improve MQTT example:

- retry connecting to broker indefinitely, indicating failure by flashing led fast.
- connect with DNS.
- publish millis() to topic every 5s while connected.
- make PubSubClient instance global variable.

Author: RoanBrand

README.md

@@ -37,9 +37,11 @@ After installing the dependencies and this library, make sure you have all 3 pre
 
 ### Details
 - Tested only on Arduino Uno. It would probably not work for the Arduino Due.
+- You cannot use the same serial port in your app that is being used by the protocol, e.g. You cannot use `Serial` (Serial0) when the library uses `NeoSerial`, etc. You can modify this lib easily to use other hardware serial ports on bigger boards than the Arduino Uno if you want to free up your USB Serial connection.
+- Your app's instance of `ArduinoSerialToTCPBridgeClient` needs to be a pointer and created with `new()`. It doesn't work otherwise and I don't know why yet.
+
 - The protocol provides the app an in order, duplicates free and error checked byte stream by adding a CRC32 and simple retry mechanism. See [this](https://en.wikibooks.org/wiki/Serial_Programming/Error_Correction_Methods) for background.
 - The **Protocol Gateway** opens a real TCP connection to a set destination on behalf of the **Protocol Client** running on the Arduino, and forwards traffic bi-directionally.
 - `ArduinoSerialToTCPBridgeClient` is derived from the standard Arduino `Client` class. This means existing code written for Ethernet/Wi-Fi shields should work with this.
 - `NeoHWSerial` is an alternative for `Serial`, used to gain access to the AVR UART interrupts.
 - The protocol cannot run over the standard Serial API or something like software serial because it needs hardware level RX interrupts when bytes arrive.
-- You cannot use the same serial port in your app that is being used by the protocol, e.g. You cannot use `Serial` (Serial0) when the library uses `NeoSerial`, etc. You can modify this lib easily to use other hardware serial ports on bigger boards than the Arduino Uno if you want to free up your USB Serial connection.

examples/MQTTClient/MQTTClient.ino

@@ -1,42 +1,78 @@
 /*
  * Basic example of MQTT Client running over the Serial To TCP Bridge Client.
- * The Arduino makes a connection to a Protocol Gateway instance on the PC that
- * is listening on the COM port that is connected to the Serial-over-USB of the Arduino.
- * The MQTT Client connects to a Broker running on localhost on the host PC.
+ *
+ * The example connects to a Broker serving on "localhost" on the host PC.
+ * It will publish millis() as a message every 5s, while connected.
+ *
+ * Using another MQTT client:
+ * Publish char '1' to topic "led" to turn on Led on Arduino Uno and '2' to turn off.
  */
- 
+
 #include <ArduinoSerialToTCPBridgeClient.h>
 #include <PubSubClient.h>
 
-ArduinoSerialToTCPBridgeClient* s;  // Protocol Client running over USB Serial
-PubSubClient* client;               // MQTT Client
+ArduinoSerialToTCPBridgeClient*      s;  // Protocol Client running over USB Serial
+PubSubClient                    client;  // MQTT Client
+
+const char* broker   = "localhost";
 const char* ledTopic = "led";
+const char* outTopic = "ArduinoOut";
+
+uint32_t lastPub = 0;
 
 void setup() {
   pinMode(13, OUTPUT);
   s = new ArduinoSerialToTCPBridgeClient();
-  client = new PubSubClient(*s);
+  client.setClient(*s);
+
   // MQTT Broker running on same PC the Arduino is connected to.
-  client->setServer(IPAddress(127,0,0,1), 1883);
-  client->setCallback(callback);
-  if (client->connect("arduinoClient")) {
-    client->publish("outTopic", "Hello world!");
-    client->subscribe(ledTopic);
-  }
+  client.setServer(broker, 1883);
+  client.setCallback(callback);
 }
 
 void loop() {
-  client->loop();
+  // If not connected, retry indefinitely.
+  while(!client.loop()) {
+    while(!connectToBroker()) {
+
+      // If connection attempt unsuccessful, flash led fast.
+      for (uint8_t fc = 0; fc < 10; fc++) {
+        digitalWrite(13, !digitalRead(13));
+        delay(200);
+      }
+    }
+  }
+
+  // Publish arduino alive time every 5s, while connected to broker.
+  uint32_t now = millis();
+  if ((now - lastPub) > 5000) {
+    String aliveTimeMsg = String(now);
+    client.publish(outTopic, aliveTimeMsg.c_str());
+    lastPub = now;
+  }
 }
 
+// MQTT incoming message from broker callback
 void callback(char* topic, byte* payload, unsigned int length) {
   // Only proceed if incoming message's topic matches.
+  // Toggle led if according to message character.
   if ((strcmp(topic, ledTopic) == 0) && (length == 1)) {
-    if (payload[0] == 0x31) {
-      digitalWrite(13, HIGH); // Turn on led if MQTT message '1'
-    } else if (payload[0] == 0x32) {
-      digitalWrite(13, LOW);  // Turn off led if MQTT message '2'
+    if (payload[0] == '1') {
+      digitalWrite(13, HIGH);
+    } else if (payload[0] == '2') {
+      digitalWrite(13, LOW);
     }
   }
 }
 
+boolean connectToBroker() {
+  if (client.connect("arduinoClient")) {
+    // Publish first message on connect and subscribe to Led controller.
+    client.publish(outTopic, "Hello world!");
+    client.subscribe(ledTopic);
+    return true;
+  } else {
+    return false;
+  }
+}
+