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Getting Started with Swift on Android

The Swift stdlib can be compiled for Android armv7 and aarch64 targets, which makes it possible to execute Swift code on a mobile device running Android. This guide explains:

  1. How to run a simple "Hello, world" program on your Android device.
  2. How to run the Swift test suite on an Android device.

If you encounter any problems following the instructions below, please file a bug using https://bugs.swift.org/.

FAQ

Let's answer a few frequently asked questions right off the bat:

Does this mean I can write Android applications in Swift?

No. Although the Swift compiler is capable of compiling Swift code that runs on an Android device, it takes a lot more than just the Swift stdlib to write an app. You'd need some sort of framework to build a user interface for your application, which the Swift stdlib does not provide.

Alternatively, one could theoretically call into Java interfaces from Swift, but unlike as with Objective-C, the Swift compiler does nothing to facilitate Swift-to-Java bridging.

Prerequisites

To follow along with this guide, you'll need:

  1. A Linux environment capable of building Swift from source, specifically Ubuntu 18.04 or Ubuntu 16.04. Before attempting to build for Android, please make sure you are able to build for Linux by following the instructions in the Swift project README.
  2. The latest version of the Android NDK (r21 at the time of this writing), available to download here: https://developer.android.com/ndk/downloads/index.html.
  3. An Android device with remote debugging enabled. We require remote debugging in order to deploy built stdlib products to the device. You may turn on remote debugging by following the official instructions: https://developer.chrome.com/devtools/docs/remote-debugging.

"Hello, world" on Android

1. Downloading (or building) the Swift Android stdlib dependencies

You may have noticed that, in order to build the Swift stdlib for Linux, you needed to apt-get install libicu-dev icu-devtools. Similarly, building the Swift stdlib for Android requires the libiconv and libicu libraries. However, you'll need versions of these libraries that work on Android devices.

The steps are as follows:

  1. Ensure you have curl, autoconf, automake, libtool, and git installed.
  2. Clone the SwiftAndroid/libiconv-libicu-android project. From the command-line, run the following command: git clone https://github.com/SwiftAndroid/libiconv-libicu-android.git.
  3. From the command-line, run which ndk-build. Confirm that the path to the ndk-build executable in the Android NDK you downloaded is displayed. If not, you may need to add the Android NDK directory to your PATH.
  4. Change directories into libiconv-libicu-android: cd libiconv-libicu-android
  5. Run the Swift build script: ./build-swift.sh
  6. Confirm that the various libicuXYZswift.so libraries are located in the armeabi-v7a directory.

2. Building the Swift stdlib for Android

Enter your Swift directory, then run the build script, passing paths to the Android NDK, as well as the directories that contain the libicuucswift.so and libicui18nswift.so you downloaded or built in step one:

$ ARM_DIR=path/to/libicu-libiconv-android
$ NDK_PATH=path/to/android-ndk21
$ utils/build-script \
    -R \                                       # Build in ReleaseAssert mode.
    --android \                                # Build for Android.
    --android-ndk $NDK_PATH \                  # Path to an Android NDK.
    --android-arch armv7 \                     # Optionally specify Android architecture, alternately aarch64
    --android-api-level 21 \                   # The Android API level to target. Swift only supports 21 or greater.
    --android-icu-uc ${ARM_DIR}/libicuucswift.so \
    --android-icu-uc-include ${ARM_DIR}/icu/source/common \
    --android-icu-i18n ${ARM_DIR}/libicui18nswift.so \
    --android-icu-i18n-include ${ARM_DIR}/icu/source/i18n \
    --android-icu-data ${ARM_DIR}/libicudataswift.so

3. Compiling hello.swift to run on an Android device

Create a simple Swift file named hello.swift:

print("Hello, Android")

Then use the built Swift compiler from the previous step to compile a Swift source file, targeting Android:

$ NDK_PATH="path/to/android-ndk21"
$ build/Ninja-ReleaseAssert/swift-linux-x86_64/bin/swiftc \     # The Swift compiler built in the previous step.
                                                                # The location of the tools used to build Android binaries
    -tools-directory ${NDK_PATH}/toolchains/llvm/prebuilt/linux-x86_64/bin/ \
    -target armv7a-none-linux-androideabi \                     # Targeting android-armv7, and supply the path to libgcc.
    -L ${NDK_PATH}/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/lib/gcc/arm-linux-androideabi/4.9.x/armv7-a \
    -sdk ${NDK_PATH}/platforms/android-21/arch-arm \            # Use the same architecture and API version as you used to build the stdlib in the previous step.
    hello.swift

This should produce a hello executable in the directory you executed the command. If you attempt to run this executable using your Linux environment, you'll see the following error:

cannot execute binary file: Exec format error

This is exactly the error we want: the executable is built to run on an Android device--it does not run on Linux. Next, let's deploy it to an Android device in order to execute it.

4. Deploying the build products to the device

You can use the adb push command to copy build products from your Linux environment to your Android device. If you haven't already installed adb, you may do so via apt-get:

$ sudo apt-get install android-tools-adb

Once you have adb installed, verify your device is connected and is listed when you run the adb devices command - currently this example works only in devices / emulators with at least Android 7.0, API 24 - then run the following commands to copy the Swift Android stdlib:

$ adb push build/Ninja-ReleaseAssert/swift-linux-x86_64/lib/swift/android/libswiftCore.so /data/local/tmp
$ adb push build/Ninja-ReleaseAssert/swift-linux-x86_64/lib/swift/android/libswiftGlibc.so /data/local/tmp
$ adb push build/Ninja-ReleaseAssert/swift-linux-x86_64/lib/swift/android/libswiftSwiftOnoneSupport.so /data/local/tmp
$ adb push build/Ninja-ReleaseAssert/swift-linux-x86_64/lib/swift/android/libswiftRemoteMirror.so /data/local/tmp

You will also need to push the icu libraries:

adb push /path/to/libicu-android/armeabi-v7a/libicudataswift.so /data/local/tmp
adb push /path/to/libicu-android/armeabi-v7a/libicui18nswift.so /data/local/tmp
adb push /path/to/libicu-android/armeabi-v7a/libicuucswift.so /data/local/tmp

In addition, you'll also need to copy the Android NDK's libc++:

$ adb push /path/to/android-ndk-r21/sources/cxx-stl/llvm-libc++/libs/armeabi-v7a/libc++_shared.so /data/local/tmp

Finally, you'll need to copy the hello executable you built in the previous step:

$ adb push hello /data/local/tmp

5. Running "Hello, world" on your Android device

You can use the adb shell command to execute the hello executable on the Android device:

$ adb shell LD_LIBRARY_PATH=/data/local/tmp /data/local/tmp/hello

You should see the following output:

Hello, Android

Congratulations! You've just run your first Swift program on Android.

Running the Swift test suite hosted on an Android device

When running the test suite, build products are automatically pushed to your device. As in part four, you'll need to connect your Android device via USB:

  1. Connect your Android device to your computer via USB. Ensure that remote debugging is enabled for that device by following the official instructions: https://developer.chrome.com/devtools/docs/remote-debugging.
  2. Confirm the device is connected by running adb devices. You should see your device listed.
  3. Run the tests using the build script:
$ utils/build-script \
  -R \                                           # Build in ReleaseAssert mode.
  -T --host-test \                               # Run all tests, including on the Android host.
  --android \                                    # Build for Android.
  --android-ndk ~/android-ndk-r21 \              # Path to an Android NDK.
  --android-arch armv7 \                         # Optionally specify Android architecture, alternately aarch64
  --android-ndk-version 21 \
  --android-icu-uc ~/libicu-android/armeabi-v7a/libicuuc.so \
  --android-icu-uc-include ~/libicu-android/armeabi-v7a/icu/source/common \
  --android-icu-i18n ~/libicu-android/armeabi-v7a/libicui18n.so \
  --android-icu-i18n-include ~/libicu-android/armeabi-v7a/icu/source/i18n/ \
  --android-icu-data ~/libicu-android/armeabi-v7a/libicudata.so