# Tutorial - Build an iOS App

Like the [Android app](android-app.md) you built in the previous step, the iOS
app is a simple mobile app that communicates with the
[backend server](backend-server.md).

Here, you'll do the following:

*   Review the source files for the app
*   Create a `BUILD` file
*   Run the build
*   Find the build outputs
*   Run the app

Note that, unlike with the Android app, you don't have to modify your
`WORKSPACE` file to add iOS-specific external dependencies.

If you're following the steps in this tutorial on Mac OS X, you can go ahead
and build the sample iOS app as described below. If you are on Linux, skip ahead
to the [next step](backend-server.md).

## Review the source files

Let's take a look at the source files for the app. These are located in
`$WORKSPACE/ios-app/UrlGet`. Again, you're just looking at these files now to
become familiar with the structure of the app. You don't have to edit any of the
source files to complete this tutorial.

## Create a BUILD file

At a command-line prompt, open your new `BUILD` file for editing:

```bash
$ vi $WORKSPACE/ios-app/BUILD
```

## Add an objc_library rule

Bazel provides several build rules that you can use to build an app for the
iOS platform. For this tutorial, you'll first use the
[`objc_library`](/docs/build-encyclopedia.html#objc_library) rule to tell Bazel
how to build an
[static library](https://developer.apple.com/library/ios/technotes/iOSStaticLibraries/Introduction.html)
from the app source code and Xib files. Then you'll use the
`objc_binary` rule to tell it how to bundle the iOS application. (Note that
this is a minimal use case of the Objective-C rules in Bazel. For example, you
have to use the `ios_application` rule to build multi-architecture iOS
apps.)

Add the following to your `BUILD` file:

```python
objc_library(
    name = "UrlGetClasses",
    srcs = [
        "UrlGet/AppDelegate.m",
        "UrlGet/UrlGetViewController.m",
    ],
    hdrs = glob(["UrlGet/*.h"]),
    xibs = ["UrlGet/UrlGetViewController.xib"],
)
```

Note the name of the rule, `UrlGetClasses`.

## Add an objc_binary rule

The [`objc_binary`](/docs/build-encyclopedia.html#objc_binary) rule creates the
bundled `.ipa` archive file for the application and also generates an Xcode
project file.

Add the following to your `BUILD` file:

```python
objc_binary(
    name = "ios-app",
    srcs = [
        "UrlGet/main.m",
    ],
    deps = [
        ":UrlGetClasses",
    ],
    infoplist = "UrlGet/UrlGet-Info.plist",
)
```

Now, save and close the file. You can compare your `BUILD` file to the
[completed example](https://github.com/bazelbuild/examples/blob/master/tutorial/ios-app/BUILD)
in the `master` branch of the GitHub repo.

Again, note how the `deps` attribute references the output of the
`UrlGetClasses` rule you added to the `BUILD` file above.

## Run the build

Make sure that your current working directory is inside your Bazel workspace:

```bash
$ cd $WORKSPACE
```

Now, enter the following to build the sample app:

```bash
$ bazel build //ios-app:ios-app
```

Bazel now launches and builds the sample app. During the build process, its
output will appear similar to the following:

```bash
INFO: Found 1 target...
Target //ios-app:ios-app up-to-date:
  bazel-bin/ios-app/ios-app_bin
  bazel-bin/ios-app/ios-app.ipa
  bazel-bin/ios-app/ios-app.xcodeproj/project.pbxproj
INFO: Elapsed time: 3.765s, Critical Path: 3.44s
```

## Find the build outputs

The `.ipa` file and other outputs are located in the
`$WORKSPACE/bazel-bin/ios-app` directory.

## Run the app

You can now run the app from Xcode using the iOS Simulator. To run the app,
open the project directory `$WORKSPACE/bazel-bin/ios-app/ios-app.xcodeproj` in
Xcode, choose an iOS Simulator as the runtime scheme and then click the **Run**
button.

**Note:** If you change anything about the project file set in Xcode (for
example, if you add or remove a file, or add or change a dependency), you must
rebuild the app using Bazel and then re-open the project.

## What's next

The next step is to build a [backend server](backend-server.md) for the two
mobile apps you built in this tutorial.