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Update Qt and KDE documentation

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- Describe the new `libsForQt5.callPackage` interface
- Emphasize that Qt dependencies must be imported unqualified
- Describe the new `kdeWrapper` wrapper generator
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Thomas Tuegel 2017-02-25 11:16:31 -06:00
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doc/languages-frameworks/qt.xml
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xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="sec-language-qt">
<title>Qt</title>
<title>Qt and KDE</title>
<para>The information in this section applies to Qt 5.5 and later.</para>
<para>Qt is an application development toolkit for C++. Although it is
not a distinct programming language, there are special considerations
for packaging Qt-based programs and libraries. A small set of tools
and conventions has grown out of these considerations.</para>
<para>Qt is a comprehensive desktop and mobile application development toolkit for C++. Legacy support is available for Qt 3 and Qt 4, but all current development uses Qt 5. The Qt 5 packages in Nixpkgs are updated frequently to take advantage of new features, but older versions are typically retained to support packages that may not be compatible with the latest version. When packaging applications and libraries for Nixpkgs, it is important to ensure that compatible versions of Qt 5 are used throughout; this consideration motivates the tools described below.</para>
<section xml:id="ssec-qt-libraries"><title>Libraries</title>
<para>Packages that provide libraries should be listed in
<varname>qt5LibsFun</varname> so that the library is built with each
Qt version. A set of packages is provided for each version of Qt; for
example, <varname>qt5Libs</varname> always provides libraries built
with the latest version, <varname>qt55Libs</varname> provides
libraries built with Qt 5.5, and so on. To avoid version conflicts, no
top-level attributes are created for these packages.</para>
<para>Libraries that depend on Qt 5 should be built with each available version to avoid linking a dependent package against incompatible versions of Qt 5. (Although Qt 5 maintains backward ABI compatibility, linking against multiple versions at once is generally not possible; at best it will lead to runtime faults.) Packages that provide libraries should be added to the top-level function <varname>mkLibsForQt5</varname>, which is used to build a set of libraries for every Qt 5 version. The <varname>callPackage</varname> provided in this scope will ensure that only one Qt version will be used throughout the dependency tree. Dependencies should be imported unqualified, i.e. <literal>qtbase</literal> not <literal>qt5.qtbase</literal>, so that <varname>callPackage</varname> can do its work. <emphasis>Do not</emphasis> import a package set such as <literal>qt5</literal> or <literal>libsForQt5</literal> into your package; although it may work fine in the moment, it could well break at the next Qt update.</para>
<para>If a library does not support a particular version of Qt 5, it is best to mark it as broken by setting its <literal>meta.broken</literal> attribute. A package may be marked broken for certain versions by testing the <literal>qtbase.version</literal> attribute, which will always give the current Qt 5 version.</para>
</section>
<section xml:id="ssec-qt-programs"><title>Programs</title>
<section xml:id="ssec-qt-applications"><title>Applications</title>
<para>Application packages do not need to be built with every Qt
version. To ensure consistency between the package's dependencies,
call the package with <literal>qt5Libs.callPackage</literal> instead
of the usual <literal>callPackage</literal>. An older version may be
selected in case of incompatibility. For example, to build with Qt
5.5, call the package with
<literal>qt55Libs.callPackage</literal>.</para>
<para>Applications generally do not need to be built with every Qt version because they do not provide any libraries for dependent packages to link against. The primary consideration is merely ensuring that the application itself and its dependencies are linked against only one version of Qt. To call your application expression, use <literal>libsForQt5.callPackage</literal> instead of <literal>callPackage</literal>. Dependencies should be imported unqualified, i.e. <literal>qtbase</literal> not <literal>qt5.qtbase</literal>. <emphasis>Do not</emphasis> import a package set such as <literal>qt5</literal> or <literal>libsForQt5</literal> into your package; although it may work fine in the moment, it could well break at the next Qt update.</para>
<para>Several environment variables must be set at runtime for Qt
applications to function correctly, including:</para>
<para>It is generally best to build an application package against the <varname>libsForQt5</varname> library set. In case a package does not build with the latest Qt version, it is possible to pick a set pinned to a particular version, e.g. <varname>libsForQt55</varname> for Qt 5.5, if that is the latest version the package supports.</para>
<itemizedlist>
<listitem><para><envar>QT_PLUGIN_PATH</envar></para></listitem>
<listitem><para><envar>QML_IMPORT_PATH</envar></para></listitem>
<listitem><para><envar>QML2_IMPORT_PATH</envar></para></listitem>
<listitem><para><envar>XDG_DATA_DIRS</envar></para></listitem>
</itemizedlist>
<para>To ensure that these are set correctly, the program must be wrapped by
invoking <literal>wrapQtProgram <replaceable>program</replaceable></literal>
during installation (for example, during
<literal>fixupPhase</literal>). <literal>wrapQtProgram</literal>
accepts the same options as <literal>makeWrapper</literal>.
</para>
<para>Qt-based applications require that several paths be set at runtime. This is accomplished by wrapping the provided executables in a package with <literal>wrapQtProgram</literal> or <literal>makeQtWrapper</literal> during the <literal>postFixup</literal> phase. To use the wrapper generators, add <literal>makeQtWrapper</literal> to <literal>nativeBuildInputs</literal>. The wrapper generators support the same options as <literal>wrapProgram</literal> and <literal>makeWrapper</literal> respectively. It is usually only necessary to generate wrappers for programs intended to be invoked by the user.</para>
</section>
<section xml:id="ssec-qt-kde"><title>KDE</title>
<para>Many of the considerations above also apply to KDE packages,
especially the need to set the correct environment variables at
runtime. To ensure that this is done, invoke <literal>wrapKDEProgram
<replaceable>program</replaceable></literal> during
installation. <literal>wrapKDEProgram</literal> also generates a
<literal>ksycoca</literal> database so that required data and services
can be found. Like its Qt counterpart,
<literal>wrapKDEProgram</literal> accepts the same options as
<literal>makeWrapper</literal>.</para>
<para>The KDE Frameworks are a set of libraries for Qt 5 which form the basis of the Plasma desktop environment and the KDE Applications suite. Packaging a Frameworks-based library does not require any steps beyond those described above for general Qt-based libraries. Frameworks-based applications should not use <literal>makeQtWrapper</literal>; instead, use <literal>kdeWrapper</literal> to create the necessary wrappers: <literal>kdeWrapper { unwrapped = <replaceable>expr</replaceable>; targets = <replaceable>exes</replaceable>; }</literal>, where <replaceable>expr</replaceable> is the un-wrapped package expression and <replaceable>exes</replaceable> is a list of strings giving the relative paths to programs in the package which should be wrapped.</para>
</section>