Add support for using std::function callbacks instead of C-pointers

Author: chris-hatton

Diff for: src/MqttClient.cpp

@@ -97,10 +97,17 @@ MqttClient::~MqttClient()
   }
 }
 
+#ifdef MQTT_CLIENT_STD_FUNCTION_CALLBACK
+void MqttClient::onMessage(MqttMessageCallback callback)
+{
+  _onMessage = callback;
+}
+#else
 void MqttClient::onMessage(void(*callback)(int))
 {
   _onMessage = callback;
 }
+#endif
 
 int MqttClient::parseMessage()
 {
@@ -550,7 +557,11 @@ void MqttClient::poll()
             _rxState = MQTT_CLIENT_RX_STATE_READ_PUBLISH_PAYLOAD;
 
             if (_onMessage) {
+#ifdef MQTT_CLIENT_STD_FUNCTION_CALLBACK
+              _onMessage(this,_rxLength);
+#else
               _onMessage(_rxLength);
+#endif
 
               if (_rxLength == 0) {
                 _rxState = MQTT_CLIENT_RX_STATE_READ_TYPE;
@@ -573,7 +584,11 @@ void MqttClient::poll()
           _rxState = MQTT_CLIENT_RX_STATE_READ_PUBLISH_PAYLOAD;
 
           if (_onMessage) {
+#ifdef MQTT_CLIENT_STD_FUNCTION_CALLBACK
+            _onMessage(this,_rxLength);
+#else
             _onMessage(_rxLength);
+#endif
           }
 
           if (_rxLength == 0) {

Diff for: src/MqttClient.h

@@ -32,12 +32,24 @@
 #define MQTT_BAD_USER_NAME_OR_PASSWORD      4
 #define MQTT_NOT_AUTHORIZED                 5
 
+// Make this definition in your application code to use std::functions for onMessage callbacks instead of C-pointers:
+// #define MQTT_CLIENT_STD_FUNCTION_CALLBACK
+
+#ifdef MQTT_CLIENT_STD_FUNCTION_CALLBACK
+#include <functional>
+#endif
+
 class MqttClient : public Client {
 public:
   MqttClient(Client& client);
   virtual ~MqttClient();
 
+#ifdef MQTT_CLIENT_STD_FUNCTION_CALLBACK
+  typedef std::function<void(MqttClient *client, int messageSize)> MessageCallback;
+  void onMessage(MessageCallback callback);
+#else
   void onMessage(void(*)(int));
+#endif
 
   int parseMessage();
   String messageTopic() const;
@@ -128,7 +140,11 @@ class MqttClient : public Client {
 private:
   Client* _client;
 
+#ifdef MQTT_CLIENT_STD_FUNCTION_CALLBACK
+  MqttClient::MessageCallback _onMessage;
+#else
   void (*_onMessage)(int);
+#endif
 
   String _id;
   String _username;