iOS is tested for iPhoneOS, WatchOS, AppleTVOS, iPhoneSimulator, AppleTVSimulator and WatchSimulator. The testing uses Xcode 10 on OS X 10.13.

The first step sets environmental variables for the cross-compile using the Travis job. A typical job with variables is shown below.

The second step for Android is to set the environmental variables `ANDROID_NDK_ROOT` and `ANDROID_SDK_ROOT`. This is an important step because the NDK and SDK use the variables internally to locate their own tools. Also see [Recommended NDK Directory?](https://groups.google.com/forum/#!topic/android-ndk/qZjhOaynHXc) on the android-ndk mailing list. (Many folks miss this step, or use incorrect variables like `ANDROID_NDK_HOME` or `ANDROID_SDK_HOME`).

If you are working from a developer machine you probably already have the necessary tools installed. You should ensure `ANDROID_NDK_ROOT` and `ANDROID_SDK_ROOT` are set properly.

The second step installs tools needed for OpenSSL, Expat and Unbound. This step is handled in by the script `contrib/android/install_tools.sh`. The tools include curl, tar, zip, unzip and java.

The third step installs the NDK and SDK. This step is handled in by the script `contrib/android/install_ndk.sh`. The script uses `ANDROID_NDK_ROOT` and `ANDROID_SDK_ROOT` to place the NDK and SDK in the `$HOME` directory.

The fourth step sets the Android cross-compile environment using the script `contrib/android/setenv_android.sh`. The script is `sourced` so the variables in the script are available to the calling shell. The script sets variables like `CC`, `CXX`, `AS` and `AR`; sets `CFLAGS` and `CXXFLAGS`; sets a `sysroot` so Android headers and libraries are found; and adds the path to the toolchain to `PATH`.

`contrib/android/setenv_android.sh` knows which toolchain and architecture to select by inspecting environmental variables set by Travis for the job. In particular, the variables `ANDROID_CPU` and `ANDROID_API` tell `contrib/android/setenv_android.sh` which tools and libraries to select.

`ANDROID_PREFIX` is the value `$HOME/android$ANDROID_API-$ANDROID_CPU`. The libraries will be installed in `$HOME/android23-armv7a`, `$HOME/android23-aarch64`, etc. For Autotools projects, the appropriate `PKG_CONFIG_PATH` is exported. `PKG_CONFIG_PATH` is the userland equivalent to sysroot, and allows Autotools to find non-system headers and libraries for an architecture. Typical `PKG_CONFIG_PATH` are `$HOME/android23-armv7a/lib/pkgconfig` and `$HOME/android23-aarch64/lib/pkgconfig`.

Travis only smoke tests an Android build using a compile, link and install. The self tests are not run. TODO: figure out how to fire up an emulator, push the tests to the device and run them.

Travis tests iOS builds for the armv7a, armv7s and aarch64 architectures for iPhoneOS, AppleTVOS and WatchOS. iPhoneOS is tested using both 32-bit builds (iPhones) and 64-bit builds (iPads). Travis also tests compiles against the simulators. The builds are trickier than other builds for several reasons. The testing requires a cross-compile, and requires OpenSSL and Expat prerequisites. The iOS cross-compiles also require care to set the Autotools triplet, the OpenSSL triplet, the toolchain path, the tool variables, and the sysroot. The discussion below detail the steps of the iOS recipes.

The first step sets environmental variables for the cross-compile using the Travis job. A typical job with variables is shown below.

The second step installs tools needed for OpenSSL, Expat and Unbound. This step is handled in by the script `contrib/ios/install_tools.sh`. The tools include autotools, curl and perl. The installation happens at the `before_script:` stage of Travis.

The third step sets the iOS cross-compile environment using the script `contrib/ios/setenv_ios.sh`. The script is `sourced` so the variables in the script are available to the calling shell. The script sets variables like `CC`, `CXX`, `AS` and `AR`; sets `CFLAGS` and `CXXFLAGS`; sets a `sysroot` so iOS headers and libraries are found; and adds the path to the toolchain to `PATH`.

`contrib/ios/setenv_ios.sh` knows which toolchain and architecture to select by inspecting environmental variables set by Travis for the job. In particular, the variables `IOS_SDK` and `IOS_CPU` tell `contrib/ios/setenv_ios.sh` which tools and libraries to select.

The `contrib/ios/setenv_ios.sh` script specifies the tools to use during the cross-compile. For Apple SDKs, the tool names are the same as a desktop. There are no special prefixes for the mobile tools.

The fourth step builds OpenSSL and Expat. OpenSSL and Expat are built for iOS using the scripts `contrib/ios/install_openssl.sh` and `contrib/ios/install_expat.sh`. The scripts download, configure and install the latest release version of the libraries. The libraries are configured with `--prefix="$IOS_PREFIX"` so the headers are placed in `$IOS_PREFIX/include` directory, and the libraries are placed in the `$IOS_PREFIX/lib` directory.

`IOS_PREFIX` is the value `$HOME/$IOS_SDK-$IOS_CPU`. The scheme handles both iOS SDKs and cpu architectures so the pair recieves a unique installation directory. The libraries will be installed in `$HOME/iPhoneOS-armv7s`, `$HOME/iPhoneOS-arm64`, `$HOME/iPhoneSimulator-i386`, etc. For Autotools projects, the appropriate `PKG_CONFIG_PATH` is exported.

`PKG_CONFIG_PATH` is an important variable. It is the userland equivalent to sysroot, and allows Autotools to find non-system headers and libraries for an architecture. Typical `PKG_CONFIG_PATH` are `$HOME/iPhoneOS-armv7s/lib/pkgconfig` and `$HOME/iPhoneOS-arm64/lib/pkgconfig`.

Finally, once OpenSSL and Expat are built, then the Travis script configures and builds Unbound. The full recipe looks as follows.

Travis only smoke tests an iOS build using a compile, link and install. The self tests are not run. TODO: figure out how to fire up an simulator, push the tests to the device and run them.