无法将文本从WKWebview复制到剪贴板

class MyScriptMessageHandler: NSObject, WKScriptMessageHandler {
    func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
        if let text = message.body as? String {
            UIPasteboard.general.string = text
        }
    }
}

let configuration = WKWebViewConfiguration()
let userContentController = WKUserContentController()
let scriptMessageHandler = MyScriptMessageHandler()
userContentController.add(scriptMessageHandler, name: "copyToClipboard")
configuration.userContentController = userContentController

let webView = WKWebView(frame: .zero, configuration: configuration)

function copyTextToClipboard(text) {
    window.webkit.messageHandlers.copyToClipboard.postMessage(text);
}

copyTextToClipboard('要复制的文本');