[SwiftUI] GlobalSpinner

public class Spinner {
    
    private var scene: UIWindowScene? {
        UIApplication
            .shared
            .connectedScenes
            .first as? UIWindowScene
    }
    
    private var root: UIViewController? {
        scene?
            .keyWindow?
            .rootViewController
    }
    
    private var activityIndicator: UIActivityIndicatorView?
    
    @MainActor public func start(
        _ style: UIActivityIndicatorView.Style = .medium,
        color: UIColor = .gray
    ) {
        if self.activityIndicator == nil {
            let indicator = UIActivityIndicatorView()
            indicator.style = style
            indicator.color = color
            indicator.hidesWhenStopped = true
            indicator.frame = .init(
                x: 0, y: 0, width: 50, height: 50
            )
            self.activityIndicator = indicator
            root?.view.addSubview(indicator)
            indicator.center = root?.view.center ?? .init()
        }
        activityIndicator?.startAnimating()
    }
    
    @MainActor public func stop() {
        self.activityIndicator?.stopAnimating()
    }
}

public struct SpinnerKey: EnvironmentKey {

    public static var defaultValue = Spinner()
}

public extension EnvironmentValues {
    
    var spinner: Spinner {
        self[SpinnerKey.self]
    }
}