Support instance-as-result children

See also: 
- #19 
- gaearon/react-hot-loader#832 
- https://github.com/gaearon/react-hot-loader/blob/master/src/proxy/utils.js#L84

Author: Thien Do

src/__tests__/index.test.js

@@ -164,4 +164,36 @@ describe('reactTreeWalker', () => {
       expect(actual).toMatchObject(expected)
     })
   })
+
+  it('works with instance-as-result component', () => {
+    // eslint-disable-next-line react/prefer-stateless-function
+    class Baz extends Component {
+      render() {
+        return (
+          <div>
+            <Foo something={1} />
+            <Foo something={2} />
+          </div>
+        )
+      }
+    }
+    const Bar = props => new Baz(props)
+    const tree = (
+      <div>
+        <Bar />
+      </div>
+    )
+    const actual = []
+    // eslint-disable-next-line no-unused-vars
+    const visitor = (element, instance, context) => {
+      if (instance && typeof instance.getSomething === 'function') {
+        const something = instance.getSomething()
+        actual.push(something)
+      }
+    }
+    return reactTreeWalker(tree, visitor).then(() => {
+      const expected = [1, 2]
+      expect(actual).toEqual(expected)
+    })
+  })
 })

src/index.js

@@ -49,6 +49,11 @@ const pMapSeries = (iterable, iterator) => {
   ).then(() => ret)
 }
 
+const ensureChild = child =>
+  child && typeof child.render === 'function'
+    ? ensureChild(child.render())
+    : child
+
 export const isPromise = x => x != null && typeof x.then === 'function'
 
 // Recurse an React Element tree, running visitor on each element.
@@ -68,7 +73,7 @@ export default function reactTreeWalker(
           resolve()
         }
 
-        const child = getChildren()
+        const child = ensureChild(getChildren())
         const theChildContext =
           typeof childContext === 'function' ? childContext() : childContext