问使用swift在WKWebView上显示活动指示器？

EN

import UIKit
import WebKit

class SupportWebView: UIViewController, WKNavigationDelegate {
    @IBOutlet var containerView : UIView? = nil

    var webView: WKWebView?

    override func loadView() {
        super.loadView()
        self.webView = WKWebView()
        self.view = self.webView
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        var dataManager = DataManager.sharedDataManager()
        var url = dataManager.myValidURL
        var req = NSURLRequest(URL:url!)
        self.webView!.loadRequest(req)
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    func webView(webView: WKWebView, didStartProvisionalNavigation navigation: WKNavigation!) {
        UIApplication.sharedApplication().networkActivityIndicatorVisible = true
    }


    func webView(webView: WKWebView, didFinishNavigation navigation: WKNavigation!) {
        UIApplication.sharedApplication().networkActivityIndicatorVisible = false
    }
}