Add back android support

Author: charpeni

README.md

@@ -52,9 +52,9 @@ class MyApp extends React.Component {
   }
 
   render() {
-    return <Router 
+    return <Router
       firstRoute={firstRoute}         
-      headerStyle={this.styles.header} 
+      headerStyle={this.styles.header}
     />
   }
 }
@@ -104,6 +104,7 @@ The **`<Router \>`** object used to initialize the navigation can take the follo
 - `rightCorner`: If you have the same occuring action buttons on the right side of your navigation bar (like the Twitter "Compose"-button), you can specify a component for that view.
 - `customAction`: A special callback prop for your action buttons (this can be handy for triggering a side menu for example). The action gets triggered from your custom `leftCorner` or `rightCorner` components by calling `this.props.customAction("someActionName")` from them. It is then picked up like this: `<Router customAction={this.doSomething} />`.
 - `hideNavigationBar`: Hide the navigation bar, always
+- `handleBackAndroid` (Boolean value): Apply a listener to the native back button on Android. On click, it will go to the previous route until it reach the first scene, then it will exit the app.
 
 The **`this.props.toRoute()`** callback prop takes one parameter (a JavaScript object) which can have the following keys:
 - `name`: The name of your route, which will be shown as the title of the navigation bar unless it is changed.

components/NavBarContainer.js

@@ -6,6 +6,8 @@ const {
   StyleSheet,
   View,
   Component,
+  Platform,
+  BackAndroid,
 } = React;
 
 class NavBarContainer extends Component {
@@ -22,6 +24,19 @@ class NavBarContainer extends Component {
     };
   }
 
+  componentDidMount() {
+    if (Platform.OS === 'android' && !!this.props.handleBackAndroid) {
+      BackAndroid.addEventListener('hardwareBackPress', () => {
+        if (this.props.currentRoute.index > 0) {
+          this.goBack();
+          return true;
+        }
+
+        return false;
+      });
+    }
+  }
+
   componentWillReceiveProps(newProps) {
     if (this.props && this.props.currentRoute.index !== newProps.currentRoute.index) {
       this.setState({
@@ -30,6 +45,12 @@ class NavBarContainer extends Component {
     }
   }
 
+  componentWillUnmount() {
+    if (Platform.OS === 'android' && !!this.props.handleBackAndroid) {
+      BackAndroid.removeEventListener('hardwareBackPress');
+    }
+  }
+
   goBack() {
     this.props.toBack(this.props.navigator);
   }

index.js

@@ -185,6 +185,7 @@ class Router extends React.Component {
           rightProps={this.state.rightProps}
           titleProps={this.state.titleProps}
           customAction={this.customAction}
+          handleBackAndroid={this.props.handleBackAndroid}
         />
       );
     }