How to draw polygons and stars
Updated for Xcode 14.0 beta 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.
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
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.
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