消息队列mqtt取消订阅的回调

消息队列的回调，可以帮助我们知道任务的结果，无论是成功还是失败，我们可以根据结果执行不同的操作,

取消订阅的sdk源码

	/**
		 * Unsubscribe for messages, stop receiving messages sent to destinations described by the filter.
		 *
		 * @name Paho.MQTT.Client#unsubscribe
		 * @function
		 * @param {string} filter - describing the destinations to receive messages from.
		 * @param {object} unsubscribeOptions - used to control the subscription
		 * @param {object} unsubscribeOptions.invocationContext - passed to the onSuccess callback
		                                      or onFailure callback.
		 * @param {function} unsubscribeOptions.onSuccess - called when the unsubscribe acknowledgement has been received from the server.
		 *                                    A single response object parameter is passed to the
		 *                                    onSuccess callback containing the following fields:
		 *                                    <ol>
		 *                                    <li>invocationContext - if set in the unsubscribeOptions.
		 *                                    </ol>
		 * @param {function} unsubscribeOptions.onFailure called when the unsubscribe request has failed or timed out.
		 *                                    A single response object parameter is passed to the onFailure callback containing the following fields:
		 *                                    <ol>
		 *                                    <li>invocationContext - if set in the unsubscribeOptions.
		 *                                    <li>errorCode - a number indicating the nature of the error.
		 *                                    <li>errorMessage - text describing the error.
		 *                                    </ol>
		 * @param {number} unsubscribeOptions.timeout - which, if present, determines the number of seconds
		 *                                    after which the onFailure callback is called. The presence of
		 *                                    a timeout does not prevent the onSuccess callback from being
		 *                                    called when the unsubscribe completes
		 * @throws {InvalidState} if the client is not in connected state.
		 */
		this.unsubscribe = function (filter, unsubscribeOptions) {
			if (typeof filter !== "string")
				throw new Error("Invalid argument:"+filter);
			unsubscribeOptions = unsubscribeOptions || {} ;
			validate(unsubscribeOptions,  {invocationContext:"object",
										   onSuccess:"function",
										   onFailure:"function",
										   timeout:"number"
										  });
			if (unsubscribeOptions.timeout && !unsubscribeOptions.onFailure)
				throw new Error("unsubscribeOptions.timeout specified with no onFailure callback.");
			client.unsubscribe(filter, unsubscribeOptions);
		};

 实际操作

 let opt={
      onSuccess(){
       console.log("退订成功")
      },
      onFailure(error){
       console.log("退订失败")
       console.log(error)
      }
      }
 this.client.unsubscribe(topic,opt);