[fixed] rendering current handlers before rendering root

when the path changes, the router figures out the
next state, but we don't want the current views
using it until the root handler is rendered again,
allowing people to put up loading indicators, or
have live data not break their apps

examples/async-data/app.js

@@ -114,7 +114,7 @@ var Index = React.createClass({
 
 var routes = (
   <Route name="contacts" path="/" handler={App}>
-    <DefaultRoute handler={Index}/>
+    <DefaultRoute name="index" handler={Index}/>
     <Route name="contact" path="contact/:id" handler={Contact}/>
   </Route>
 );

modules/__tests__/Router-test.js

@@ -198,10 +198,6 @@ describe('Router.run', function () {
     });
   });
 
-  describe('RouteHandler', function () {
-    it('throws if called after the router transitions to a new state');
-  });
-
   describe('locations', function () {
     it('defaults to HashLocation', function (done) {
       var routes = <Route path="/" handler={Foo}/>

modules/components/__tests__/RouteHandler-test.js

@@ -1,14 +1,72 @@
 /** @jsx React.DOM */
-
 var assert = require('assert');
 var expect = require('expect');
 var React = require('react');
 var Router = require('../../index');
 var Route = require('../Route');
 var RouteHandler = require('../RouteHandler');
+var TestLocation = require('../../locations/TestLocation');
+var {
+  Bar,
+  Foo
+} = require('../../__tests__/TestHandlers');
+
 
 describe('RouteHandler', function () {
 
+  it('uses the old handler until the top-level component is rendered again', function (done) {
+    var updateComponentBeforeNextRender;
+    TestLocation.history = [ '/foo' ];
+
+    var Root = React.createClass({
+      componentDidMount: function () {
+        updateComponentBeforeNextRender = function (cb) {
+          this.forceUpdate(cb);
+        }.bind(this);
+      },
+
+      render: function() {
+        return (
+          <div>
+            <h1>Root</h1>
+            <RouteHandler/>
+          </div>
+        );
+      }
+    });
+
+    var routes = (
+      <Route name="root" handler={Root} path='/'>
+        <Route name="foo" handler={Foo} path='/foo'/>
+        <Route name="bar" handler={Bar} path='/bar'/>
+      </Route>
+    );
+
+    var div = document.createElement('div');
+    var steps = [];
+
+    steps.push(function(Handler, state) {
+      React.render(<Handler/>, div, function () {
+        expect(div.innerHTML).toMatch(/Foo/);
+        TestLocation.push('/bar');
+      });
+    });
+
+    steps.push(function(Handler, state) {
+      updateComponentBeforeNextRender(function() {
+        expect(div.innerHTML).toMatch(/Foo/);
+        React.render(<Handler/>, div, function () {
+          expect(div.innerHTML).toMatch(/Bar/);
+          done();
+        });
+      });
+    });
+
+    Router.run(routes, TestLocation, function () {
+      steps.shift().apply(this, arguments);
+    });
+  });
+
   it('renders after an update', function (done) {
     var Nested = React.createClass({
       componentDidMount: function () {

modules/utils/createRouter.js

@@ -138,6 +138,12 @@ function createRouter(options) {
   var onError = options.onError || defaultErrorHandler;
   var onAbort = options.onAbort || defaultAbortHandler;
   var state = {};
+  var nextState = {};
+
+  function updateState() {
+    state = nextState;
+    nextState = {};
+  }
 
   // Automatically fall back to full page refreshes in
   // browsers that don't support the HTML history API.
@@ -302,11 +308,11 @@ function createRouter(options) {
             if (error || transition.isAborted)
               return callback.call(router, error, transition);
 
-            state.path = path;
-            state.action = action;
-            state.routes = nextRoutes;
-            state.params = nextParams;
-            state.query = nextQuery;
+            nextState.path = path;
+            nextState.action = action;
+            nextState.routes = nextRoutes;
+            nextState.params = nextParams;
+            nextState.query = nextQuery;
 
             callback.call(router, null, transition);
           });
@@ -327,7 +333,7 @@ function createRouter(options) {
           } else if (transition.isAborted) {
             onAbort.call(router, transition.abortReason, location);
           } else {
-            callback.call(router, router, state);
+            callback.call(router, router, nextState);
           }
         }
 
@@ -384,10 +390,12 @@ function createRouter(options) {
     },
 
     getInitialState: function () {
+      updateState();
       return state;
     },
 
     componentWillReceiveProps: function () {
+      updateState();
       this.setState(state);
     },