CircleImageRenderer.swift

final class CircleImageRenderer {
    let image: Observable<UIImage>
    
    init(radius: CGFloat, fillColor: UIColor = .white) {
        self.image = Observable.create { observer in
            let format = UIGraphicsImageRendererFormat.default()
            format.scale = UIScreen.main.scale
            let width = radius * 2
            let size = CGSize.square(width)
            let renderer = UIGraphicsImageRenderer(
                size: size,
                format: format
            )
            let image = renderer.image(actions: { context in
                let rect = CGRect(origin: .zero, size: size)
                fillColor.setFill()
                context.cgContext.fillEllipse(in: rect)
            })
            observer.onNext(image)
            observer.onCompleted()
            return Disposables.create()
        }
        .subscribeOn(ConcurrentDispatchQueueScheduler(qos: .utility))
        .share(replay: 1, scope: .forever)
    }
}