Mobile Forge

This is a forge-like environment that can be used to build wheels for mobile platforms. It is currently only tested for iOS, but in theory, it should also be usable for Android. Contributions to verify Android support, tvOS and watchOS support, and to add more package recipes, are enthusiastically encouraged.

Usage

This repo contains an activation script that will configure your environment so it's ready to use. To set up a build environment:

1. Clone this repository:

   $ git clone https://github.com/beeware/mobile-forge.git
   $ cd mobile-forge
   

2. Run the script for the Python version you want to use, providing the support revision:

   $ source ./setup-iOS.sh 3.11
   

Running this script will create a Python virtual environment, install Mobile Forge and some other required tools, and provide some hints at forge commands you can run.

If a virtual environment already exists, it will be activated, and the same hints displayed.

lru-dict is a good first package to try compiling:

(venv3.11) $ forge iOS lru-dict

Or, to build a wheel for a single architecture:

(venv3.11) $ forge iphonesimulator:12.0:arm64 lru-dict

Once this command completes, there should be a wheel for each platform in the dist folder. A log for each successful build will be in the logs folder; a log for each unsuccessful build (if there are any) will be in the errors folder.

Local support package builds

By default, the Mobile Forge setup script will download a support revision and use the binaries in the downloaded package. However, you can also use a local build of the support package.

After cloning and building Python-Apple-support, set the PYTHON_APPLE_SUPPORT environment variable to the root of the Python-Apple-support checkout. Then run the setup-iOS.sh script to configure your environment.

Specific support package builds

The Mobile Forge setup script will download a support package for any supported Python version. The version that is downloaded is hard-coded in the setup script. To use a specific revision rather than the default, add the revision number as an additional argument to the setup script. For example, to use revision 4 of the 3.11 support package, run:

$ source ./setup-iOS.sh 3.11 4

The special snowflakes

Mobile Forge is trying to support multiple packages, building on multiple Python versions, for multiple architectures; and some of those Python versions were released before the release of ARM64 macOS hardware. As a result, some versions of some packages have some quirks that must be taken into account.

Pandas

Pandas uses a meta-package named oldest-supported-numpy to ensure ABI compatibility during compilation. However, this can install a different version of numpy, depending on the platform. This is especially problematic for Python 3.9, because the minimum supported version for Python 3.9 on ARM64 is different to the version that is installed for x86_64. Mobile-forge produces a replacement oldest-supported-numpy package, tagged as version 2999.1.1, which ensures that consistent versions are available for build purposes; however, this wheel should not be published.

Cryptography

Cryptography currently builds a very old version (3.4.8). This is the last version that could be built without a Rust compiler.

What now?

To include these wheels in a test project, you can add the dist folder as a links source in your requires definition in your Briefcase pyproject.toml. For example, the following will install the lru-dict wheels you've just compiled:

requires = [
    "--find-links", "/path/to/mobile-forge/dist",
    "lru-dict",
]

Adding your own packages

If there's a package that you want that doesn't have an existing recipe, you can add a recipe for that package.

Create a directory in recipes. The name of the directory must be in PyPI normalized form (PEP 503). Alternatively, you can create this directory somewhere else, and pass its path when calling forge.

Inside the recipe directory, add the following files.

• A meta.yaml file. This supports a subset of Conda syntax, defined in meta-schema.yaml.

• A test.py file (or test package), to run on a target installation. This should contain a pytest suite which imports the package and does some basic checks.

• Optionally, one or more patch files in a folder named patches. These patches will be applied when the source code is unpacked for a given platform.

• For non-Python packages, a build.sh script. This is the script that will be executed in the build environment build the package. This script should invoke any configure, make, or any other compilation steps needed to build the package. This script will be executed in an environment that defines the following environment variables:

  • AR - the AR value used to compile the host Python, as determined from sysconfig
  • CC - the CC value used to compile the host Python, as determined from sysconfig.
  • CFLAGS - the CFLAGS value used to compile the host Python, as determined from sysconfig, augmented with the include paths for the SDK, and opt/include in the host environment's site-packages.
  • LDFLAGS - the CFLAGS value used to compile the host Python, as determined from sysconfig, augmented with the library paths for the SDK, and opt/lib in the host environment's site-packages.
  • CPU_COUNT - The number of CPUs that are available, as determined by multiprocessing.cpu_count()
  • HOST_TRIPLET - the GCC compiler triplet for the host platform (e.g., aarch64-apple-ios12.0-simulator)
  • BUILD_TRIPLET - the GCC compiler triplet for the build platform (e.g., aarch64-apple-darwin)
  • PREFIX - a location where the compiled package can be installed in preparation for packaging.

  This script should install the package into $PREFIX. Mobile Forge will package any content installed into $PREFIX into a "wheel" that can be installed as a host requirement.

Python-based projects

All Python projects are compiled using python -m build, using a clean crossenv virtual environment for each platform of a package. Any PEP518 build requirements will be included in both the host and build environments.

If you're lucky, all you'll need to do is define a meta.yaml that describes the package name and version: e.g.,:

package:
  name: blis
  version: 0.4.1

If this doesn't result in a successful build, it will likely be for one of the following reasons:

1. The build process has a dependency on a system library. For example, Pillow has a dependency on libjpeg. libjpeg isn't available on PyPI; but it is possible to build a "wheel" for libjpeg, so it can be specified as a requirement.

   A non-python "wheel" is constructed by compiling the package for your target platform, then installing it into a folder named opt. As a result of this "install", you'll usually end up with an opt/include and opt/lib folder; Mobile Forge will then wrap up this opt folder in a wheel, along with Python wheel metadata.

   When this "wheel" is specified as a host requirement, the "wheel" will be unpacked into the site packages folder of your cross-compilation host environment. This path the include and lib paths will be automatically included in the CFLAGS/LDFLAGS environment variables when the Python build is executed.

2. The build process has a dependency on external tooling. Mobile Forge will configure a C and C++ compiler using the same configuration that was used to compile the support libraries; however a package may require addition build tooling (e.g., a Fortran compiler) to complete the build. If this is the case, you'll need to find a version of the tool that can target mobile platforms, and work out how to modify the build process to apply any necessary compiler flags.

3. The build script has platform-specific logic. For example, if the setup.py file contain an if sys.platform == ... clauses, it is unlikely that a mobile platform will trigger the right logic.

If you need to make any alterations to a project's source code for a build to succeed, you can provide those patches by putting them in one or more files in a folder named patches in the recipe folder. These patches will be applied once the source code has been unpacked.

Configure-based projects

If the project includes a configure script, you will likely need to provide a patch for config.sub. config.sub is the tools used by configure to identify the architecture and machine type; however, it doesn't currently recognize the host triples used by Apple. If you get the error:

checking host system type... Invalid configuration `arm64-apple-ios': machine `arm64-apple' not recognized
configure: error: /bin/sh config/config.sub arm64-apple-ios failed

you will need to patch config.sub. There are several examples of patched config.sub scripts in the packages contained in this repository, and in the Python-Apple-support project; it is quite possible one of those patches can be used for the library you are trying to compile. The config.sub script has a datestamp at the top of the file; that can be used to identify which patch you will need.

Community

Mobile Forge is part of the BeeWare suite. You can talk to the community through:

• @beeware@fosstodon.org on Mastodon
• Discord
• The Mobile Forge Github Discussions forum

We foster a welcoming and respectful community as described in our BeeWare Community Code of Conduct.

Contributing

If you experience problems with Mobile Forge, log them on GitHub. If you want to contribute code, please fork the code and submit a pull request.

Acknowledgements

This project draws significantly on the implementation and knowledge developed in the Chaquopy package builder. Although this is largely a "clean room" reimplementation of that project, many details from that project have been used in the development of this one.