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How to control the user interface of a WKWebView using WKUIDelegate

Paul Hudson       @twostraws

By default WKWebView works sort of like Safari, albeit in a single view rather than having tabs. If you want something more advanced – being able to monitor opening and closing windows, override behavior for JavaScript user interface, and so on – then the WKUIDelegate protocol is for you.

First, make your view controller conform to it:

class ViewController: UIViewController, WKUIDelegate {

Second, assign your view controller to the uiDelegate property of your web view:

webView.uiDelegate = self

Finally, implement whichever of the optional methods of WKUIDelegate takes your interest. For example, you can make WKWebView show a custom alert controller when any web page uses the alert() JavaScript function:

func webView(_ webView: WKWebView, runJavaScriptAlertPanelWithMessage message: String, initiatedByFrame frame: WKFrameInfo, completionHandler: @escaping () -> Void) {
    let ac = UIAlertController(title: "Hey, listen!", message: message, preferredStyle: .alert)
    ac.addAction(UIAlertAction(title: "OK", style: .default, handler: nil))
    present(ac, animated: true)
    completionHandler()
}

There’s also runJavaScriptConfirmPanelWithMessage for showing confirm and deny UI, runJavaScriptTextInputPanelWithPrompt for requesting user text input, and so on.

Note: You must call the completion handler. JavaScript’s alerts are blocking, which means JavaScript execution will not continue until the alert finishes. As a result, WebKit will complain if you don’t let it know when you’re done.

Available from iOS 10.0

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