add count() method to ReactChildren

added ReactChildren.count() to count the number of children, and a test case.

src/utils/ReactChildren.js

@@ -129,9 +129,25 @@ function mapChildren(children, func, context) {
   return mapResult;
 }
 
+/**
+* Count the number of children that are typically specified as
+* `props.children`.
+*/
+
+function forEachSingleChildDummy(traverseContext, child, name, i) {
+  return null;
+}
+
+function countChildren(children,  context) {
+  var numberOfChildren = traverseAllChildren(children, forEachSingleChildDummy,
+    null);
+  return numberOfChildren;
+}
+
 var ReactChildren = {
   forEach: forEachChildren,
-  map: mapChildren
+  map: mapChildren,
+  count: countChildren
 };
 
 module.exports = ReactChildren;

src/utils/__tests__/ReactChildren-test.js

@@ -335,4 +335,67 @@ describe('ReactChildren', function() {
     expect(console.warn.calls.length).toEqual(1);
     expect(mapped).toEqual({'.$something': zero});
   });
+
+  it('should return 0 for null children', function() {
+    var numberOfChildren = ReactChildren.count(null);
+    expect(numberOfChildren).toBe(0);
+  });
+
+  it('should return 0 for undefined children', function() {
+    var numberOfChildren = ReactChildren.count(undefined);
+    expect(numberOfChildren).toBe(0);
+  });
+
+  it('should return 1 for single child', function() {
+    var simpleKid = <span key="simple" />;
+    var instance = <div>{simpleKid}</div>;
+    var numberOfChildren = ReactChildren.count(instance.props.children);
+    expect(numberOfChildren).toBe(1);
+  });
+
+  it('should count the number of children in flat structure', function() {
+    var zero = <div key="keyZero" />;
+    var one = null;
+    var two = <div key="keyTwo" />;
+    var three = null;
+    var four = <div key="keyFour" />;
+
+    var instance = (
+      <div>
+        {zero}
+        {one}
+        {two}
+        {three}
+        {four}
+      </div>
+    );
+    var numberOfChildren = ReactChildren.count(instance.props.children);
+    expect(numberOfChildren).toBe(5);
+  });
+
+  it('should count the number of children in nested structure', function() {
+    var zero = <div key="keyZero" />;
+    var one = null;
+    var two = <div key="keyTwo" />;
+    var three = null;
+    var four = <div key="keyFour" />;
+    var five = <div key="keyFiveInner" />;
+    // five is placed into a JS object with a key that is joined to the
+    // component key attribute.
+    // Precedence is as follows:
+    // 1. If grouped in an Object, the object key combined with `key` prop
+    // 2. If grouped in an Array, the `key` prop, falling back to array index
+
+    var instance = (
+      <div>{
+        [{
+          firstHalfKey: [zero, one, two],
+          secondHalfKey: [three, four],
+          keyFive: five
+        }]
+      }</div>
+    );
+    var numberOfChildren = ReactChildren.count(instance.props.children);
+    expect(numberOfChildren).toBe(6);
+  });
 });

src/utils/traverseAllChildren.js

@@ -180,10 +180,14 @@ var traverseAllChildrenImpl =
  * @param {?*} children Children tree object.
  * @param {!function} callback To invoke upon traversing each child.
  * @param {?*} traverseContext Context for traversal.
+ * @return {!number} The number of children in this subtree.
  */
 function traverseAllChildren(children, callback, traverseContext) {
   if (children !== null && children !== undefined) {
-    traverseAllChildrenImpl(children, '', 0, callback, traverseContext);
+    return traverseAllChildrenImpl(children, '', 0, callback, traverseContext);
+  }
+  else {
+    return 0;
   }
 }