Cross-platform apps

Zero Install supports two ways to ship a single user-facing feed that works on multiple operating systems:

1. One feed, multiple <implementation> elements, each with its own arch. Use this when all platforms can be updated in lockstep, e.g. when one upstream release has builds for every OS.
2. One top-level feed, sub-feeds per OS. The top-level feed contains only metadata and <feed arch="..."> references. Each sub-feed is published on its own schedule. Use this when upstream releases are staggered per platform, or when the per-platform builds have meaningfully different versioning.

Both are illustrated below.

Single feed, per-arch implementations

This is the simplest pattern when upstream ships per-OS archives at the same version. See CMake's template for a full example; abridged:

<group license="BSD-3-Clause">
  <command name="run" path="bin/cmake"/>
  <command name="run-gui" path="bin/cmake-gui"/>

  <implementation arch="Linux-x86_64" version="{version}" released="{released}" stability="stable">
    <manifest-digest/>
    <archive href="https://cmake.org/files/v{minor-version}/cmake-{version}-Linux-x86_64.tar.gz"
             extract="cmake-{version}-Linux-x86_64"/>
  </implementation>
</group>

<group license="BSD-3-Clause">
  <command name="run" path="CMake.app/Contents/MacOS/CMake"/>
  <command name="run-gui" path="CMake.app/Contents/bin/cmake-gui"/>

  <implementation arch="Darwin-*" version="{version}" released="{released}" stability="stable">
    <manifest-digest/>
    <archive href="https://cmake.org/files/v{minor-version}/cmake-{version}-macos-universal.tar.gz"
             extract="cmake-{version}-macos-universal"/>
  </implementation>
</group>

<group license="BSD-3-Clause">
  <command name="run" path="bin/cmake.exe"/>
  <command name="run-gui" path="bin/cmake-gui.exe"/>

  <implementation arch="Windows-x86_64" version="{version}" released="{released}" stability="stable">
    <manifest-digest/>
    <archive href="https://cmake.org/files/v{minor-version}/cmake-{version}-win64-x64.msi"
             type="application/x-msi" extract="SourceDir/CMake"/>
  </implementation>
</group>

Note how each <group> overrides the <command> paths to match the platform's binary layout (bin/cmake, CMake.app/Contents/MacOS/CMake, bin/cmake.exe). The arch on the <implementation> is what 0install uses to pick the right one.

Adding a new version with 0template creates a new XML file containing all of these implementations at once.

Top-level feed with sub-feeds

When per-OS releases are independent, publish a small top-level feed that pulls in OS-specific sub-feeds:

<?xml version="1.0"?>
<interface xmlns="http://zero-install.sourceforge.net/2004/injector/interface"
           uri="https://example.com/myapp.xml">
  <name>MyApp</name>
  <summary>does something useful</summary>
  <homepage>https://example.com/myapp</homepage>

  <feed src="https://example.com/myapp-linux.xml"   arch="Linux-*"/>
  <feed src="https://example.com/myapp-macos.xml"   arch="MacOSX-*"/>
  <feed src="https://example.com/myapp-windows.xml" arch="Windows-*"/>

  <entry-point binary-name="myapp" command="run"/>
</interface>

Each sub-feed is a normal feed of its own, declared <feed-for interface="https://example.com/myapp.xml"/>. Users add the top-level feed; 0install fetches whichever sub-feed matches their platform.

This is how the apps repository ships Blender, Calibre, Node.js and many others. Each sub-feed has its own template and its own 0watch script, so they can be updated independently.

Choosing between the two

If upstream releases...	Use
One archive per OS, all at the same version, on one schedule	Single feed
Per-OS releases on different schedules, sometimes skipping platforms	Sub-feeds
Different version numbers per OS	Sub-feeds
You only care about one OS	Single feed (no sub-feeds)

Tip

The two patterns can be mixed: a top-level feed can both reference sub-feeds and include cross-platform implementations directly, e.g. for source builds.