修复NavBar在Android上的一些兼容问题。reviewed by yuliang。

@@ -35,6 +35,7 @@ import {
   TouchableOpacity,
   View,
   Dimensions,
+  BackAndroid,
 } from 'react-native';
 import {Actions} from 'react-native-router-flux';
 import _drawerImage from '../images/menu_burger.png';
@@ -50,7 +51,7 @@ const styles = StyleSheet.create({
     fontWeight: '500',
     color: '#0A0A0A',
     position: 'absolute',
-    top: Platform.OS === 'ios' || Platform.Version > 19 ? 20 : 0,
+    top: Platform.OS === 'ios' ? 20 : 5,
     left: 0,
     right: 0,
     backgroundColor: 'transparent',
@@ -59,7 +60,7 @@ const styles = StyleSheet.create({
     backgroundColor: '#EFEFF2',
     paddingTop: 0,
     top: 0,
-    height: Platform.OS === 'ios' || Platform.Version > 19 ? 64 : 44,
+    height: Platform.OS === 'ios' ? 64 : 50,
     right: 0,
     left: 0,
     borderBottomWidth: 0.5,
@@ -175,6 +176,27 @@ class NavBar extends React.Component {
     this.renderTitle = this.renderTitle.bind(this);
   }
 
+  componentWillMount() {
+      if (Platform.OS === 'android') {
+          BackAndroid.addEventListener('hardwareBackPress', this.onBackAndroid);
+      }
+  }
+
+  componentWillUnmount() {
+      if (Platform.OS === 'android') {
+          BackAndroid.removeEventListener('hardwareBackPress', this.onBackAndroid);
+      }
+  }
+
+  onBackAndroid = () => {
+      let sceneKey = this.props.navigationState.sceneKey;
+      if (CONFIG.sceneKey.root === sceneKey) {
+          Actions.pop();
+          return true;
+      }
+      return false;
+  }
+
   renderBackButton() {
     const state = this.props.navigationState;
     const childState = state.children[state.index];
@@ -407,13 +429,13 @@ class NavBar extends React.Component {
     if (selected.hideNavBar) {
       return null;
     }
-    
+
     while (selected.hasOwnProperty('children')) {
       state = selected;
       selected = selected.children[selected.index];
     }
 
-     
+
 
     const renderLeftButton = selected.renderLeftButton ||
       selected.component.renderLeftButton ||
@@ -437,10 +459,11 @@ class NavBar extends React.Component {
           this.props.navigationBarStyle,
           state.navigationBarStyle,
           selected.navigationBarStyle,
+          CONFIG.sceneKey.user === sceneKey && styles.imageMy,
         ]}
       >
-        <Image 
-          source={CONFIG.sceneKey.user === sceneKey ? require('../images/mybg.png') : require('../images/navbar.png')} 
+        <Image
+          source={CONFIG.sceneKey.user === sceneKey ? require('../images/mybg.png') : require('../images/navbar.png')}
           style={[styles.image, CONFIG.sceneKey.user === sceneKey && styles.imageMy]}
         >
           {renderTitle ? renderTitle(selected) : state.children.map(this.renderTitle, this)}