在swift 2中json数据拉取时如何启动和停止活动指示器？

let activityIndicator = UIActivityIndicatorView(style: .gray)
activityIndicator.center = view.center
view.addSubview(activityIndicator)

activityIndicator.startAnimating()

activityIndicator.stopAnimating()

let activityIndicator = UIActivityIndicatorView(style: .gray)
activityIndicator.center = view.center
view.addSubview(activityIndicator)

// 启动活动指示器
activityIndicator.startAnimating()

// 模拟JSON数据的拉取过程
DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
    // 停止活动指示器
    activityIndicator.stopAnimating()
}