Merge the Objective-C and Swift iOS existing-app guides

Use `<Tabs>` to display the appropriate-language content.

Author: mhjacobson

docs/_integration-with-exisiting-apps-java.md renamed to docs/_integration-with-exisiting-apps-android.md

@@ -244,14 +244,61 @@ When you build a React Native application, you use the [Metro bundler][metro] to
 
 We will, for debugging purposes, log that the event handler was invoked. Then, we will create a string with the location of our React Native code that exists inside the `index.bundle`. Finally, we will create the main `RCTRootView`. Notice how we provide `RNHighScores` as the `moduleName` that we created [above](#the-react-native-component) when writing the code for our React Native component.
 
+<Tabs groupId="language" defaultValue="objective-c" values={[ {label: 'Objective-C', value: 'objective-c'}, {label: 'Swift', value: 'swift'}, ]}>
+<TabItem value="objective-c">
+
+First `import` the `RCTRootView` header.
+
+```objectivec
+#import <React/RCTRootView.h>
+```
+
+</TabItem>
+<TabItem value="swift">
+
 First `import` the `React` module.
 
 ```swift
 import React
 ```
 
+</TabItem>
+</Tabs>
+
 > The `initialProperties` are here for illustration purposes so we have some data for our high score screen. In our React Native component, we will use `this.props` to get access to that data.
 
+<Tabs groupId="language" defaultValue="objective-c" values={[ {label: 'Objective-C', value: 'objective-c'}, {label: 'Swift', value: 'swift'}, ]}>
+<TabItem value="objective-c">
+
+```objectivec
+- (IBAction)highScoreButtonPressed:(id)sender {
+    NSURL *jsCodeLocation = [NSURL URLWithString:@"http://localhost:8081/index.bundle?platform=ios"];
+    NSDictionary *mockData = @{
+        @"scores" : @[
+            @{ @"name" : @"Alex", @"value": @"42" },
+            @{ @"name" : @"Joel", @"value": @"10" },
+        ],
+    };
+
+    RCTRootView *rootView =
+        [[RCTRootView alloc] initWithBundleURL:jsCodeLocation
+                                    moduleName:@"RNHighScores"
+                             initialProperties:mockData
+                                 launchOptions:nil];
+
+    UIViewController *vc = [[UIViewController alloc] init];
+    vc.view = rootView;
+    [self presentViewController:vc animated:YES completion:nil];
+}
+```
+
+> Note that `-[RCTRootView initWithBundleURL:...]` starts up a new JavaScript VM. To save resources and simplify the communication between React Native views in different parts of your native app, you can have multiple views powered by React Native that are associated with a single JS runtime. To do that, instead of using `-[RCTRootView initWithBundleURL:...]`, use [`-[RCTBridge initWithBundleURL:...]`](https://github.com/facebook/react-native/blob/master/React/Base/RCTBridge.h#L93) to create a bridge, and then use `-[RCTRootView initWithBridge:...]`.
+
+> When moving your app to production, the `NSURL` can point to a pre-bundled file on disk via something like `[[NSBundle mainBundle] URLForResource:@"main" withExtension:@"jsbundle"];`. You can use the `react-native-xcode.sh` script in `node_modules/react-native/scripts/` to generate that pre-bundled file.
+
+</TabItem>
+<TabItem value="swift">
+
 ```swift
 @IBAction func highScoreButtonTapped(sender: UIButton) {
     let jsCodeLocation = URL(string: "http://localhost:8081/index.bundle?platform=ios")
@@ -279,6 +326,9 @@ import React
 
 > When moving your app to production, the `URL` can point to a pre-bundled file on disk via something like `Bundle.main.url(forResource: "main", withExtension: "jsbundle")`. You can use the `react-native-xcode.sh` script in `node_modules/react-native/scripts/` to generate that pre-bundled file.
 
+</TabItem>
+</Tabs>
+
 ##### 3. Wire Up
 
 Wire up the new link in the main menu to the newly added event handler method.