Javascript 如何使用WKScriptMessageHandler读取发送到父元素的postmessage？

我正在WKWebView中加载一个URL，该URL正在向父元素发送postmessage：

window.parent.test.postMessage('A message')

我正在实现WKScriptMessageHandler并尝试使用userController（didReceiveMessage）获取该消息：

message.body是一个类似于

{
   command = "015.1";
}

---

我想将这个值015.1读入字符串变量

首先，消息应该作为字典传递

window.parent.test.postMessage({"command":id})

func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
        print("chegou mensagem do javascript ")
        print(message.body)
        print(message.name)
        let dict = message.body as? NSDictionary;
        let str = dict?["command"] as? String
        print(str ?? "")

window.parent.test.postMessage({"command":id})

func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
        print("chegou mensagem do javascript ")
        print(message.body)
        print(message.name)
        let dict = message.body as? NSDictionary;
        let str = dict?["command"] as? String
        print(str ?? "")