How to draw lines in Core Graphics: move(to:) and addLine(to:)

Paul Hudson September 20th 2017 @twostraws

You can draw lines in Core Graphics using move(to:) and addLine(to:). The first function moves the Core Graphics path to a CGPoint of your choosing, and the second function moves the path to a new point while also adding a line. Once you add in the required code to set up a context and choose a color, you can draw a triangle with this code:

let renderer1 = UIGraphicsImageRenderer(size: CGSize(width: 500, height: 500))
let img1 = renderer1.image { ctx in
    ctx.cgContext.setStrokeColor(UIColor.white.cgColor)
    ctx.cgContext.setLineWidth(3)

    ctx.cgContext.move(to: CGPoint(x: 50, y: 450))
    ctx.cgContext.addLine(to: CGPoint(x: 250, y: 50))
    ctx.cgContext.addLine(to: CGPoint(x: 450, y: 450))
    ctx.cgContext.addLine(to: CGPoint(x: 50, y: 450))

    let rectangle = CGRect(x: 0, y: 0, width: 512, height: 512)
    ctx.cgContext.addRect(rectangle)
    ctx.cgContext.drawPath(using: .fillStroke)
}

Once you've mastered drawing basic lines, you can create neat effects by rotating the context as you draw, like this:

let renderer2 = UIGraphicsImageRenderer(size: CGSize(width: 512, height: 512))
let img2 = renderer2.image { ctx in
    ctx.cgContext.setStrokeColor(UIColor.black.cgColor)

    ctx.cgContext.translateBy(x: 256, y: 256)

    var first = true
    var length: CGFloat = 256

    for _ in 0 ..< 256 {
        ctx.cgContext.rotate(by: CGFloat.pi / 2)

        if first {
            ctx.cgContext.move(to: CGPoint(x: length, y: 50))
            first = false
        } else {
            ctx.cgContext.addLine(to: CGPoint(x: length, y: 50))
        }

        length *= 0.99
    }

    ctx.cgContext.strokePath()
}

Available from iOS 4.0 – see Hacking with Swift tutorial 27

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