid using C style pointers and unsafeBitCast, you should use the much more convenient instance method applyWithBlock. For example, if you want to get an array of CGPoints of a CGPath you can add an extension to CGPath, in this example a computed property(points) that collects the CGPoints of the CGPath:

/// Extension to collect CGPath points
extension CGPath {

  /// this is a computed property, it will hold the points we want to extract
  var points: [CGPoint] {

     /// this is a local transient container where we will store our CGPoints
     var arrPoints: [CGPoint] = []

     // applyWithBlock lets us examine each element of the CGPath, and decide what to do
     self.applyWithBlock { element in

        switch element.pointee.type
        {
        case .moveToPoint, .addLineToPoint:
          arrPoints.append(element.pointee.points.pointee)

        case .addQuadCurveToPoint:
          arrPoints.append(element.pointee.points.pointee)
          arrPoints.append(element.pointee.points.advanced(by: 1).pointee)

        case .addCurveToPoint:
          arrPoints.append(element.pointee.points.pointee)
          arrPoints.append(element.pointee.points.advanced(by: 1).pointee)
          arrPoints.append(element.pointee.points.advanced(by: 2).pointee)

        default:
          break
        }
     }

    // We are now done collecting our CGPoints and so we can return the result
    return arrPoints

  }
}

Answer 4

A CGPath is an opaque data type and does not necessarily store all the points used. In addition to this, a path may not actually draw all the points used as input (for example, consider Bézier control points).

The only two documented ways of getting information out of a path is to use CGPathGetBoundingBox to get the bounding box, or the more complicated method of using CGPathApply to call a callback function that will give you a sequence if CGPathElement types.