建立应用侧与前端页面数据通道
==============

前端页面和应用侧之间可以用[createWebMessagePorts()](https://developer.huawei.com/consumer/cn/doc/harmonyos-references/arkts-apis-webview-webviewcontroller#createwebmessageports)接口创建消息端口来实现两端的通信。

在下面的示例中，应用侧页面中通过createWebMessagePorts方法创建两个消息端口，再把其中一个端口通过[postMessage()](https://developer.huawei.com/consumer/cn/doc/harmonyos-references/arkts-apis-webview-webviewcontroller#postmessage)接口发送到前端页面，便可以在前端页面和应用侧之间互相发送消息。端口使用完毕后或Webview对象销毁前通过[close](https://developer.huawei.com/consumer/cn/doc/harmonyos-references/arkts-apis-webview-webmessageport#close)接口关闭端口。

*   应用侧代码。
    
    // xxx.ets
    import { webview } from '@kit.ArkWeb';
    import { BusinessError } from '@kit.BasicServicesKit';
    
    @Entry
    @Component
    struct WebComponent {
      controller: webview.WebviewController = new webview.WebviewController();
      ports: webview.WebMessagePort\[\] = \[\];
      @State sendFromEts: string = 'Send this message from ets to HTML';
      @State receivedFromHtml: string = 'Display received message send from HTML';
    
      build() {
        Column() {
          // 展示接收到的来自HTML的内容
          Text(this.receivedFromHtml);
          // 输入框的内容发送到HTML
          TextInput({ placeholder: 'Send this message from ets to HTML' })
            .onChange((value: string) => {
              this.sendFromEts = value;
            })
    
          // 该内容可以放在onPageEnd生命周期中调用。
          Button('postMessage')
            .onClick(() => {
              try {
                // 1、创建两个消息端口。
                this.ports = this.controller.createWebMessagePorts();
                if (this.ports && this.ports\[0\] && this.ports\[1\]) {
                  // 2、在应用侧的消息端口(如端口1)上注册回调事件。
                  this.ports\[1\].onMessageEvent((result: webview.WebMessage) => {
                    let msg = 'Got msg from HTML:';
                    if (typeof (result) === 'string') {
                      console.info(\`received string message from html5, string is: ${result}\`);
                      msg = msg + result;
                    } else if (typeof (result) === 'object') {
                      if (result instanceof ArrayBuffer) {
                        console.info(\`received arraybuffer from html5, length is: ${result.byteLength}\`);
                        msg = msg + 'length is ' + result.byteLength;
                      } else {
                        console.info('not support');
                      }
                    } else {
                      console.info('not support');
                    }
                    this.receivedFromHtml = msg;
                  })
                  // 3、将另一个消息端口(如端口0)发送到HTML侧，由HTML侧保存并使用。
                  this.controller.postMessage('\_\_init\_port\_\_', \[this.ports\[0\]\], '\*');
                } else {
                  console.error(\`ports is null, Please initialize first\`);
                }
              } catch (error) {
                console.error(\`ErrorCode: ${(error as BusinessError).code},  Message: ${(error as BusinessError).message}\`);
              }
            })
    
          // 4、使用应用侧的端口给另一个已经发送到html的端口发送消息。
          Button('SendDataToHTML')
            .onClick(() => {
              try {
                if (this.ports && this.ports\[1\]) {
                  this.ports\[1\].postMessageEvent(this.sendFromEts);
                } else {
                  console.error(\`ports is null, Please initialize first\`);
                }
              } catch (error) {
                console.error(\`ErrorCode: ${(error as BusinessError).code},  Message: ${(error as BusinessError).message}\`);
              }
            })
    
          // 5、关闭端口。
          Button('closePort')
          .onClick(() => {
            try {
              if (this.ports && this.ports.length == 2) {
                this.ports\[0\].close();
                this.ports = \[\];
              } else {
                console.error("ports is null, not need close");
              }
            } catch (error) {
              console.error(\`ErrorCode: ${(error as BusinessError).code},  Message: ${(error as BusinessError).message}\`);
            }
          })
          Web({ src: $rawfile('index.html'), controller: this.controller })
        }
      }
    }
    
*   前端页面代码。
    
    <!--index.html-->
    <!DOCTYPE html>
    <html>
    <head>
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <title>WebView Message Port Demo</title>
    </head>
    <body>
        <h1>WebView Message Port Demo</h1>
        <div>
            <input type="button" value="SendToEts" onclick="PostMsgToEts(msgFromJS.value);"/><br/>
            <input id="msgFromJS" type="text" value="send this message from HTML to ets"/><br/>
        </div>
        <p class="output">display received message send from ets</p>
    </body>
    <script>
    var h5Port;
    var output = document.querySelector('.output');
    window.addEventListener('message', function (event) {
        if (event.data === '\_\_init\_port\_\_') {
            if (event.ports\[0\] !== null) {
                h5Port = event.ports\[0\]; // 1. 保存从应用侧发送过来的端口。
                h5Port.onmessage = function (event) {
                  // 2. 接收ets侧发送过来的消息。
                  var msg = 'Got message from ets:';
                  var result = event.data;
                  if (typeof(result) === 'string') {
                    console.info(\`received string message from ets, string is: ${result}\`);
                    msg = msg + result;
                  } else if (typeof(result) === 'object') {
                    if (result instanceof ArrayBuffer) {
                      console.info(\`received arraybuffer from ets, length is: ${result.byteLength}\`);
                      msg = msg + 'length is ' + result.byteLength;
                    } else {
                      console.info('not support');
                    }
                  } else {
                    console.info('not support');
                  }
                  output.innerHTML = msg;
                }
            }
        }
    })
    
    // 3. 使用h5Port向应用侧发送消息。
    function PostMsgToEts(data) {
        if (h5Port) {
          h5Port.postMessage(data);
        } else {
          console.error('h5Port is null, Please initialize first');
        }
    }
    </script>
    </html>
    

#### 常见问题

#### \[h2\]为什么H5向应用侧发送消息接收不到？

检查传递的数据类型是否正确，WebMessage支持的数据类型有string和ArrayBuffer。

如果想要传递对象类型则需要将对象类型通过JSON.stringify方法转换为string类型再进行传递。示例如下：

  function PostMsgToEts(data) {
      if (h5Port) {
        let obj = {name:'exampleName',id:10}
        h5Port.postMessage(JSON.stringify(obj));
      } else {
        console.error('h5Port is null. Please initialize it first.');
      }
  }

#### \[h2\]onControllerAttached与javaScriptOnDocumentStart的执行顺序是什么？

[javaScriptOnDocumentStart](https://developer.huawei.com/consumer/cn/doc/harmonyos-references/arkts-basic-components-web-attributes#javascriptondocumentstart11)在[onControllerAttached](https://developer.huawei.com/consumer/cn/doc/harmonyos-references/arkts-basic-components-web-events#oncontrollerattached10)之后执行。