mqtt android demo

以下是一个基本的MQTT Android应用程序的示例代码，包括连接到MQTT服务器、发布消息和订阅主题：

public class MainActivity extends AppCompatActivity {
    private MqttAndroidClient mqttAndroidClient;
    private static final String TAG = "MQTT Example";
    private static final String MQTT_BROKER_URL = "tcp://mqtt.example.com:1883";
    private static final String MQTT_USERNAME = "username";
    private static final String MQTT_PASSWORD = "password";
    private static final String PUBLISH_TOPIC = "mytopic";
    private static final String SUBSCRIBE_TOPIC = "mytopic";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        String clientId = MqttClient.generateClientId();
        mqttAndroidClient = new MqttAndroidClient(this, MQTT_BROKER_URL, clientId);
        mqttAndroidClient.setCallback(new MqttCallback() {
            @Override
            public void connectionLost(Throwable cause) {
                Log.d(TAG, "Connection lost");
            }

            @Override
            public void messageArrived(String topic, MqttMessage message) throws Exception {
                Log.d(TAG, "Message received: " + new String(message.getPayload()));
            }

            @Override
            public void deliveryComplete(IMqttDeliveryToken token) {
                Log.d(TAG, "Delivery complete");
            }
        });

        MqttConnectOptions mqttConnectOptions = new MqttConnectOptions();
        mqttConnectOptions.setUserName(MQTT_USERNAME);
        mqttConnectOptions.setPassword(MQTT_PASSWORD.toCharArray());

        try {
            IMqttToken token = mqttAndroidClient.connect(mqttConnectOptions);
            token.setActionCallback(new IMqttActionListener() {
                @Override
                public void onSuccess(IMqttToken asyncActionToken) {
                    Log.d(TAG, "Connected to broker");
                    subscribeToTopic();
                }

                @Override
                public void onFailure(IMqttToken asyncActionToken, Throwable exception) {
                    Log.d(TAG, "Connection failed: " + exception.getMessage());
                }
            });
        } catch (MqttException e) {
            e.printStackTrace();
        }
    }

    private void subscribeToTopic() {
        try {
            mqttAndroidClient.subscribe(SUBSCRIBE_TOPIC, 0, null, new IMqttActionListener() {
                @Override
                public void onSuccess(IMqttToken asyncActionToken) {
                    Log.d(TAG, "Subscribed to topic: " + SUBSCRIBE_TOPIC);
                }

                @Override
                public void onFailure(IMqttToken asyncActionToken, Throwable exception) {
                    Log.d(TAG, "Failed to subscribe to topic: " + SUBSCRIBE_TOPIC);
                }
            });
        } catch (MqttException e) {
            e.printStackTrace();
        }
    }

    public void publishMessage(View view) {
        String message = "Hello, MQTT!";
        try {
            MqttMessage mqttMessage = new MqttMessage();
            mqttMessage.setPayload(message.getBytes());
            mqttAndroidClient.publish(PUBLISH_TOPIC, mqttMessage);
            Log.d(TAG, "Message published: " + message);
        } catch (MqttException e) {
            e.printStackTrace();
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        try {
            mqttAndroidClient.disconnect();
        } catch (MqttException e) {
            e.printStackTrace();
        }
    }
}

此示例中，`MqttAndroidClient`是用于连接到MQTT服务器的客户端。在`onCreate`方法中，我们首先创建一个`MqttAndroidClient`实例并设置它的回调方法。然后，我们使用`MqttConnectOptions`来设置MQTT连接的用户名和密码，并使用`connect`方法连接到MQTT服务器。如果连接成功，我们将调用`subscribeToTopic`方法来订阅一个主题。

在`subscribeToTopic`方法中，我们使用`mqttAndroidClient`实例来订阅一个主题。如果订阅成功，我们将在日志中打印一条消息。

在`publishMessage`方法中，我们使用`mqttAndroidClient`实例来发布一条消息到指定的主题。这条消息将会在MQTT服务器上发送给所有订阅该主题的设备。

最后，在`onDestroy`方法中，我们使用`mqttAndroidClient`实例来断开MQTT连接。

请注意，这只是一个基本示例，并且可能需要根据您的应用程序的需求进行修改。