参考: http://www.jianshu.com/p/367952852c6d
webView.addObserver(self, forKeyPath: "estimatedProgress", options: .new, context: nil) //
override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) { if ((object as AnyObject).isEqual(webView) && (keyPath == "estimatedProgress")) { progeress!.setProgress(Float(webView.estimatedProgress), animated: true) if(webView.estimatedProgress>=1){ progeress.isHidden = true; progeress!.setProgress(0, animated: false); }else{ progeress.isHidden = false; progeress!.setProgress(Float(webView.estimatedProgress), animated: true); } print(webView.estimatedProgress); } }