应用订阅了 MQTT 主题，但缺少发布到该主题的一些消息

Question

我在 Android 上使用 AWS Iot。我想获得特定事物的影子状态。这是代码：

mqttManager = new AWSIotMqttManager(clientId, CUSTOMER_SPECIFIC_ENDPOINT);

String subTopic = "$aws/things/<thingName>/shadow/get/accepted";
mqttManager.subscribeToTopic(subTopic, AWSIotMqttQos.QOS0, new AWSIotMqttNewMessageCallback() {
                        @Override
                        public void onMessageArrived(String topic, byte[] data) {
                            // do something
                        });

String pubTopic = "$aws/things/<thingName>/shadow/get";                        
mqttManager.publishString("", pubTopic, AWSIotMqttQos.QOS0);

奇怪的是它运行一次，然后我重新 运行 代码但它不起作用，我重新 运行 它再次运行。这是为什么？

Answer 1

问题是客户端在订阅完成前发布。幸运的是，当订阅完成时，subscribeToTopic 方法接受另一个 returns 回调。那是我们可以安全发布的时候。代码如下所示：

String subTopic = "$aws/things/<thingName>/shadow/get/accepted";
mqttManager.subscribeToTopic(subTopic, 
                        AWSIotMqttQos.QOS0, 
                        new AWSIotMqttSubscriptionStatusCallback() {
                        
                            @Override
                            public void onSuccess() {
                                String pubTopic = "$aws/things/<thingName>/shadow/get";                        
                                mqttManager.publishString("", pubTopic, AWSIotMqttQos.QOS0);
                            }

                            @Override
                            public void onFailure(Throwable exception) {

                            }
                            
                        },
                        new AWSIotMqttNewMessageCallback() {
                            @Override
                            public void onMessageArrived(String topic, byte[] data) {
                                // do something
                            } 
                        });