Add an i2c request handler

Marlin/Marlin_main.cpp

 
 #if ENABLED(EXPERIMENTAL_I2CBUS) && I2C_SLAVE_ADDRESS > 0
 
-  void i2c_listener(int bytes) {
-    i2c.receive(bytes);        // just echo all bytes received to serial
+  void i2c_on_receive(int bytes) { // just echo all bytes received to serial
+    i2c.receive(bytes);
+  }
+
+  void i2c_on_request() {          // just send dummy data for now
+    static const char *msg = "Hello World!\n";
+    const char *adr = msg;
+    char c;
+    i2c.reset();
+    while (c = *adr++) i2c.addbyte(c);
+    i2c.send();
   }
 
 #endif
   #endif
 
   #if ENABLED(EXPERIMENTAL_I2CBUS) && I2C_SLAVE_ADDRESS > 0
-    i2c.onReceive(i2c_listener);
+    i2c.onReceive(i2c_on_receive);
+    i2c.onRequest(i2c_on_request);
   #endif
 }
 

Marlin/twibus.h

 // Print debug messages with M111 S2 (Uses 236 bytes of PROGMEM)
 //#define DEBUG_TWIBUS
 
-typedef void (*twiSlaveFunc_t)(int bytes);
+typedef void (*twiReceiveFunc_t)(int bytes);
+typedef void (*twiRequestFunc_t)();
 
 /**
  * TWIBUS class
       inline void receive(uint8_t bytes) { relaydata(bytes); }
 
       /**
-       * @brief Register a slave handler
-       * @details Set a handler to receive data from the bus,
-       *          so we can act as a slave.
+       * @brief Register a slave receive handler
+       * @details Set a handler to receive data addressed to us.
        *
        * @param handler A function to handle receiving bytes
        */
-      inline void onReceive(const twiSlaveFunc_t handler) { Wire.onReceive(handler); }
+      inline void onReceive(const twiReceiveFunc_t handler) { Wire.onReceive(handler); }
+
+      /**
+       * @brief Register a slave request handler
+       * @details Set a handler to send data requested from us.
+       *
+       * @param handler A function to handle receiving bytes
+       */
+      inline void onRequest(const twiRequestFunc_t handler) { Wire.onRequest(handler); }
 
     #endif