Feat(BackAndroid): Add handler for Back Key in Android (#1049)

* Feat(TabBar): add pressOpacity for TabBar

* Feat(Doc): add doc for pressOpacity

* Feat(BackAndroid): Add handler for Back Key in Android

* Fix(Lint): fix lint error

* Fix(Lint): fix lint error

docs/API_CONFIGURATION.md

@@ -22,6 +22,9 @@
 | children | | required (if no scenes property passed)| Scene root element |
 | scenes | `object` | optional | scenes for Router created with Actions.create. This will allow to create all actions BEFORE React processing. If you don't need it you may pass Scene root element as children |
 | getSceneStyle | `function` | optional | Optionally override the styles for NavigationCard's Animated.View rendering the scene. |
+| backAndroidHandler | `function` | optional | Optionally override the handler for `BackAndroid`, return `true` to stay in the app or return `false` to exit the app. Default handler will pop a scene and exit the app at last when the back key is pressed on Android. |
+| onBackAndroid | `function` | optional | Get called after back key is pressed and a scene is poped, won't affect the default behavior. |
+| onExitApp | `function` | optional | Optionally override the default action after back key is pressed on root scene. Return `true` to stay, or return `false` to exit the app. |
 
 ## Scene:
 

src/Reducer.js

@@ -13,6 +13,7 @@ import * as ActionConst from './ActionConst';
 import { ActionMap } from './Actions';
 import { assert } from './Util';
 import { getInitialState } from './State';
+import { Platform } from 'react-native';
 
 // WARN: it is not working correct. rewrite it.
 function checkPropertiesEqual(action, lastAction) {
@@ -90,6 +91,11 @@ function inject(state, action, props, scenes) {
     case ActionConst.BACK:
     case ActionConst.BACK_ACTION: {
       assert(!state.tabs, 'pop() operation cannot be run on tab bar (tabs=true)');
+
+      if (Platform.OS === 'android') {
+        assert(state.index > 0, 'You are already in the root scene.');
+      }
+
       if (state.index === 0) {
         return state;
       }

src/Router.js

@@ -10,6 +10,7 @@ import React, {
   Component,
   PropTypes,
 } from 'react';
+import { BackAndroid } from 'react-native';
 import NavigationExperimental from 'react-native-experimental-navigation';
 
 import Actions, { ActionMap } from './Actions';
@@ -25,6 +26,9 @@ const {
 
 const propTypes = {
   dispatch: PropTypes.func,
+  backAndroidHandler: PropTypes.func,
+  onBackAndroid: PropTypes.func,
+  onExitApp: PropTypes.func,
 };
 
 class Router extends Component {
@@ -34,16 +38,49 @@ class Router extends Component {
     this.state = {};
     this.renderNavigation = this.renderNavigation.bind(this);
     this.handleProps = this.handleProps.bind(this);
+    this.handleBackAndroid = this.handleBackAndroid.bind(this);
   }
 
   componentDidMount() {
     this.handleProps(this.props);
+
+    BackAndroid.addEventListener('hardwareBackPress', this.handleBackAndroid);
   }
 
   componentWillReceiveProps(props) {
     this.handleProps(props);
   }
 
+  componentWillUnmount() {
+    BackAndroid.removeEventListener('hardwareBackPress', this.handleBackAndroid);
+  }
+
+  handleBackAndroid() {
+    const {
+      backAndroidHandler,
+      onBackAndroid,
+      onExitApp,
+    } = this.props;
+    // optional for customizing handler
+    if (backAndroidHandler) {
+      return backAndroidHandler();
+    }
+
+    try {
+      Actions.pop();
+      if (onBackAndroid) {
+        onBackAndroid();
+      }
+      return true;
+    } catch (err) {
+      if (onExitApp) {
+        return onExitApp();
+      }
+
+      return false;
+    }
+  }
+
   handleProps(props) {
     let scenesMap;