Add onbeforeunload support

README.md

@@ -171,6 +171,22 @@ var appComponent = Application({
 
 ```
 
+## onbeforeunload Support
+
+The `History` API does not allow `popstate` events to be cancelled, which results in `window.onbeforeunload()` methods not being triggered.  This is problematic for users, since application state could be lost when they navigate to a certain page without knowing the consequences.
+
+Our solution is to check for a `window.onbeforeunload()` method, prompt the user with `window.confirm()`, and then navigate to the correct route based on the confirmation.  If a route is cancelled by the user, we reset the page URL back to the original URL by using  the `History` `pushState()` method.
+
+To implement the `window.onbeforeunload()` method, you need to set it within the components that need user verification before leaving a page.  Here is an example:
+
+```javascript
+componentDidMount: function() {
+  window.onbeforeunload = function () {
+    return 'Make sure to save your changes before leaving this page!';
+  }
+}
+```
+
 
 ## Polyfills
 `addEventListener` and `removeEventListener` polyfills are provided by:

lib/NavLink.js

@@ -100,12 +100,21 @@ NavLink = React.createClass({
 
         e.preventDefault();
         e.stopPropagation();
-        context.executeAction(navigateAction, {
-            type: navType,
-            url: href,
-            preserveScrollPosition: this.props.preserveScrollPosition,
-            params: this.props.navParams
-        });
+
+        var onBeforeUnloadText = typeof window.onbeforeunload === 'function' ? window.onbeforeunload() : '';
+        var confirmResult = onBeforeUnloadText ? window.confirm(onBeforeUnloadText) : true;
+
+        if (confirmResult) {
+            // Removes the window.onbeforeunload method so that the next page will not be affected
+            window.onbeforeunload = null;
+
+            context.executeAction(navigateAction, {
+                type: navType,
+                url: href,
+                preserveScrollPosition: this.props.preserveScrollPosition,
+                params: this.props.navParams
+            });
+        }
     },
     render: function() {
         return React.createElement(

lib/RouterMixin.js

@@ -83,11 +83,37 @@ RouterMixin = {
         }
 
         self._historyListener = function (e) {
+            debug('history listener invoked', e, url, self.state.route.url);
+
             if (context) {
+                var state = self.state || {};
                 var url = self._history.getUrl();
-                debug('history listener invoked', e, url, self.state.route.url);
-                if (url !== self.state.route.url) {
-                    context.executeAction(navigateAction, {type: TYPE_POPSTATE, url: url, params: (e.state && e.state.params)});
+                var currentUrl = state.route.url;
+                var route = state.route || {};
+                var onBeforeUnloadText = typeof window.onbeforeunload === 'function' ? window.onbeforeunload() : '';
+                var confirmResult = onBeforeUnloadText ? window.confirm(onBeforeUnloadText) : true;
+                var nav = route.navigate || {};
+                var navParams = nav.params || {};
+                var enableScroll = self._enableScroll && nav.preserveScrollPosition;
+                var historyState = {
+                    params: (nav.params || {}),
+                    scroll: {
+                        x: (enableScroll ? window.scrollX : 0),
+                        y: (enableScroll ? window.scrollY : 0)
+                    }
+                };
+                var pageTitle = navParams.pageTitle || null;
+
+                if (!confirmResult) {
+                    // Pushes the previous history state back on top to set the correct url
+                    self._history.pushState(historyState, pageTitle, currentUrl);
+                } else {
+                    if (url !== currentUrl) {
+                        // Removes the window.onbeforeunload method so that the next page will not be affected
+                        window.onbeforeunload = null;
+
+                        context.executeAction(navigateAction, {type: TYPE_POPSTATE, url: url, params: (e.state && e.state.params)});
+                    }
                 }
             }
         };

tests/unit/lib/NavLink-test.js

@@ -196,6 +196,26 @@ describe('NavLink', function () {
                 done();
             }, 10);
         });
+
+        describe('window.onbeforeunload', function () {
+            beforeEach(function () {
+                global.window.confirm = function () { return false; };
+                global.window.onbeforeunload = function () {
+                    return 'this is a test';
+                };
+            });
+
+            it ('should not call context.executeAction when a user does not confirm the onbeforeunload method', function (done) {
+                var navParams = {a: 1, b: true};
+                var link = ReactTestUtils.renderIntoDocument(NavLink( {href:"/foo", navParams:navParams}, React.DOM.span(null, "bar")));
+                link.context = contextMock;
+                ReactTestUtils.Simulate.click(link.getDOMNode(), {button: 0});
+                window.setTimeout(function () {
+                    expect(testResult).to.deep.equal({});
+                    done();
+                }, 10);
+            });
+        });
     });
 
     describe('click type', function () {

tests/unit/lib/RouterMixin-test.js

@@ -163,6 +163,31 @@ describe ('RouterMixin', function () {
                 done();
             }, 10);
         });
+
+        describe('window.onbeforeunload', function () {
+            beforeEach(function () {
+                global.window.confirm = function () { return false; };
+                global.window.onbeforeunload = function () {
+                    return 'this is a test';
+                };
+            });
+
+            it ('should change the url back to the oldRoute if there is a window.onbeforeunload method', function (done) {
+                routerMixin.props = {context: contextMock, historyCreator: function() { return historyMock(); }};
+                var origPushState = window.history.pushState;
+                routerMixin.state = {
+                    route: {
+                        url: '/the_path_from_history'
+                    }
+                };
+                routerMixin.componentDidMount();
+                window.setTimeout(function() {
+                    expect(testResult.dispatch).to.equal(undefined, JSON.stringify(testResult.dispatch));
+                    done();
+                }, 10);
+            });
+        });
+
     });
 
     describe('componentWillUnmount()', function () {