custom back button handler

Summary:sometimes it is nessesary to handle back button
specifically for component, by changing its state.
 For ex. exit from edit mode.
Closes https://github.com/facebook/react-native/pull/5062

Differential Revision: D3084590

Pulled By: ericvicenti

fb-gh-sync-id: 13a4ea1d64ce725daa5d3af0d629a0c132a3f3d5
shipit-source-id: 13a4ea1d64ce725daa5d3af0d629a0c132a3f3d5

Examples/UIExplorer/BackAndroidExample.js

@@ -0,0 +1,66 @@
+/**
+ * The examples provided by Facebook are for non-commercial testing and
+ * evaluation purposes only.
+ *
+ * Facebook reserves all rights not expressly granted.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+ * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NON INFRINGEMENT. IN NO EVENT SHALL
+ * FACEBOOK BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ *
+ */
+
+'use strict';
+
+var React = require('react-native');
+var {
+  Text,
+  View,
+  BackAndroid,
+} = React;
+
+var BackAndroidExample = React.createClass({
+
+  getInitialState: function() {
+    return {
+      defaultBackHandler: false
+    };
+  },
+
+  componentDidMount: function() {
+    BackAndroid.addEventListener('hardwareBackPress', this._handleBackButton);
+  },
+
+  componentWillUnmount: function() {
+    BackAndroid.removeEventListener('hardwareBackPress', this._handleBackButton);
+  },
+
+  _handleBackButton: function() {
+    if (!this.state.defaultBackHandler) {
+      this.setState({defaultBackHandler: true});
+      return true;
+    }
+
+    return false;
+  },
+
+  render: function() {
+    return (
+      <View>
+        <Text>{this.state.defaultBackHandler ? 'Ok, I\'ll quit.' : 'I won\'t quit.'}</Text>
+      </View>
+    );
+  },
+});
+
+exports.title = 'BackAndroid';
+exports.description = 'Custom back button handler';
+exports.examples = [
+  {
+    title: 'Custom back button handler',
+    render(): ReactElement { return <BackAndroidExample />; }
+  }
+];