Record scroll position in dispatch

Recording scroll position in dispatch method is necessary for clients using alternative batching strategies, such as react-raf-batching[1].
With an alternative batching strategy, by the time componentWillUpdate is called, browser may have tried to restore position itself (not always correctly), and the recorded position is thus wrong.

Also, don't record position for REPLACE action since there is no way to get back to it.

[1]: https://github.com/petehunt/react-raf-batching

Author: gaearon

modules/mixins/Scrolling.js

@@ -32,38 +32,43 @@ function shouldUpdateScroll(state, prevState) {
  */
 var Scrolling = {
 
+  statics: {
+    /**
+     * Records curent scroll position as the last known position for the given URL path.
+     */
+    recordScrollPosition: function (path) {
+      if (!this.scrollHistory)
+        this.scrollHistory = {};
+
+      this.scrollHistory[path] = getWindowScrollPosition();
+    },
+
+    /**
+     * Returns the last known scroll position for the given URL path.
+     */
+    getScrollPosition: function (path) {
+      if (!this.scrollHistory)
+        this.scrollHistory = {};
+
+      return this.scrollHistory[path] || null;
+    }
+  },
+
   componentWillMount: function () {
     invariant(
       this.getScrollBehavior() == null || canUseDOM,
       'Cannot use scroll behavior without a DOM'
     );
-
-    this._scrollHistory = {};
   },
 
   componentDidMount: function () {
     this._updateScroll();
   },
 
-  componentWillUpdate: function () {
-    this._scrollHistory[this.state.path] = getWindowScrollPosition();
-  },
-
   componentDidUpdate: function (prevProps, prevState) {
     this._updateScroll(prevState);
   },
 
-  componentWillUnmount: function () {
-    delete this._scrollHistory;
-  },
-
-  /**
-   * Returns the last known scroll position for the given URL path.
-   */
-  getScrollPosition: function (path) {
-    return this._scrollHistory[path] || null;
-  },
-
   _updateScroll: function (prevState) {
     if (!shouldUpdateScroll(this.state, prevState)) {
       return;
@@ -73,7 +78,7 @@ var Scrolling = {
 
     if (scrollBehavior)
       scrollBehavior.updateScrollPosition(
-        this.getScrollPosition(this.state.path),
+        this.constructor.getScrollPosition(this.state.path),
         this.state.action
       );
   }

modules/utils/createRouter.js

@@ -4,6 +4,7 @@ var invariant = require('react/lib/invariant');
 var canUseDOM = require('react/lib/ExecutionEnvironment').canUseDOM;
 var ImitateBrowserBehavior = require('../behaviors/ImitateBrowserBehavior');
 var RouteHandler = require('../components/RouteHandler');
+var LocationActions = require('../actions/LocationActions');
 var HashLocation = require('../locations/HashLocation');
 var HistoryLocation = require('../locations/HistoryLocation');
 var RefreshLocation = require('../locations/RefreshLocation');
@@ -264,9 +265,14 @@ function createRouter(options) {
        * hooks wait, the transition is fully synchronous.
        */
       dispatch: function (path, action, callback) {
-        if (state.path === path)
+        var prevPath = state.path;
+        if (prevPath === path)
           return; // Nothing to do!
 
+        if (prevPath && action !== LocationActions.REPLACE) {
+          this.recordScrollPosition(prevPath);
+        }
+
         var match = this.match(path);
 
         warning(