https://stackoverflow.com/a/61884774/1271826

CGPoint+Centroid.swift

//
//  CGPoint+Centroid.swift
//
//  Created by Robert Ryan on 5/19/20.
//  Copyright © 2020 Robert Ryan. All rights reserved.
//
//  See https://stackoverflow.com/a/61884774/1271826

import Foundation

extension Sequence where Element == CGPoint {
    /// Calculate centroid
    ///
    /// See https://en.wikipedia.org/wiki/Centroid#Of_a_polygon
    ///
    /// - Note: If the area of the polygon is zero, this returns `nil`.
    ///
    /// - Parameter points: Unclosed points of polygon.
    /// - Returns: Centroid point.

    func centroid() -> CGPoint? {
        var iterator = makeIterator()

        guard let firstPoint = iterator.next() else { return nil }

        var area: CGFloat = 0
        var sumPoint: CGPoint = .zero
        var lastPoint = firstPoint
        var nextPoint: CGPoint?

        repeat {
            nextPoint = iterator.next()
            let point = nextPoint ?? firstPoint

            area += lastPoint.x * point.y - point.x * lastPoint.y

            let factor = lastPoint.x * point.y - point.x * lastPoint.y
            sumPoint.x += (lastPoint.x + point.x) * factor
            sumPoint.y += (lastPoint.y + point.y) * factor

            lastPoint = point
        } while nextPoint != nil

        area /= 2
        if area == 0 { return nil }

        return sumPoint / 6 / area
    }

    func mean() -> CGPoint? {
        var sum: CGPoint = .zero
        var count = 0

        for point in self {
            count += 1
            sum = sum + point
        }

        return count == 0 ? nil : sum / CGFloat(count)
    }
}

extension CGPoint {
    static func + (lhs: CGPoint, rhs: CGPoint) -> CGPoint {
        CGPoint(x: lhs.x + rhs.x, y: lhs.y + rhs.y)
    }

    static func - (lhs: CGPoint, rhs: CGPoint) -> CGPoint {
        CGPoint(x: lhs.x - rhs.x, y: lhs.y - rhs.y)
    }

    static func / (lhs: CGPoint, rhs: CGFloat) -> CGPoint {
        CGPoint(x: lhs.x / rhs, y: lhs.y / rhs)
    }

    static func * (lhs: CGPoint, rhs: CGFloat) -> CGPoint {
        CGPoint(x: lhs.x * rhs, y: lhs.y * rhs)
    }
}

This is a Sequence based rendition of https://stackoverflow.com/a/61884774/1271826, which makes it a little more flexible (e.g. it can be used with slices). I’m also calculating the signed area and the centroid in a single loop.