NEW: Nominations are now open for the 2019 Swift Community Awards! >>

How to draw polygons and stars

Paul Hudson    @twostraws   

Fully updated for Xcode 11.1

Once you understand SwiftUI’s basic path drawing system, you can add all sorts of shapes easily. For example, we could create a Star shape that is capable of representing a variety of star shapes, or even other polygons, with just a little mathematics.

For example:

struct Star: Shape {
    // store how many corners the star has, and how smooth/pointed it is
    let corners: Int
    let smoothness: CGFloat

    func path(in rect: CGRect) -> Path {
        // ensure we have at least two corners, otherwise send back an empty path
        guard corners >= 2 else { return Path() }

        // draw from the center of our rectangle
        let center = CGPoint(x: rect.width / 2, y: rect.height / 2)

        // start from directly upwards (as opposed to down or to the right)
        var currentAngle = -CGFloat.pi / 2

        // calculate how much we need to move with each star corner
        let angleAdjustment = .pi * 2 / CGFloat(corners * 2)

        // figure out how much we need to move X/Y for the inner points of the star
        let innerX = center.x * smoothness
        let innerY = center.y * smoothness

        // we're ready to start with our path now
        var path = Path()

        // move to our initial position
        path.move(to: CGPoint(x: center.x * cos(currentAngle), y: center.y * sin(currentAngle)))

        // track the lowest point we draw to, so we can center later
        var bottomEdge: CGFloat = 0

        // loop over all our points/inner points
        for corner in 0..<corners * 2  {
            // figure out the location of this point
            let sinAngle = sin(currentAngle)
            let cosAngle = cos(currentAngle)
            let bottom: CGFloat

            // if we're a multiple of 2 we are drawing the outer edge of the star
            if corner.isMultiple(of: 2) {
                // store this Y position
                bottom = center.y * sinAngle

                // …and add a line to there
                path.addLine(to: CGPoint(x: center.x * cosAngle, y: bottom))
            } else {
                // we're not a multiple of 2, which means we're drawing an inner point

                // store this Y position
                bottom = innerY * sinAngle

                // …and add a line to there
                path.addLine(to: CGPoint(x: innerX * cosAngle, y: bottom))
            }

            // if this new bottom point is our lowest, stash it away for later
            if bottom > bottomEdge {
                bottomEdge = bottom
            }

            // move on to the next corner
            currentAngle += angleAdjustment
        }

        // figure out how much unused space we have at the bottom of our drawing rectangle
        let unusedSpace = (rect.height / 2 - bottomEdge) / 2

        // create and apply a transform that moves our path down by that amount, centering the shape vertically
        let transform = CGAffineTransform(translationX: center.x, y: center.y + unusedSpace)
        return path.applying(transform)
    }
}

We can then use our Star shape like this:

struct ContentView: View {
    var body: some View {
        Star(corners: 5, smoothness: 0.45)
            .fill(Color.red)
            .frame(width: 200, height: 200)
            .background(Color.green)

    }
}

As stars are just polygons, if you increase smoothness to 1 you’ll find you can draw shapes like hexagons and octagons without having to change the code.

SAVE 20% ON iOS CONF SG The largest iOS conference in Southeast Asia is back in Singapore for the 5th time in January 2020, now with two days of workshops plus two days of talks on SwiftUI, Combine, GraphQL, and more! Save a massive 20% on your tickets by clicking on this link.

Similar solutions…

MASTER SWIFT NOW
Buy Testing Swift Buy Practical iOS 12 Buy Pro Swift Buy Swift Design Patterns Buy Swift Coding Challenges Buy Server-Side Swift (Vapor Edition) Buy Server-Side Swift (Kitura Edition) Buy Hacking with macOS Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with Swift Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let us know!

Average rating: 5.0/5