Add begin API

I’ve made calling this mandatory for now.

NTPClient.cpp

@@ -48,6 +48,16 @@ NTPClient::NTPClient(UDP& udp, const char* poolServerName, int timeOffset, int u
   this->_updateInterval = updateInterval;
 }
 
+void NTPClient::begin() {
+  this->begin(NTP_DEFAULT_LOCAL_PORT);
+}
+
+void NTPClient::begin(int port) {
+  this->_port = port;
+
+  this->_udp->begin(this->_port);
+}
+
 void NTPClient::forceUpdate() {
   #ifdef DEBUG_NTPClient
     Serial.println("Update from NTP Server");
@@ -81,7 +91,6 @@ void NTPClient::forceUpdate() {
 void NTPClient::update() {
   if ((millis() - this->_lastUpdate >= this->_updateInterval)     // Update after _updateInterval
     || this->_lastUpdate == 0) {                                // Update if there was no update yet.
-    if (this->_lastUpdate == 0) this->_udp->begin(this->_port);  // Start _udp if there was no update yet.
     this->forceUpdate();
   }
 }

NTPClient.h

@@ -6,13 +6,14 @@
 
 #define SEVENZYYEARS 2208988800UL
 #define NTP_PACKET_SIZE 48
+#define NTP_DEFAULT_LOCAL_PORT 1337
 
 class NTPClient {
   private:
     UDP*          _udp;
 
     const char*   _poolServerName = "time.nist.gov"; // Default time server
-    int           _port           = 1337;
+    int           _port           = NTP_DEFAULT_LOCAL_PORT;
     int           _timeOffset     = 0;
 
     unsigned int  _updateInterval = 60000;  // In ms
@@ -31,6 +32,16 @@ class NTPClient {
     NTPClient(UDP& udp, const char* poolServerName, int timeOffset);
     NTPClient(UDP& udp, const char* poolServerName, int timeOffset, int updateInterval);
 
+    /**
+     * Starts the underlying UDP client with the default local port
+     */
+    void begin();
+
+    /**
+     * Starts the underlying UDP client with the specified local port
+     */
+    void begin(int port);
+
     /**
      * This should be called in the main loop of your application. By default an update from the NTP Server is only
      * made every 60 seconds. This can be configured in the NTPClient constructor.

README.md

@@ -31,6 +31,7 @@ void setup(){
     Serial.print ( "." );
   }
 
+  timeClient.begin();
 }
 
 void loop() {

examples/Advanced/Advanced.ino

@@ -23,6 +23,8 @@ void setup(){
     delay ( 500 );
     Serial.print ( "." );
   }
+
+  timeClient.begin();
 }
 
 void loop() {

examples/Basic/Basic.ino

@@ -20,6 +20,8 @@ void setup(){
     delay ( 500 );
     Serial.print ( "." );
   }
+
+  timeClient.begin();
 }
 
 void loop() {