Get the start point of a UIBezierPath

Question

I created a UIBezierPath but I don't know how to access the start point of it. I tried doing this:

let startPoint = path.currentPoint

The property currentPoint gives me the last point, not the start point. The reason I need the start point is because I want to place an image at the starting point of the path.

Any ideas?

score 8 · Accepted Answer · edited May 23 '17 at 11:46

You need to drop down to CGPath and use CGPathApply to walk the elements. You only want the first one, but you have to look at them all.

I'm assuming that your path is well-formed, and starts with a "move." That should always be true for a UIBezierPath (I'm not aware of any way to make it not be true.)

You'll need some help from rob mayoff's CGPath.forEach, which is quite tricky, but with that in place it's pretty straightforward:

// rob mayoff's CGPath.foreach
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)
        }
        let unsafeBody = unsafeBitCast(body, UnsafeMutablePointer<Void>.self)
        CGPathApply(self, unsafeBody, callback)
    }
}

// Finds the first point in a path
extension UIBezierPath {
    func firstPoint() -> CGPoint? {
        var firstPoint: CGPoint? = nil

        self.CGPath.forEach { element in
            // Just want the first one, but we have to look at everything
            guard firstPoint == nil else { return }
            assert(element.type == .MoveToPoint, "Expected the first point to be a move")
            firstPoint = element.points.memory
        }
        return firstPoint
    }
}

In Swift 3, it's basically the same:

// rob mayoff's CGPath.foreach
extension CGPath {
    func forEach( body: @convention(block) (CGPathElement) -> Void) {
        typealias Body = @convention(block) (CGPathElement) -> Void
        func callback(info: UnsafeMutableRawPointer?, element: UnsafePointer<CGPathElement>) {
            let body = unsafeBitCast(info, to: Body.self)
            body(element.pointee)
        }
        let unsafeBody = unsafeBitCast(body, to: UnsafeMutableRawPointer.self)
        self.apply(info: unsafeBody, function: callback)
    }
}

// Finds the first point in a path
extension UIBezierPath {
    func firstPoint() -> CGPoint? {
        var firstPoint: CGPoint? = nil

        self.cgPath.forEach { element in
            // Just want the first one, but we have to look at everything
            guard firstPoint == nil else { return }
            assert(element.type == .moveToPoint, "Expected the first point to be a move")
            firstPoint = element.points.pointee
        }
        return firstPoint
    }
}

Thanks it works!!! I have yet another problem and asked a different question. could you look at it and see if you can answer? Heres the link: http://stackoverflow.com/questions/36705072/dynamically-change-position-based-on-scrollview — JEL, Apr 18 '16 at 22:05
Hi, I used Xcode to convert this code to Swift 3.0. I am now getting a crash on this line, which Xcode changed: `apply(info: unsafeBody, function: callback as! CGPathApplierFunction)`. Unfortunately there is nothing in the debugger but this appears `thread 1 exc_bad_instruction(code=exc_1386_invop subcode=0x0)`. My research says that it is due to force unwrapping (in this case force casting). If I try to safely cast, then I get a `Segmentation fault: 11`. Any suggestions? — JEL, Dec 02 '16 at 21:18
The type of `CGPathApplierFunction` changed. It now takes a `UnsafeMutableRawPointer?`. You have to change the signature of `callback` to match, not just force-cast it. — Rob Napier, Dec 02 '16 at 23:08

Surya Subenthiran · Answer 2 · 2016-04-18T20:10:52.990

2

Simplest way is

-Subclass the UIBezierPath

-Create the custom property as startPoint

-Override the moveToPoint method and set value for startPoint there

class MyBezierPath: UIBezierPath {
    var startPoint :CGPoint?

    override func moveToPoint(point: CGPoint) {
        super.moveToPoint(point)
        startPoint=point;
    }
}

    var myBezier = MyBezierPath()
    myBezier.moveToPoint(CGPoint(x: 0, y: 0))
    myBezier.addLineToPoint(CGPoint(x: 100, y: 0))
    myBezier.addLineToPoint(CGPoint(x: 50, y: 100))

edited Apr 18 '16 at 20:10

answered Apr 18 '16 at 20:05

Surya Subenthiran

2,217
1
15
22

Why do we search for start point if we jus set its value in the moveToPoint function? – Alex Kosyakov Apr 30 '16 at 18:23
@AlexKosyakov you might have start with path.addArc(...) for example. – Vladimir Shutyuk Nov 28 '16 at 07:09
@VladimirShutyuk got it, tnx! – Alex Kosyakov Dec 08 '16 at 14:06
Is there any similar workaround for finding the end point of a bezierPath. – Frankenxtein Jan 04 '21 at 05:22
@Frankenxtein uibezierpath.currentPoint will give you the end point. – Ramesh Sanghar Aug 26 '22 at 11:15

Get the start point of a UIBezierPath

2 Answers2