fpillet · May 7, 2013 13:46 · May 7, 2013
diff --git a/PageFlipMonitor.js b/PageFlipMonitor.js
@@ -0,0 +1,85 @@
+/* PageFlipMonitor v1.0
+ * Copyright 2013 CommandFusion, public domain
+ * 
+ * Utility object that monitors entering and exiting pages,
+ * and can call any number of callbacks specifically registered to observe
+ * when entering or exiting a page
+ *
+ * You callback functions should be of the form:
+ *
+ function callback(fromPage, toPage) {
+ 	// do your processing here
+ }
+ *
+ * To monitor page flips, call watchEnter or watchExit. Each function returns a unique ID that allows
+ * you to later unregister the watch
+ *
+ * var watch1 = PageFlipMonitor.watchEnter("Page 1", myEnterCallback);	// be notified when entering "Page 1"
+ * var watch2 = PageFlipMonitor.watchExit("Page 1", myExitCallback);		// be notified when exiting "Page 1"
+ *
+ * // to stop watching:
+ *
+ * PageFlipMonitor.unwatch(watch1);
+ * PageFlipMonitor.unwatch(watch2);
+ *
+ */
+var PageFlipMonitor = {
+	nextID: 0,
+    enter: {},
+    exit: {},
+	watchEnter: function(pageName, callback) {
+		return this.register(this.enter, pageName, callback);
+	},
+	watchExit: function(pageName, callback) {
+		return this.register(this.exit, pageName, callback);
+	},
+	unwatch: function(id) {
+		function filter(watchSet) {
+			for (var pageName in watchSet) {
+				if (watchSet.hasOwnProperty(pageName)) {
+					watchSet[pageName] = watchSet[pageName].filter(function(e) {
+						return e.id != id;
+					});
+				}
+			}
+		}
+		filter(this.enter);
+		filter(this.exit);
+	},
+
+	// internal function, do not use
+	register: function(watchSet, pageName, callback) {
+		function doCallbacks(array, from, to) {
+			if (array != null) {
+				array.forEach(function(elem) {
+					try {
+						elem.cb.apply(null, from, to);
+					}
+					catch(e) {
+						CF.log("Warning: exception caught while calling Page Flip callback from " + from + " to " + to + ": " + e);
+					}
+				});
+			}
+		}
+	    var array = watchSet[pageName];
+		if (array == null) {
+			array = [];
+			watchSet[pageName] = array;
+		}
+		if (this.nextID == 0) {
+			var self = this;
+			CF.watch(CF.PageFlipEvent, function(from, to) {
+				if (from != null)
+					doCallbacks(self.exit[from], from, to);
+				if (to != null)
+					doCallbacks(self.enter[to], from, to);
+			});
+		}
+		var id = this.nextID++;
+		array.push({
+			id: id,
+			cb: callback
+		});
+		return id;
+	}
+};