[Edit] Add mct-before-unload directive

Add directive for exposing expressions which should be
evaluated for the browser's onbeforeunload event, to
prevent user-initiated navigation from causing a loss
of unsaved changes. WTD-1035.

platform/commonUI/edit/src/directives/MCTBeforeUnload.js

@@ -0,0 +1,68 @@
+/*global define*/
+
+define(
+    [],
+    function () {
+        "use strict";
+
+        /**
+         * Defines the `mct-before-unload` directive. The expression bound
+         * to this attribute will be evaluated during page navigation events
+         * and, if it returns a truthy value, will be used to populate a
+         * prompt to the user to confirm this navigation.
+         * @constructor
+         * @param $window the window
+         */
+        function MCTBeforeUnload($window) {
+            var unloads = [],
+                oldBeforeUnload = $window.onbeforeunload;
+
+            // Run all unload functions, returning the first returns truthily.
+            function checkUnloads() {
+                var result;
+                unloads.forEach(function (unload) {
+                    result = result || unload();
+                });
+                return result;
+            }
+
+            // Link function for an mct-before-unload directive usage
+            function link(scope, element, attrs) {
+                // Invoke the
+                function unload() {
+                    return scope.$eval(attrs.mctBeforeUnload);
+                }
+
+                // Stop using this unload expression
+                function removeUnload() {
+                    unloads = unloads.filter(function (callback) {
+                        return callback !== unload;
+                    });
+                    if (unloads.length === 0) {
+                        $window.onbeforeunload = oldBeforeUnload;
+                    }
+                }
+
+                // Include this instance of the directive's unload function
+                unloads.push(unload);
+
+                // If this is the first active instance of this directive,
+                // register as the window's beforeunload handler
+                $window.onbeforeunload = checkUnloads;
+
+                // Remove it when the scope is destroyed
+                scope.$on("$destroy", removeUnload);
+            }
+
+            return {
+                // Applicable as an attribute
+                restrict: "A",
+                // Link with the provided function
+                link: link
+            };
+        }
+
+        return MCTBeforeUnload;
+
+    }
+);