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// 首先添加监听
webView.addObserver(
    self, 
    forKeyPath: #keyPath(WKWebView.estimatedProgress), 
    options: .new, 
    context: nil
)

// 监听回调
override func observeValue(
    forKeyPath keyPath: String?,
    of object: Any?,
    change: [NSKeyValueChangeKey : Any]?,
    context: UnsafeMutableRawPointer?) {
    
    if keyPath == "estimatedProgress" {
        // do something 
    }
                             
}