Eclipse Paho - MQTT Javascript - 如何从外部访问 onMessageArrived

Question

我正在使用 Eclipse Paho - Mqtt javascript 库。

试图从外部函数访问onMessageArrived：

  mqttConnect(){
    var client = new Paho.MQTT.Client("wss://test.mqtt.address", "myClientId");
    this.connectionInfo = client;
    client.onConnectionLost = onConnectionLost;
    client.onMessageArrived = onMessageArrived;
    client.connect({onSuccess:onConnect});

    function onConnect() {
        client.subscribe("some/path/to/subscribe/");
    }
    function onConnectionLost(responseObject) {
        if (responseObject.errorCode !== 0) {
            console.log("onConnectionLost:"+responseObject.errorMessage);
        }
    }
    function onMessageArrived(message) {
        console.log("onMessageArrived: "+message.payloadString);
        return message.payloadString; //this does not work of course.
    }
  }

尝试从外部访问消息，例如：

_constract(){
  var message = mqttConnect();
  console.log(message);

}

也许注册一些全局变量并像this.global = message.payloadString一样把它放在那里？

Answer 1

您可以将消息负载存储在一个全局变量中，以便稍后再次访问它，或者您可以在 onMessageArrived() 回调中进行所有您需要的处理。

必须这样，因为onMessageArrived() 是异步的，它可能永远不会被调用，或者它可能会被调用多次，完全取决于该主题的消息速率。

编辑（示例）：

var client;
var lastMsg;

function connect(){
  client = new Paho.MQTT.Client("wss://test.mqtt.address", "myClientId");

  client.onConnectionLost = onConnectionLost;
  client.onMessageArrived = onMessageArrived;
  client.connect({onSuccess:onConnect});
}

function onConnect() {
  client.subscribe("some/path/to/subscribe/");
}

function onConnectionLost(responseObject) {
  if (responseObject.errorCode !== 0) {
    console.log("onConnectionLost:"+responseObject.errorMessage);
  }
}

function onMessageArrived(message) {
  console.log("onMessageArrived: "+message.payloadString);
  lastMsg = message.payloadString;
}

connect();

setInterval(function() {
  document.findElementById("foo").innerHTML = lastMsg;
}, 5000);