use a callback to send RDM data

protocol/esta/rdm/responder.cpp

@@ -118,7 +118,8 @@ Responder::~Responder()
  */
 void Responder::send(const std::vector<uint8_t> &data)
 {
-  (void)data;
+  if (auto sp = sender_.lock())
+    (*sp)(data);                            // sender still exists
 }
 
 
@@ -146,6 +147,22 @@ void Responder::send(MsgPtr response)
 }
 
 
+/**
+ * @brief Set the message sending callback function.
+ * @param cb Callback function.
+ * @return Token that the owner of the callback must keep for it's lifetime.
+ */
+std::shared_ptr<void> Responder::setSender(std::function<void(const std::vector<uint8_t>&)> cb)
+{
+  // wrap the callback with a shared pointer
+  auto sp = std::make_shared<std::function<void(const std::vector<uint8_t>&)>>(std::move(cb));
+  // store callback (as a weak pointer)
+  sender_ = sp;
+  // return token that caller must keep throughout it's scope
+  return sp;
+}
+
+
 /**
  * @brief Responder::receive
  * @param data

protocol/esta/rdm/responder.h

@@ -40,6 +40,8 @@ public:
   explicit Responder(UID id, Device* parent = nullptr);
   virtual ~Responder();
 
+  std::shared_ptr<void> setSender(std::function<void(const std::vector<uint8_t>&)> cb);
+
   union {
     uint16_t control_field;
     struct {
@@ -78,6 +80,7 @@ private:
   uint16_t checksum_fail_counter_   = 0;
   MsgPtr   last_status_message_;
 
+  std::weak_ptr<std::function<void(const std::vector<uint8_t>&)>> sender_;
 };