[Fixed Position] Add handles for line end points

Add handles for moving line end points in fixed position view,
WTD-882.

platform/features/layout/src/elements/LineHandle.js

@@ -0,0 +1,61 @@
+/*global define*/
+define(
+    [],
+    function () {
+        'use strict';
+
+        /**
+         * Handle for changing x/y position of a line's end point.
+         * This is used to support drag handles for line elements
+         * in a fixed position view. Field names for opposite ends
+         * are provided to avoid zero-length lines.
+         * @constructor
+         * @param element the line element
+         * @param {string} xProperty field which stores x position
+         * @param {string} yProperty field which stores x position
+         * @param {string} xOther field which stores x of other end
+         * @param {string} yOther field which stores y of other end
+         */
+        function LineHandle(element, xProperty, yProperty, xOther, yOther) {
+            return {
+                /**
+                 * Get/set the x position of the lower-right corner
+                 * of the handle-controlled element, changing size
+                 * as necessary.
+                 */
+                x: function (value) {
+                    if (arguments.length > 0) {
+                        // Ensure we stay in view
+                        value = Math.max(value, 0);
+                        // Make sure end points will still be different
+                        if (element[yOther] !== element[yProperty] ||
+                                element[xOther] !== value) {
+                            element[xProperty] = value;
+                        }
+                    }
+                    return element[xProperty];
+                },
+                /**
+                 * Get/set the y position of the lower-right corner
+                 * of the handle-controlled element, changing size
+                 * as necessary.
+                 */
+                y: function (value) {
+                    if (arguments.length > 0) {
+                        // Ensure we stay in view
+                        value = Math.max(value, 0);
+                        // Make sure end points will still be different
+                        if (element[xOther] !== element[xProperty] ||
+                                element[yOther] !== value) {
+                            element[yProperty] = value;
+                        }
+                    }
+                    return element[yProperty];
+                }
+            };
+        }
+
+        return LineHandle;
+
+    }
+);

platform/features/layout/src/elements/LineProxy.js

@@ -1,8 +1,8 @@
 /*global define*/
 
 define(
-    ['./ElementProxy'],
+    ['./ElementProxy', './LineHandle'],
-    function (ElementProxy) {
+    function (ElementProxy, LineHandle) {
         'use strict';
 
         /**
@@ -15,7 +15,11 @@ define(
          * @param {Array} elements the full array of elements
          */
         function LineProxy(element, index, elements) {
-            var proxy = new ElementProxy(element, index, elements);
+            var proxy = new ElementProxy(element, index, elements),
+                handles = [
+                    new LineHandle(element, 'x', 'y', 'x2', 'y2'),
+                    new LineHandle(element, 'x2', 'y2', 'x', 'y')
+                ];
 
             /**
              * Get the top-left x coordinate, in grid space, of
@@ -105,6 +109,15 @@ define(
                 return element.y2 - proxy.y();
             };
 
+            /**
+             * Get element handles for changing the position of end
+             * points of this line.
+             * @returns {LineHandle[]} line handles for both end points
+             */
+            proxy.handles = function () {
+                return handles;
+            };
+
             return proxy;
         }