设置UIActivityIndi​​catorView的框架

import UIKit

class ViewController: UIViewController {
    private var activityIndicatorView: UIActivityIndicatorView!

    override func viewDidLoad() {
        super.viewDidLoad()

        // 创建UIActivityIndicatorView
        activityIndicatorView = UIActivityIndicatorView(style: .gray)
        activityIndicatorView.center = view.center
        view.addSubview(activityIndicatorView)

        // 开始加载
        activityIndicatorView.startAnimating()

        // 模拟网络请求
        DispatchQueue.main.asyncAfter(deadline: .now() + 3) {
            self.activityIndicatorView.stopAnimating()
        }
    }
}