This is largely based on a presentation I gave a couple of weeks ago. If you are too lazy to read, go watch it instead 😉

For 20 years KDE has been building free software for the world. As part of this endeavor, we created a collection of libraries to assist in high-quality C++ software development as well as building highly integrated graphic applications on any operating system. We call them the KDE Frameworks.

With the recent advance of software bundling systems such as Snapcraft and Flatpak, KDE software maintainers are however a bit on the spot. As our software is building on such a vast collection of frameworks and supporting technology, the individual size of a distributable application can be quite abysmal.

When we tried to package our calculator KCalc as a snap bundle, we found that even a relatively simple application like this, makes for a good 70 MiB snap to be in a working state (most of this is the graphical stack required by our underlying C++ framework, Qt).

Since then a lot of effort was put into devising a system that would allow us to more efficiently deal with this. We now have a reasonably suitable solution on the table.

The KDE Frameworks 5 content snap.

A content snap is a special bundle meant to be mounted into other bundles for the purpose of sharing its content. This allows us to share a common core of libraries and other content across all applications, making the individual applications just as big as they need to be. KCalc is only 312 KiB without translations.

The best thing is that beside some boilerplate definitions, the snapcraft.yaml file defining how to snap the application is like a regular snapcraft file.

Let’s look at how this works by example of KAlgebra, a calculator and mathematical function plotter:

Any snapcraft.yaml has some global attributes we’ll want to set for the snap

name: kalgebra version: 16.08.2 summary: ((TBD)) description: ((TBD)) confinement: strict grade: devel

We’ll want to define an application as well. This essentially allows snapd to expose and invoke our application properly. For the purpose of content sharing we will use a special start wrapper called kf5-launch that allows us to use the content shared Qt and KDE Frameworks. Except for the actual application/binary name this is fairly boilerplate stuff you can use for pretty much all KDE applications.

apps: kalgebra: command: kf5-launch kalgebra plugs: - kde-frameworks-5-plug # content share itself - home # give us a dir in the user home - x11 # we run with xcb Qt platform for now - opengl # Qt/QML uses opengl - network # gethotnewstuff needs network IO - network-bind # gethotnewstuff needs network IO - unity7 # notifications - pulseaudio # sound notifications

To access the KDE Frameworks 5 content share we’ll then want to define a plug our application can use to access the content. This is always the same for all applications.

plugs: kde-frameworks-5-plug: interface: content content: kde-frameworks-5-all default-provider: kde-frameworks-5 target: kf5

Once we got all that out of the way we can move on to actually defining the parts that make up our snap. For the most part parts are build instructions for the application and its dependencies. With content shares there are two boilerplate parts you want to define.

The development tarball is essentially a fully built kde frameworks tree including development headers and cmake configs. The tarball is packed by the same tech that builds the actual content share, so this allows you to build against the correct versions of the latest share.

kde-frameworks-5-dev: plugin: dump snap: [-*] source: http://build.neon.kde.org/job/kde-frameworks-5-release_amd64.snap/lastSuccessfulBuild/artifact/kde-frameworks-5-dev_amd64.tar.xz

The environment rigging provide the kf5-launch script we previously saw in the application’s definition, we’ll use it to execute the application within a suitable environment. It also gives us the directory for the content share mount point.

kde-frameworks-5-env: plugin: dump snap: [kf5-launch, kf5] source: http://github.com/apachelogger/kf5-snap-env.git

Lastly, we’ll need the actual application part, which simply instructs that it will need the dev part to be staged first and then builds the tarball with boilerplate cmake config flags.

kalgebra: after: [kde-frameworks-5-dev] plugin: cmake source: http://download.kde.org/stable/applications/16.08.2/src/kalgebra-16.08.2.tar.xz configflags: - "-DKDE_INSTALL_USE_QT_SYS_PATHS=ON" - "-DCMAKE_INSTALL_PREFIX=/usr" - "-DCMAKE_BUILD_TYPE=Release" - "-DENABLE_TESTING=OFF" - "-DBUILD_TESTING=OFF" - "-DKDE_SKIP_TEST_SETTINGS=ON"

Putting it all together we get a fairly standard snapcraft.yaml with some additional boilerplate definitions to wire it up with the content share. Please note that the content share is using KDE neon’s Qt and KDE Frameworks builds, so, if you want to try this and need additional build-packages or stage-packages to build a part you’ll want to make sure that KDE neon’s User Edition archive is present in the build environments sources.list deb http://archive.neon.kde.org/user xenial main . This is going to get a more accessible centralized solution for all of KDE soon™.

name: kalgebra version: 16.08.2 summary: ((TBD)) description: ((TBD)) confinement: strict grade: devel apps: kalgebra: command: kf5-launch kalgebra plugs: - kde-frameworks-5-plug # content share itself - home # give us a dir in the user home - x11 # we run with xcb Qt platform for now - opengl # Qt/QML uses opengl - network # gethotnewstuff needs network IO - network-bind # gethotnewstuff needs network IO - unity7 # notifications - pulseaudio # sound notifications plugs: kde-frameworks-5-plug: interface: content content: kde-frameworks-5-all default-provider: kde-frameworks-5 target: kf5 parts: kde-frameworks-5-dev: plugin: dump snap: [-*] source: http://build.neon.kde.org/job/kde-frameworks-5-release_amd64.snap/lastSuccessfulBuild/artifact/kde-frameworks-5-dev_amd64.tar.xz kde-frameworks-5-env: plugin: dump snap: [kf5-launch, kf5] source: http://github.com/apachelogger/kf5-snap-env.git kalgebra: after: [kde-frameworks-5-dev] plugin: cmake source: http://download.kde.org/stable/applications/16.08.2/src/kalgebra-16.08.2.tar.xz configflags: - "-DKDE_INSTALL_USE_QT_SYS_PATHS=ON" - "-DCMAKE_INSTALL_PREFIX=/usr" - "-DCMAKE_BUILD_TYPE=Release" - "-DENABLE_TESTING=OFF" - "-DBUILD_TESTING=OFF" - "-DKDE_SKIP_TEST_SETTINGS=ON"

Now to install this we’ll need the content snap itself. Here is the content snap. To install it a command like sudo snap install --force-dangerous kde-frameworks-5_*_amd64.snap should get you going. Once that is done one can install the kalgebra snap. If you are a KDE developer and want to publish your snap on the store get in touch with me so we can get you set up.

The kde-frameworks-5 content snap is also available in the edge channel of the Ubuntu store. You can try the games kblocks and ktuberling like so:

sudo snap install --edge kde-frameworks-5 sudo snap install --edge --devmode kblocks sudo snap install --edge --devmode ktuberling

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