Add a method to start a new session (close snowplow#515)

Author: mhadam

src/js/tracker.js

@@ -1322,7 +1322,57 @@
 				};
 			}
 		}
-		
+
+		/**
+		 * Expires current session and starts a new session.
+		 */
+		function newSession() {
+			// If cookies are enabled, base visit count and session ID on the cookies
+			var nowTs = Math.round(new Date().getTime() / 1000),
+				ses = getSnowplowCookieValue('ses'),
+				id = loadDomainUserIdCookie(),
+				cookiesDisabled = id[0],
+				_domainUserId = id[1], // We could use the global (domainUserId) but this is better etiquette
+				createTs = id[2],
+				visitCount = id[3],
+				currentVisitTs = id[4],
+				lastVisitTs = id[5],
+				sessionIdFromCookie = id[6];
+
+			// When cookies are enabled
+			if (cookiesDisabled === '0') {
+				memorizedSessionId = sessionIdFromCookie;
+
+				// When cookie/local storage is enabled - make a new session
+				if (configStateStorageStrategy != 'none') {
+					// New session (aka new visit)
+					visitCount++;
+					// Update the last visit timestamp
+					lastVisitTs = currentVisitTs;
+					// Regenerate the session ID
+					memorizedSessionId = uuid.v4();
+				}
+
+				memorizedVisitCount = visitCount;
+
+				// Create a new session cookie
+				setSessionCookie()
+
+			} else {
+				memorizedSessionId = uuid.v4();
+				memorizedVisitCount++;
+			}
+
+			// Update cookies
+			if (configStateStorageStrategy != 'none') {
+				setDomainUserIdCookie(_domainUserId, createTs, memorizedVisitCount, nowTs,
+					lastVisitTs, memorizedSessionId);
+				setSessionCookie();
+			}
+
+			lastEventTime = new Date().getTime();
+		}
+
 		/**
 		 * Attempts to create a context using the geolocation API and add it to commonContexts
 		 */
@@ -1612,6 +1662,11 @@
 				return getPageViewId();
 			},
 
+			/**
+			 * Expires current session and starts a new session.
+			 */
+			newSession: newSession,
+
 			/**
 			 * Get the cookie name as cookieNamePrefix + basename + . + domain.
 			 *