Add support for movementX/Y in SyntheticMouseEvent

Register screenX/Y after calculating values

Fix movementY variable name error

Linting fix

Return 0 in instanes where previous values are null

Author: Brandon Dail

src/renderers/dom/client/syntheticEvents/SyntheticMouseEvent.js

@@ -13,6 +13,7 @@
 
 var SyntheticUIEvent = require('SyntheticUIEvent');
 var ViewportMetrics = require('ViewportMetrics');
+var MouseMetrics = require('MouseMetrics');
 
 var getEventModifierState = require('getEventModifierState');
 
@@ -63,6 +64,28 @@ var MouseEventInterface = {
       event.pageY :
       event.clientY + ViewportMetrics.currentScrollTop;
   },
+  movementX: function(event) {
+    if ('movementX' in event) {
+      return event.movementX;
+    }
+    var previousScreenX = MouseMetrics.previousScreenX;
+    MouseMetrics.setPreviousScreenX(event.screenX);
+    if (previousScreenX === null) {
+      return 0;
+    }
+    return event.screenX - previousScreenX;
+  },
+  movementY: function(event) {
+    if ('movementY' in event) {
+      return event.movementY;
+    }
+    var previousScreenY = MouseMetrics.previousScreenY;
+    MouseMetrics.setPreviousScreenY(event.screenY);
+    if (previousScreenY === null) {
+      return 0;
+    }
+    return event.screenY - previousScreenY;
+  },
 };
 
 /**

src/renderers/dom/client/utils/MouseMetrics.js

@@ -0,0 +1,30 @@
+/**
+ * Copyright 2013-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @providesModule MouseMetrics
+ */
+
+'use strict';
+
+var MouseMetrics = {
+
+  previousScreenX: null,
+
+  previousScreenY: null,
+
+  setPreviousScreenX: function(value) {
+    MouseMetrics.previousScreenX = value;
+  },
+
+  setPreviousScreenY: function(value) {
+    MouseMetrics.previousScreenY = value;
+  },
+
+};
+
+module.exports = MouseMetrics;