How to draw polygons and stars

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How to draw polygons and stars 관련

SwiftUI by Example

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How to draw polygons and stars | SwiftUI by Example

How to draw polygons and stars

Updated for Xcode 15

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.

Here’s that in code:

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

    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 / Double(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: Double = 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: Double

            // 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)
    }
}

// Draw our star shape in a view
struct ContentView: View {
    var body: some View {
        Star(corners: 5, smoothness: 0.45)
            .fill(.red)
            .frame(width: 200, height: 200)
            .background(.green)

    }
}

Download this as an Xcode project

A red five pointed star on a green square.

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.