NEW: Master Swift design patterns with my latest book! >>

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

Written by Paul Hudson    @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

Did this solution work for you? Please pass it on!

Other people are reading…

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

Test your Swift today

Think you know Swift? My book Swift Coding Challenges will put your Swift skills to the test – it's perfect for job interviews and more!

Click here to visit the Hacking with Swift store >>