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.. _`cross_building_flatpaks`:

Cross-Building Flatpaks

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.. contents::

Since the Librem 5 phone is based on a processor architecture (aarch64) that is
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different to the architectures used by most development workstations, the way
that applications are built for the phone is a bit different to the way that
applications are built and tested on a workstation.
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Installing Runtimes and SDKs

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The first difference is that we need to explicitly install the aarch64 versions
of the SDK and runtime used to build flatpaks.

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For example, when using the runtimes from the `Nightly GNOME Apps`_ repository
(``gnome-nightly``), we need to install versions of those for the aarch64
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   flatpak install gnome-nightly org.gnome.Platform/aarch64 org.gnome.Sdk/aarch64

This ensures that the resources needed for applications on that architecture
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are available when they are built to target it.
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Installing Emulators

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As mentioned in the :ref:`dev_env_flatpak` section of the
:ref:`development_environment` chapter, a version of the `qemu`_ emulator is
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used to help Flatpak with cross-building. This needs to be installed before
applications are built for the target architecture. On Debian-based systems,
this command will install the relevant package::

   sudo apt -y install qemu-user-static

It should not be necessary to perform any further configuration on your
workstation if you are using a recent distribution of GNU/Linux.

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.. Some people mention binfmt and restarting systemd-binfmt.service.

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.. _`cross_building_flatpaks_app`:

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Cross-Building an Application

As in the :ref:`building_flatpaks_building` section, ``flatpak-builder`` is
used to build an application for deployment in a flatpak. The command used to
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do this now includes the ``--arch`` option::
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   flatpak-builder --arch=<arch> --repo=<repo> <build-dir> <manifest>

In the case of the Librem 5, ``<arch>`` is ``aarch64``.

The example used previously is the example program supplied with the `libhandy
library`_, which we now build with the additional option::

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   flatpak-builder --arch=aarch64 --repo=myrepo _flatpak sm.puri.Handy.Demo.json

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The result is stored in the ``myrepo`` directory, which is a local repository.
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Note that we did not try to use the ``--install`` option to build and install
the application locally. This would only make sense if the workstation we are
using has an aarch64 architecture.

Creating a Bundle

Following the general advice for building flatpaks given earlier, we can
export a binary bundle from the local repository so that it can be tested on
the target system::

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   flatpak build-bundle --arch=aarch64 myrepo app.flatpak sm.puri.Handy.Demo
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This builds a bundle called ``app.flatpak`` from the application referred to by
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``sm.puri.Handy.Demo`` from the ``myrepo`` directory. This bundle can be
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copied to the phone or development board for testing.

Deploying a Bundle for Testing

Before the application can be installed on the target system, both Flatpak and
the appropriate runtime must be installed. Flatpak should already be installed
on the Librem 5 and the Librem 5 development board. However, if you need to
install it again, the following command will do this::

   sudo apt -y install flatpak

Flatpak on the target system will need to know about the remotes you used to
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build the application. For example, if you used the `Nightly GNOME Apps`_
repository (``gnome-nightly``) then this will need to be registered::

   flatpak --user remote-add --if-not-exists gnome-nightly
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We do this with the ``--user`` option for convenience. It can also be added to
the system-wide list of remotes if ``sudo`` is used instead.

This bundle can now be installed, again as a normal user::

   flatpak --user install app.flatpak

Flatpak will resolve the dependencies of the bundle using the remote we
registered and begin to install them if they are not already present, before
installing the application itself.

The application can then be run using ``flatpak`` in the usual way::

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   flatpak run sm.puri.Handy.Demo
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The commands for managing applications, runtimes and remotes are all the same
on the target system as they are on the workstation.

.. include:: /links.txt