Merge pull request react-navigation#285 from eriveltonelias/source

add how determine if the drawer is open or closed

Author: brentvatne

docs/drawer-based-navigation.md

@@ -84,3 +84,10 @@ this.props.navigation.dispatch(DrawerActions.openDrawer());
 this.props.navigation.dispatch(DrawerActions.closeDrawer());
 this.props.navigation.dispatch(DrawerActions.toggleDrawer());
 ```
+
+If you would like to determine if drawer is open or closed, you can do the following:
+
+```js
+const parent = this.props.navigation.dangerouslyGetParent();
+const isDrawerOpen = parent && parent.state && parent.state.isDrawerOpen;
+```