[changed] Remove preserveScrollPosition, add scrollStrategy

This removes support for `preserveScrollPosition` due to its unforunate naming.
Instead, we introduce three scrolling strategies:

* `none`: Router doesn't scroll the window when routes change
* `scrollToTop` (default): Router always scrolls to top when routes change
* `imitateBrowser`: Router tries to act like browser acts with server-rendered pages: it scrolls to top when clicking on links, but tries to restore position when navigating back and forward

You can only specify these on <Routes />.
Per-route overrides are not supported, but you can supply a custom strategy object.

This also fixes #252.

Migration path:

The default changed from what corresponded to `imitateBrowser`, to `scrollToTop`.
If router's server-rendered scrolling imitation worked well for you, you must now specify it explicitly:

```
// before
<Routes>
<Routes preserveScrollPosition={false}>

// after
<Routes scrollStrategy='imitateBrowser'>
```

If you wish router to not try to manage scrolling, you must opt out:

```
// before
<Routes preserveScrollPosition={true}>

// after
<Routes scrollStrategy='none'>
```

Also, as a third option, you may now use the simple `scrollToTop` strategy.

modules/actions/LocationActions.js

@@ -1,20 +1,96 @@
+var supportsHistory = require('../utils/supportsHistory');
+var HistoryLocation = require('../locations/HistoryLocation');
+var RefreshLocation = require('../locations/RefreshLocation');
 var LocationDispatcher = require('../dispatchers/LocationDispatcher');
+var ActionTypes = require('../constants/ActionTypes');
+var warning = require('react/lib/warning');
 var isAbsoluteURL = require('../utils/isAbsoluteURL');
 var makePath = require('../utils/makePath');
 
 function loadURL(url) {
   window.location = url;
 }
 
+var _location = null;
+var _isDispatching = false;
+var _previousPath = null;
+
+function dispatchAction(actionType, operation) {
+  if (_isDispatching)
+    throw new Error('Cannot handle ' + actionType + ' in the middle of another action.');
+
+  _isDispatching = true;
+
+  var scrollPosition = {
+    x: window.scrollX,
+    y: window.scrollY
+  };
+
+  if (typeof operation === 'function')
+    operation(_location);
+
+  var path = _location.getCurrentPath();
+  LocationDispatcher.handleViewAction({
+    type: actionType,
+    path: path,
+    scrollPosition: scrollPosition
+  });
+
+  _isDispatching = false;
+  _previousPath = path;
+}
+
+function handleChange() {
+  var path = _location.getCurrentPath();
+
+  // Ignore changes inside or caused by dispatchAction
+  if (!_isDispatching && path !== _previousPath) {
+    dispatchAction(ActionTypes.POP);
+  }
+}
+
 /**
  * Actions that modify the URL.
  */
 var LocationActions = {
 
-  PUSH: 'push',
-  REPLACE: 'replace',
-  POP: 'pop',
-  UPDATE_SCROLL: 'update-scroll',
+  getLocation: function () {
+    return _location;
+  },
+
+  setup: function (location) {
+    // When using HistoryLocation, automatically fallback
+    // to RefreshLocation in browsers that do not support
+    // the HTML5 history API.
+    if (location === HistoryLocation && !supportsHistory())
+      location = RefreshLocation;
+
+    if (_location !== null) {
+      warning(
+        _location === location,
+        'Cannot use location %s, already using %s', location, _location
+      );
+      return;
+    }
+
+    _location = location;
+
+    if (_location !== null) {
+      dispatchAction(ActionTypes.SETUP, function (location) {
+        if (typeof location.setup === 'function')
+          location.setup(handleChange);
+      });
+    }
+  },
+
+  teardown: function () {
+    if (_location !== null) {
+      if (typeof _location.teardown === 'function')
+        _location.teardown();
+
+      _location = null;
+    }
+  },
 
   /**
    * Transitions to the URL specified in the arguments by pushing
@@ -24,9 +100,9 @@ var LocationActions = {
     if (isAbsoluteURL(to)) {
       loadURL(to);
     } else {
-      LocationDispatcher.handleViewAction({
-        type: LocationActions.PUSH,
-        path: makePath(to, params, query)
+      dispatchAction(ActionTypes.PUSH, function (location) {
+        var path = makePath(to, params, query);
+        location.push(path);
       });
     }
   },
@@ -39,9 +115,9 @@ var LocationActions = {
     if (isAbsoluteURL(to)) {
       loadURL(to);
     } else {
-      LocationDispatcher.handleViewAction({
-        type: LocationActions.REPLACE,
-        path: makePath(to, params, query)
+      dispatchAction(ActionTypes.REPLACE, function (location) {
+        var path = makePath(to, params, query);
+        location.replace(path);
       });
     }
   },
@@ -50,18 +126,8 @@ var LocationActions = {
    * Transitions to the previous URL.
    */
   goBack: function () {
-    LocationDispatcher.handleViewAction({
-      type: LocationActions.POP
-    });
-  },
-
-  /**
-   * Updates the window's scroll position to the last known position
-   * for the current URL path.
-   */
-  updateScroll: function () {
-    LocationDispatcher.handleViewAction({
-      type: LocationActions.UPDATE_SCROLL
+    dispatchAction(ActionTypes.POP, function (location) {
+      location.pop();
     });
   }
 

modules/components/Routes.js

@@ -7,6 +7,7 @@ var Route = require('../components/Route');
 var ActiveDelegate = require('../mixins/ActiveDelegate');
 var PathListener = require('../mixins/PathListener');
 var RouteStore = require('../stores/RouteStore');
+var ScrollStore = require('../stores/ScrollStore');
 var Path = require('../utils/Path');
 var Promise = require('../utils/Promise');
 var Redirect = require('../utils/Redirect');
@@ -51,9 +52,11 @@ function maybeUpdateScroll(routes) {
     return;
 
   var currentRoute = routes.getCurrentRoute();
+  var scrollPosition = ScrollStore.getScrollPosition();
 
-  if (!routes.props.preserveScrollPosition && currentRoute && !currentRoute.props.preserveScrollPosition)
-    LocationActions.updateScroll();
+  if (currentRoute && scrollPosition) {
+    window.scrollTo(scrollPosition.x, scrollPosition.y);
+  }
 }
 
 /**

modules/constants/ActionTypes.js

@@ -0,0 +1,10 @@
+var keyMirror = require('react/lib/keyMirror');
+
+var ActionTypes = keyMirror({
+  SETUP: null,
+  PUSH: null,
+  REPLACE: null,
+  POP: null
+});
+
+module.exports = ActionTypes;

modules/locations/HistoryLocation.js

@@ -34,12 +34,10 @@ var HistoryLocation = {
 
   push: function (path) {
     window.history.pushState({ path: path }, '', path);
-    _onChange();
   },
 
   replace: function (path) {
     window.history.replaceState({ path: path }, '', path);
-    _onChange();
   },
 
   pop: function () {

modules/locations/MemoryLocation.js

@@ -2,26 +2,19 @@ var warning = require('react/lib/warning');
 
 var _lastPath = null;
 var _currentPath = null;
-var _onChange;
 
 /**
  * A Location that does not require a DOM.
  */
 var MemoryLocation = {
 
-  setup: function (onChange) {
-    _onChange = onChange;
-  },
-
   push: function (path) {
     _lastPath = _currentPath;
     _currentPath = path;
-    _onChange();
   },
 
   replace: function (path) {
     _currentPath = path;
-    _onChange();
   },
 
   pop: function () {
@@ -32,7 +25,6 @@ var MemoryLocation = {
 
     _currentPath = _lastPath;
     _lastPath = null;
-    _onChange();
   },
 
   getCurrentPath: function () {

modules/mixins/PathListener.js

@@ -1,9 +1,13 @@
-var React = require('react');
+var LocationActions = require('../actions/LocationActions');
 var DefaultLocation = require('../locations/DefaultLocation');
 var HashLocation = require('../locations/HashLocation');
 var HistoryLocation = require('../locations/HistoryLocation');
 var RefreshLocation = require('../locations/RefreshLocation');
+var NoneStrategy = require('../strategies/NoneStrategy');
+var ScrollToTopStrategy = require('../strategies/ScrollToTopStrategy');
+var ImitateBrowserStrategy = require('../strategies/ImitateBrowserStrategy');
 var PathStore = require('../stores/PathStore');
+var ScrollStore = require('../stores/ScrollStore');
 
 /**
  * A hash of { name, location } pairs.
@@ -14,6 +18,15 @@ var NAMED_LOCATIONS = {
   refresh: RefreshLocation
 };
 
+/**
+ * A hash of { name, scrollStrategy } pairs.
+ */
+var NAMED_SCROLL_STRATEGIES = {
+  none: NoneStrategy,
+  scrollToTop: ScrollToTopStrategy,
+  imitateBrowser: ImitateBrowserStrategy
+};
+
 /**
  * A mixin for components that listen for changes to the current
  * URL path.
@@ -26,12 +39,20 @@ var PathListener = {
 
       if (typeof location === 'string' && !(location in NAMED_LOCATIONS))
         return new Error('Unknown location "' + location + '", see ' + componentName);
-    }
+    },
+
+    scrollStrategy: function (props, propName, componentName) {
+      var scrollStrategy = props[propName];
+
+      if (typeof scrollStrategy === 'string' && !(scrollStrategy in NAMED_SCROLL_STRATEGIES))
+        return new Error('Unknown scrollStrategy "' + scrollStrategy + '", see ' + componentName);
+    },
   },
 
   getDefaultProps: function () {
     return {
-      location: DefaultLocation
+      location: DefaultLocation,
+      scrollStrategy: ScrollToTopStrategy
     };
   },
 
@@ -48,8 +69,22 @@ var PathListener = {
     return location;
   },
 
+  /**
+   * Gets the scroll strategy object this component uses to
+   * restore scroll position when the path changes.
+   */
+  getScrollStrategy: function () {
+    var scrollStrategy = this.props.scrollStrategy;
+
+    if (typeof scrollStrategy === 'string')
+      return NAMED_SCROLL_STRATEGIES[scrollStrategy];
+
+    return scrollStrategy;
+  },
+
   componentWillMount: function () {
-    PathStore.setup(this.getLocation());
+    ScrollStore.setup(this.getScrollStrategy());
+    LocationActions.setup(this.getLocation());
 
     if (this.updatePath)
       this.updatePath(PathStore.getCurrentPath());
@@ -61,6 +96,8 @@ var PathListener = {
 
   componentWillUnmount: function () {
     PathStore.removeChangeListener(this.handlePathChange);
+    ScrollStore.teardown();
+    LocationActions.teardown();
   },
 
   handlePathChange: function () {