qt_add_qml_module.qdoc

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// Copyright (C) 2021 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
\page qt-add-qml-module.html
\ingroup cmake-commands-qtqml
 
\title qt_add_qml_module
\target qt6_add_qml_module
 
\brief Defines a QML module.
 
\cmakecommandsince 6.2
 
\section1 Synopsis
 
\badcode
qt_add_qml_module(
    target
    URI uri
    [VERSION version]
    [PAST_MAJOR_VERSIONS ...]
    [STATIC | SHARED]
    [PLUGIN_TARGET plugin_target]
    [OUTPUT_DIRECTORY output_dir]
    [RESOURCE_PREFIX resource_prefix]
    [CLASS_NAME class_name]
    [TYPEINFO typeinfo]
    [IMPORTS ...]
    [OPTIONAL_IMPORTS ...]
    [DEFAULT_IMPORTS ...]
    [DEPENDENCIES ...]
    [IMPORT_PATH ...]
    [SOURCES ...]
    [QML_FILES ...]
    [RESOURCES ...]
    [OUTPUT_TARGETS out_targets_var]
    [DESIGNER_SUPPORTED]
    [FOLLOW_FOREIGN_VERSIONING]
    [NAMESPACE namespace]
    [NO_PLUGIN]
    [NO_PLUGIN_OPTIONAL]
    [NO_CREATE_PLUGIN_TARGET]
    [NO_GENERATE_PLUGIN_SOURCE]
    [NO_GENERATE_QMLTYPES]
    [NO_GENERATE_QMLDIR]
    [NO_LINT]
    [NO_CACHEGEN]
    [NO_RESOURCE_TARGET_PATH]
    [NO_IMPORT_SCAN]
    [ENABLE_TYPE_COMPILER]
    [TYPE_COMPILER_NAMESPACE namespace]
    [QMLTC_EXPORT_DIRECTIVE export_macro]
    [QMLTC_EXPORT_FILE_NAME header_defining_export_macro]
 
)
 
\endcode
 
\versionlessCMakeCommandsNote qt6_add_qml_module()
 
See \l {Building a QML application} and \l {Building a reusable QML module}
for examples that define QML modules.
 
\section1 Description
 
This command defines a QML module that can consist of C++ sources, \c{.qml}
files, or both. It ensures that essential module details are provided and that
they are consistent. It also sets up and coordinates things like cached
compilation of \c{.qml} sources, resource embedding, linting checks, and
auto-generation of some key module files.
 
\section2 Target Structure
 
A QML module can be structured in a few different ways. The following scenarios
are the typical arrangements:
 
\section3 Separate backing and plugin targets
 
This is the recommended arrangement for most QML modules. All of the module's
functionality is implemented in the \e backing target, which is given as the
first command argument. C++ sources, \c{.qml} files, and resources should all
be added to the backing target. The backing target is a library that should be
installed in the same location as any other library defined by the project.
 
The source directory structure under which the backing target is created should
match the target path of the QML module (the target path is the module's URI
with dots replaced by forward slashes). If the source directory structure
doesn't match the target path, \c{qt_add_qml_module()} will issue a warning.
 
The following example shows a suitable source directory structure for a QML
module with a URI of \c{MyThings.Panels}. The call to \c{qt_add_qml_module()}
would be in the \c{CMakeLists.txt} file shown.
 
\badcode
src
 +-- MyThings
      +-- Panels
           +-- CMakeLists.txt
\endcode
 
A separate \e plugin target is associated with the QML module. It is used at
runtime to load the module dynamically when the application doesn't already
link to the backing target. The plugin target will also be a library and is
normally installed to the same directory as the module's
\l{Module Definition qmldir Files}{qmldir} file.
 
The plugin target should ideally contain nothing more than a trivial
implementation of the plugin class. This allows the plugin to be designated as
optional in the \c qmldir file. Other targets can then link directly to the
backing target and the plugin will not be needed at runtime, which can improve
load-time performance. By default, a C++ source file that defines a minimal
plugin class will be automatically generated and added to the plugin target.
For cases where the QML module needs a custom plugin class implementation, the
\l{NO_GENERATE_PLUGIN_SOURCE} and usually the \l{NO_PLUGIN_OPTIONAL} options
will be needed.
 
\note
When using static linking, it might be necessary to use
\c Q_IMPORT_QML_PLUGIN to ensure that the QML plugin is correctly linked.
 
\section3 Plugin target with no backing target
 
A QML module can be defined with the plugin target serving as its own backing
target. In this case, the module must be loaded dynamically at runtime and
cannot be linked to directly by other targets. To create this arrangement,
the \c PLUGIN_TARGET keyword must be used, with the \c target repeated as the
plugin target name. For example:
 
\badcode
qt_add_qml_module(someTarget
    PLUGIN_TARGET someTarget
    ...
)
\endcode
 
While this arrangement may seem marginally simpler to deploy, a separate
backing target should be preferred where possible due to the potentially better
load-time performance.
 
\section3 Executable as a QML module
 
An executable target can act as a backing target for a QML module. In this case,
there will be no plugin library, since the QML module will always be loaded
directly as part of the application. The \c{qt_add_qml_module()} command will
detect when an executable is used as the backing target and will automatically
disable the creation of a separate plugin. Do not use any of the options with
\c{PLUGIN} in their name when using this arrangement.
 
When an executable is used as the backing target, the source directory structure
is not expected to match the QML module's target path.
See \l{qmlcachegen-auto}{Caching compiled QML sources} for additional target
path differences for compiled-in resources.
 
 
\target qmldir-autogeneration
\section2 Auto-generating \c{qmldir} and typeinfo files
 
By default, a \l{Module Definition qmldir Files}{qmldir} file and a typeinfo
file will be auto-generated for the QML module being defined. The contents of
those files are determined by the various arguments given to this command, as
well as the sources and \c{.qml} files added to the backing target.
The \l OUTPUT_DIRECTORY argument determines where the \c qmldir and typeinfo
files will be written to. If the QML module has a plugin, that plugin will also
be created in the same directory as the \c qmldir file.
 
If using a statically built Qt, the backing target's \c{.qml} files will be
scanned during the CMake configure run to determine the imports used by the
module and to set up linking relationships (the \c{NO_IMPORT_SCAN} keyword
can be given to disable this). When a \c{.qml} file is added to or
removed from the module, CMake will normally re-run automatically and the
relevant files will be re-scanned, since a \c{CMakeLists.txt} file will have
been modified. During the course of development, an existing \c{.qml} file may
add or remove an import or a type. On its own, this would not cause CMake to
re-run automatically, so you should explicitly re-run CMake to force the
\c qmldir file to be regenerated and any linking relationships to be updated.
 
The backing target's C++ sources are scanned at build time to generate a
typeinfo file and a C++ file to register the associated types. The generated
C++ file is automatically added to the backing target as a source.
This requires \c AUTOMOC to be enabled on the target. The project is
responsible for ensuring this, usually by setting the \c CMAKE_AUTOMOC variable
to \c TRUE before calling \c qt_add_qml_module(), or by passing in an existing
target with the \c AUTOMOC target property already set to \c TRUE. It isn't an
error to have \c AUTOMOC disabled on the target, but the project is then
responsible for handling the consequences. This may include having to manually
generate the typeinfo file instead of allowing it to be auto-generated with
missing details, and adding C++ code to register the types.
 
Projects should prefer to use the auto-generated typeinfo and \c qmldir files
where possible. They are easier to maintain and they don't suffer from the same
susceptibility to errors that hand-written files do. Nevertheless, for
situations where the project needs to provide these files itself, the
auto-generation can be disabled. The \c NO_GENERATE_QMLDIR option disables the
\c qmldir auto-generation and the \c NO_GENERATE_QMLTYPES option disables the
typeinfo and C++ type registration auto-generation. If the auto-generated
typeinfo file is acceptable, but the project wants to use a different name for
that file, it can override the default name with the \c TYPEINFO option (but
this should not typically be needed).
 
\target qmlcachegen-auto
\section2 Caching compiled QML sources
 
All \c{.qml}, \c{.js}, and \c{.mjs} files added to the module via the
\c QML_FILES argument will be compiled to bytecode and cached directly in the
backing target. This improves load-time performance of the module. The original
uncompiled files are also stored in the backing target's resources, as these
may still be needed in certain situations by the QML engine.
 
The resource path of each file is determined by its path relative to the
current source directory (\c CMAKE_CURRENT_SOURCE_DIR). This resource path is
appended to a prefix formed by concatenating the \l{RESOURCE_PREFIX} and
the target path (but see \l NO_RESOURCE_TARGET_PATH for an exception to this).
 
If \l{QTP0001} policy is set to \c NEW, the \l{RESOURCE_PREFIX} defaults
to \c{/qt/qml/} which is the default import path of the QML engine.
This ensures that modules are put into the \l{QML Import Path} and can be
found without further setup.
 
Ordinarily, the project should aim to place \c{.qml} files in
the same relative location as they would have in the resources. If the \c{.qml}
file is in a different relative directory to its desired resource path, its
location in the resources needs to be explicitly specified. This is done by
setting the \c QT_RESOURCE_ALIAS source file property, which must be set before
the \c{.qml} file is added. For example:
 
\badcode
set_source_files_properties(path/to/somewhere/MyFrame.qml PROPERTIES
    QT_RESOURCE_ALIAS MyFrame.qml
)
 
qt_add_qml_module(someTarget
    URI MyCo.Frames
    RESOURCE_PREFIX /my.company.com/imports
    QML_FILES
        path/to/somewhere/MyFrame.qml
        AnotherFrame.qml
)
\endcode
 
In the above example, the target path will be \c{MyCo/Frames}. After
taking into account the source file properties, the two \c{.qml} files will be
found at the following resource paths:
 
\list
\li \c{/my.company.com/imports/MyCo/Frames/MyFrame.qml}
\li \c{/my.company.com/imports/MyCo/Frames/AnotherFrame.qml}
\endlist
 
In the rare case that you want to override the automatic selection of the
qmlcachegen program to be used, you may set the \c QT_QMLCACHEGEN_EXECUTABLE
target property on the module target. For example:
 
\badcode
set_target_properties(someTarget PROPERTIES
    QT_QMLCACHEGEN_EXECUTABLE qmlcachegen
)
\endcode
 
This explicitly selects qmlcachegen as the program to be used, even if
better alternatives are available.
 
Furthermore, you can pass extra arguments to qmlcachegen, by setting the
\c QT_QMLCACHEGEN_ARGUMENTS option. In particular, the \c --only-bytecode
option will turn off compilation of QML script code to C++. For example:
 
\badcode
set_target_properties(someTarget PROPERTIES
    QT_QMLCACHEGEN_ARGUMENTS "--only-bytecode"
)
\endcode
 
Another important argument is \c{--direct-calls}. You can use it to enable the
direct mode of \l{The QML script compiler} in case the Qt Quick Compiler
Extensions are installed. If the extensions are not installed, the argument is
ignored. There is a shorthand called \c {QT_QMLCACHEGEN_DIRECT_CALLS} for it.
 
\badcode
set_target_properties(someTarget PROPERTIES
    QT_QMLCACHEGEN_DIRECT_CALLS ON
)
\endcode
 
Finally, the \c --verbose argument can be used to see diagnostic output from
qmlcachegen:
 
\badcode
set_target_properties(someTarget PROPERTIES
    QT_QMLCACHEGEN_ARGUMENTS "--verbose"
)
\endcode
 
With this flag set, qmlcachegen will output warnings for each function it
cannot compile to C++. Some of these warnings will point to problems in your
QML code and some will tell you that certain features of the QML language are
not implemented in the C++ code generator. In both cases, qmlcachegen will
still generate byte code for such functions. If you want to see only the
problems in your QML code, you should use qmllint and the targets generated
for it instead.
 
\target qmllint-auto
\section2 Linting QML sources
 
A separate linting target will be automatically created if any \c{.qml} files
are added to the module via the \c QML_FILES keyword, or by a later call to
\l{qt6_target_qml_sources}{qt_target_qml_sources()}. The name of the linting
target will be the \c target followed by \c{_qmllint}. An \c{all_qmllint}
target which depends on all the individual \c{*_qmllint} targets is also
provided as a convenience.
 
\target qml-naming-js-files
\section2 Naming conventions for \c{.js} files
 
JavaScript file names that are intended to be addressed as components should
start with an uppercase letter.
 
Alternatively, you may use lowercase file names and set the source file
property \l QT_QML_SOURCE_TYPENAME to the desired type name.
 
\target qml-cmake-singletons
\section2 Singletons
 
If a QML module has \c{.qml} files which provide singleton types, these files
need to have their \c QT_QML_SINGLETON_TYPE source property set to \c TRUE, to
ensure that the \c singleton command is written into the
\l{Module Definition qmldir Files}{qmldir} file. This must be done in addition
to the QML file containing the \c {pragma Singleton} statement.
 
See \l{qt_target_qml_sources_example}{qt_target_qml_sources()} for an example on
how to set the \c QT_QML_SINGLETON_TYPE property.
 
\target qmltc-cmake
\section2 Compiling QML to C++ with QML type compiler
 
If a QML module has \c{.qml} files, you can compile them to C++ using \l{QML
type compiler}{qmltc}. Unlike \l{qmlcachegen-auto}{bytecode compilation}, you
have to explicitly enable qmltc via \l{ENABLE_TYPE_COMPILER} argument. In which
case, \c{.qml} files specified under \c{QML_FILES} would be compiled. Files
ending with \c{.js} and \c{.mjs} are ignored as qmltc does not compile
JavaScript code. Additionally, files marked with QT_QML_SKIP_TYPE_COMPILER
source file property are also skipped.
 
By default, qmltc creates lower-case \c{.h} and \c{.cpp} files for a given
\c{.qml} file. For example, \c{Foo.qml} ends up being compiled into \c{foo.h}
and \c{foo.cpp}.
 
The created C++ files are placed into a dedicated \c{.qmltc/<target>/}
sub-directory of the \c BINARY_DIR of the \c target. These files are then
automatically added to the target sources and compiled as Qt C++ code along with
other source files.
 
While processing QML_FILES, the following source file properties are respected:
\list
    \li \c{QT_QMLTC_FILE_BASENAME}: use this source file property to specify a
        non-default .h and .cpp file name, which might be useful to e.g. resolve
        conflicting file names (imagine you have main.qml that is being
        compiled, but main.h already exists, so #include "main.h" might not do
        what you expect it to do). QT_QMLTC_FILE_BASENAME is expected to be a
        file name (without extension), so any preceding directory is ignored.
        Unlike in the case of default behavior, the QT_QMLTC_FILE_BASENAME is
        not lower-cased.
    \li \c{QT_QML_SKIP_TYPE_COMPILER}: use this source file property to
        specify that a QML file must be ignored by qmltc.
\endlist
 
\section1 Arguments
 
\section2 Required arguments
 
The \c target specifies the name of the backing target for the QML module.
By default, it is created as a shared library if Qt was built as shared
libraries, or as a static library otherwise. This choice can be explicitly
overridden with the \c STATIC or \c SHARED options.
 
Every QML module must define a \c URI. It should be specified in dotted URI
notation, such as \c{QtQuick.Layouts}. Each segment must be a well-formed
ECMAScript Identifier Name. This means, for example, the segments
must not start with a number and they must not contain \e{-} (minus)
characters. As the \c URI will be translated into directory names, you
should restrict it to alphanumeric characters of the latin alphabet,
underscores, and dots. Other QML modules may use this name in
\l{qtqml-syntax-imports.html}{import statements} to import the module. The
\c URI will be used in the \c module line of the generated
\l{Module Definition qmldir Files}{qmldir} file. The \c URI is also used to
form the \e{target path} by replacing dots with forward slashes.
 
See \l{qtqml-modules-identifiedmodules.html}{Identified Modules} for further
in-depth discussion of the module URI.
 
\section2 Versions
 
A QML module can also define a \c VERSION in the form \c{Major.Minor}, where
both \c Major and \c Minor must be integers. An additional \c{.Patch}
component may be appended, but will be ignored. A list of earlier major
versions the module provides types for can also optionally be given after the
\c PAST_MAJOR_VERSIONS keyword (see below).
See \l{qtqml-modules-identifiedmodules.html}{Identified Modules} for further
in-depth discussion of version numbering,
\l{Registering past major versions} for registering past major versions, and
\l{Keeping module versions in sync} for keeping module versions in sync.
 
If you don't need versions you should omit the \c VERSION argument. It defaults
to the highest possible version. Internal versioning of QML modules has some
fundamental flaws. You should use an external package management mechanism to
manage different versions of your QML modules.
 
\section2 Adding sources and resources to the module
 
\c SOURCES specifies a list of non-QML sources to be added to the backing
target. It is provided as a convenience and is equivalent to adding the sources
to the backing target with the built-in \c{target_sources()} CMake command.
 
\c QML_FILES lists the \c{.qml}, \c{.js} and \c{.mjs} files for the module.
These will be automatically \l{qmlcachegen-auto}{compiled into bytecode} and
embedded in the backing target unless the \c NO_CACHEGEN option is given.
The uncompiled file is always stored in the embedded resources of the backing
target, even if \c NO_CACHEGEN is specified. Unless the \c NO_LINT option is
given, the uncompiled files will also be
\l{Linting QML sources}{processed by \c qmllint} via a separate custom build
target. The files will also be used to populate type information in the
generated \l{Module Definition qmldir Files}{qmldir} file by default.
\c NO_GENERATE_QMLDIR can be given to disable the automatic generation of the
\c qmldir file. This should normally be avoided, but for cases where the
project needs to provide its own \c qmldir file, this option can be used.
 
\note See \l{qt6_target_qml_sources}{qt_target_qml_sources()} for further details on
how to add qmlfiles after \c qt_add_qml_module() was called.
For example, you may wish to add files conditionally based on an if statement
expression, or from subdirectories that will only be added if certain criteria
are met.
Furthermore, files added with \l{qt6_target_qml_sources}{qt_target_qml_sources()}
also can specify if they should be skipped for the linting,
\l{qmlcachegen-auto}{bytecode compilation} or \c qmldir file generation.
 
\c RESOURCES lists any other files needed by the module, such as images
referenced from the QML code. These files will be added as compiled-in
resources (see \l RESOURCE_PREFIX for an explanation of the base point they
will be located under). If needed, their relative location can
be controlled by setting the \c QT_RESOURCE_ALIAS source property, just as for
\c{.qml} files (see \l{qmlcachegen-auto}{Caching compiled QML sources}).
 
\target RESOURCE_PREFIX
\c RESOURCE_PREFIX is intended to encapsulate a namespace for the project and
will often be the same for all QML modules that the project defines. It should
be chosen to avoid clashing with the resource prefix of anything else used by
the project or likely to be used by any other project that might consume it.
A good choice is to incorporate the domain name of the organization the project
belongs to. A common convention is to append \c{/imports} to the domain name to
form the resource prefix. For example:
 
\badcode
qt_add_qml_module(someTarget
    RESOURCE_PREFIX /my.company.com/imports
    ...
)
\endcode
 
\target NO_RESOURCE_TARGET_PATH
When various files are added to the compiled-in resources, they are placed
under a path formed by concatenating the \c RESOURCE_PREFIX and the target path.
For the special case where the backing target is an executable, it may be
desirable to place the module's \c{.qml} files and other resources directly
under the \c RESOURCE_PREFIX instead. This can be achieved by specifying the
\c NO_RESOURCE_TARGET_PATH option, which may only be used if the backing target
is an executable.
 
\target PAST_MAJOR_VERSIONS
\section2 Registering past major versions
 
\c PAST_MAJOR_VERSIONS contains a list of additional major version that the module
provides. For each of those versions and each QML file
without a \c QT_QML_SOURCE_VERSIONS setting an additional entry in the
\l{Module Definition qmldir Files}{qmldir} file will be generated to specify
the extra version. Furthermore, the generated module registration code will
register the past major versions using \l{qmlRegisterModule()} on the C++ side.
The module registration code is automatically generated for your QML module,
unless you specify \c{NO_GENERATE_QMLTYPES} (but use of this option is strongly
discouraged). Usage of \c PAST_MAJOR_VERSIONS adds some overhead when your
module is imported. You should increment the major version of your module as
rarely as possible. Once you can rely on all QML files importing this module to
omit the version in their imports, you can safely omit \c{PAST_MAJOR_VERSIONS}.
All the QML files will then import the latest version of your module. If you
have to support versioned imports, consider supporting only a limited number of
past major versions.
 
\section2 Declaring module dependencies
 
\c IMPORTS provides a list of other QML modules that this module imports. Each
module listed here will be added as an \c{import} entry in the generated
\l{Module Definition qmldir Files}{qmldir} file. If a QML file imports
this module, it also imports all the modules listed under \c{IMPORTS}.
Optionally, a version can be specified by appending it after a slash, such as
\c{QtQuick/2.0}. Omitting the version will cause the greatest version available
to be imported. You may only specify the major version, as in \c{QtQuick/2}. In
that case the greatest minor version available with the given major version will
be imported. Finally, \c{auto} may be given as version (\c{QtQuick/auto}). If
\c{auto} is given, the version that the current module is being imported with is
propagated to the module to be imported. Given an entry \c{QtQuick/auto} in a
module \c{YourModule}, if a QML file specifies \c{import YourModule 3.14}, this
results in importing  version \c{3.14} of \c{QtQuick}. For related modules that
follow a common versioning scheme, you should use \c{auto}.
 
\c OPTIONAL_IMPORTS provides a list of other QML modules that this module
\e may import at run-time. These are not automatically imported by the QML
engine when importing the current module, but rather serve as hints to tools
like \c qmllint. Versions can be specified in the same way as for \c IMPORTS.
Each module listed here will be added as an \c{optional import} entry in the
generated \l{Module Definition qmldir Files}{qmldir} file.
 
\c DEFAULT_IMPORTS specifies which of the optional imports are the default entries
that should be loaded by tooling. One entry should be specified for every group of
\c OPTIONAL_IMPORTS in the module. As optional imports are only resolved at runtime,
tooling like qmllint cannot in general know which of the optional imports should
be resolved. To remedy this, you can specify one of the optional imports as the
default import;  tooling will then pick it. If you have one optional import that
gets used at runtime without any further configuration, that is an ideal candidate
for the default import.
 
\c DEPENDENCIES provides a list of other QML modules that this module depends
on, but doesn't necessarily import. It would typically be used for dependencies
that only exist at the C++ level, such as a module registering a class to QML
which is a subclass of one defined in another module.
 
For example, if one would like to subclass \c QQuickItem as following:
 
\badcode
class MyItem: public QQuickItem { ... };
\endcode
 
then one has to make sure that the module containing \c QQuickItem, called
\c Quick, is declared as a dependency via the \c DEPENDENCIES option. Not doing
so might result in errors during type compilation with
\l{QML type compiler}{qmltc} or during binding and function compilation to C++
with \l{qmlcachegen-auto}{qmlcachegen}.
 
\note Adding the module to \c DEPENDENCIES is not necessary if the module
is already imported via the \c IMPORTS option. The recommended way is to
use the lighter alternative \c DEPENDENCIES over \c IMPORTS.
 
The module version of the
dependencies must be specified along with the module name, in the same form as
used for \c IMPORTS and \c OPTIONAL_IMPORTS. Each module listed here will be
added as a \c{depends} entry in the generated
\l{Module Definition qmldir Files}{qmldir} file.
 
\target IMPORT_PATH
\c IMPORT_PATH can be used to add to the search paths where other QML modules
that this one depends on can be found. The other modules must have their
\c qmldir file under their own target path below one of the search paths.
 
If the backing target is a static library and that static library will be
installed, \c OUTPUT_TARGETS should be given to provide a variable in which to
store a list of additional targets that will also need to be installed.
These additional targets are generated internally by \c{qt_add_qml_module()}
and are referenced by the backing target's linking requirements as part of
ensuring that resources are set up and loaded correctly.
 
\target PLUGIN_TARGET
\section2 Targets and plugin targets
 
\c PLUGIN_TARGET specifies the plugin target associated with the QML module.
The \c PLUGIN_TARGET can be the same as the backing
\c target, in which case there will be no separate backing target.
If \c PLUGIN_TARGET is not given, it defaults to \c target with \c plugin
appended. For example, a backing target called \c mymodule would have a default
plugin name of \c mymoduleplugin. The plugin target's name will be used to
populate a \c{plugin} line in the generated
\l{Module Definition qmldir Files}{qmldir} file. Therefore, you must not try to
change the plugin's output name by setting target properties like
\c OUTPUT_NAME or any of its related properties.
 
The backing \c target and the plugin target (if different) will be created by
the command, unless they already exist. Projects should generally let them be
created by the command so that they are created as the appropriate target type.
If the backing \c target is a static library, the plugin will also be created
as a static library. If the backing \c target is a shared library, the plugin
will be created as a module library. If an existing \c target is passed in and
it is an executable target, there will be no plugin. If you intend to always
link directly to the backing target and do not need a plugin, it can be
disabled by adding the \c NO_PLUGIN option. Specifying both \c NO_PLUGIN and
\c PLUGIN_TARGET is an error.
 
\target NO_CREATE_PLUGIN_TARGET
In certain situations, the project may want to delay creating the plugin target
until after the call. The \c NO_CREATE_PLUGIN_TARGET option can be given in
that situation. The project is then expected to call
\l{qt6_add_qml_plugin}{qt_add_qml_plugin()} on the plugin target once it has
been created. When  \c NO_CREATE_PLUGIN_TARGET is given, \c PLUGIN_TARGET must
also be provided to explicitly name the plugin target.
 
\target CLASS_NAME
\target NO_GENERATE_PLUGIN_SOURCE
By default, \c{qt_add_qml_module()} will auto-generate a \c{.cpp} file that
implements the plugin class named by the \c CLASS_NAME argument. The generated
\c{.cpp} file will be automatically added to the plugin target as a source file
to be compiled. If the project wants to provide its own implementation of the
plugin class, the \c NO_GENERATE_PLUGIN_SOURCE option should be given. Where no
\c CLASS_NAME is provided, it defaults to the \c URI with dots replaced by
underscores, then \c Plugin appended. Unless the QML module has no plugin, the
class name will be recorded as a \c classname line in the generated
\l{Module Definition qmldir Files}{qmldir} file. You need to add any C++ files
with custom plugin code to the plugin target. Since the plugin then likely
contains functionality that goes beyond simply loading the backing library, you
will probably want to add \l{NO_PLUGIN_OPTIONAL}, too. Otherwise the QML engine
may skip loading the plugin if it detects that the backing library is already
linked.
 
\target NO_PLUGIN
If the \c NO_PLUGIN keyword is given, then no plugin will be built. This
keyword is thus incompatible with all the options that customize the plugin
target, in particular \l{NO_GENERATE_PLUGIN_SOURCE}, \l{NO_PLUGIN_OPTIONAL},
\l{PLUGIN_TARGET}, \l{NO_CREATE_PLUGIN_TARGET}, and \l{CLASS_NAME}. If you do
not provide a plugin for your module, it will only be fully usable if its
backing library has been linked into the executable. It is generally hard to
guarantee that a linker preserves the linkage to a library it considers unused.
 
\target NO_PLUGIN_OPTIONAL
If the \c NO_PLUGIN_OPTIONAL keyword is given, then the plugin is recorded in
the generated \c qmldir file as non-optional. If all of a QML module's
functionality is implemented in its backing target and the plugin target is
separate, then the plugin can be optional, which is the default and recommended
arrangement. The auto-generated plugin source file satisfies this requirement.
Where a project provides its own \c{.cpp} implementation for the plugin, that
would normally mean the \c NO_PLUGIN_OPTIONAL keyword is also needed because
the plugin will almost certainly contain functionality that the QML module
requires.
 
\section2 Automatic type registration
 
Type registration is automatically performed for the backing target's C++
sources that are processed by AUTOMOC. This will generate a typeinfo file in the
\l{OUTPUT_DIRECTORY}{output directory}, the file name being the \c target name
with \c{.qmltypes} appended. This file name can be changed using the
\c TYPEINFO option if desired, but this should not normally be necessary.
The file name is also recorded as a \c typeinfo entry in the generated
\l{Module Definition qmldir Files}{qmldir} file. Automatic type registration
can be disabled using the \c NO_GENERATE_QMLTYPES option, in which case no
typeinfo file will be generated, but the project will still be expected to
generate a typeinfo file and place it in the same directory as the generated
\c qmldir file.
 
\target OUTPUT_DIRECTORY
\c OUTPUT_DIRECTORY specifies where the plugin library, \c qmldir and typeinfo
files are generated. When this keyword is not given, the default value will be
the target path (formed from the \c URI) appended to the value of the
\l QT_QML_OUTPUT_DIRECTORY variable.
If that variable is not defined, the default depends on the type of backing
target. For executables, the value will be the target path appended to
\c{${CMAKE_CURRENT_BINARY_DIR}}, whereas for other targets it will be just
\c{${CMAKE_CURRENT_BINARY_DIR}}. When the structure of the source tree
matches the structure of QML module target paths (which is highly recommended),
\l QT_QML_OUTPUT_DIRECTORY often isn't needed. In order to match the structure
of the target paths, you have to call your directories \e exactly like the
segments of your module URI. For example, if your module URI is
\c{MyUpperCaseThing.mylowercasething}, you need to put this in a directory
called \c{MyUpperCaseThing/mylowercasething/}.
 
The need for specifying the \c OUTPUT_DIRECTORY keyword should be rare, but if
it is used, it is likely that the caller will also need to add to the
\l IMPORT_PATH to ensure that \l{qmllint-auto}{linting},
\l{qmlcachegen-auto}{cached compilation} of qml sources,
\l{qt6_import_qml_plugins}{automatic importing} of plugins in static builds,
and \l{qt_deploy_qml_imports}{deploying imported QML modules} for non-static
builds all work correctly.
 
\section2 Qt Quick Designer compatibility
 
\c DESIGNER_SUPPORTED should be given if the QML module supports
Qt Quick Designer. When present, the generated \c qmldir file will contain
a \c designersupported line. See \l{Module Definition qmldir Files} for how
this affects the way Qt Quick Designer handles the plugin.
 
\section2 Keeping module versions in sync
 
The \c FOLLOW_FOREIGN_VERSIONING keyword relates to base types of your own
C++-defined QML types that live in different QML modules. Typically, the
versioning scheme of your module does not match that of the module providing
the base types. Therefore, by default all revisions of the base types are
made available in any import of your module. If \c FOLLOW_FOREIGN_VERSIONING
is given, the version information attached to the base types and their
properties is respected. So, an \c {import MyModule 2.8} will then only make
available versioned properties up to version \c{2.8} of any base types outside
\c{MyModule}.
This is mostly useful if you want to keep your module version in sync
with other modules you're basing types on. In that case you might want your custom
types to not expose properties from a module's base type version greater than the one being
imported.
 
\section2 C++ namespaces of generated code
 
If a namespace is given with the \c NAMESPACE keyword, the plugin and registration
code will be generated into a C++ namespace of this name.
 
\section2 qmlimportscanner and NO_IMPORT_SCAN
 
For static Qt builds, \c{qmlimportscanner} is run at configure time to scan the
\c{.qml} files of a QML module and identify the QML imports it uses (see
\l{qt6_import_qml_plugins}{qt_import_qml_plugins()}). For non-static Qt builds,
if the target is an executable, a similar scan is performed at build time to
provide the information needed by deployment scripts (see
\l{qt6_deploy_qml_imports}{qt_deploy_qml_imports()}). Both scans can be
disabled by providing the \c{NO_IMPORT_SCAN} option. Doing so means the project
takes on the responsibility of ensuring all required plugins are instantiated
and linked for static builds. For non-static builds the project must manually
work out and deploy all QML modules used by an executable target.
 
\section2 Arguments for qmltc
 
\target ENABLE_TYPE_COMPILER
\c ENABLE_TYPE_COMPILER can be used to compile \c{.qml} files to C++ source code
with \l{QML type compiler}{qmltc}. Files with the source property
\c{QT_QML_SKIP_TYPE_COMPILER} are not compiled to C++.
 
\c TYPE_COMPILER_NAMESPACE argument allows to override the namespace in which
\l{QML type compiler}{qmltc} generates code.
By default, the namespace of the generated code follows the module
hierarchy as depicted in the URI,
e.g., \c MyModule for a module with URI \c MyModule or
\c com::example::Module for URI \c com.example.MyModule.
By specifying the \c TYPE_COMPILER_NAMESPACE option, the generated code
can be put instead in a custom namespace, where different subnamespaces are to
be separated by a "::", e.g. "MyNamespace::MySubnamespace" for the namespace MySubnamespace that
is inside the MyNamespace. Apart from the "::", C++ namespace naming rules
apply.
 
\c QMLTC_QMLTC_EXPORT_DIRECTIVE should be used with \c QMLTC_EXPORT_FILE_NAME when
the classes generated by \l{QML type compiler}{qmltc} should be exported from
the qml library. By default, classes generated by qmltc are not exported from
their library.
The header defining the export macro for the current library
can be specified as an optional argument to \c QMLTC_EXPORT_FILE_NAME while the
exporting macro name should be specified as an argument to
\c QMLTC_QMLTC_EXPORT_DIRECTIVE. If no additional include is required or wanted,
e.g. when the header of the export macro is already indirectly included by a base
class, then the \c QMLTC_EXPORT_FILE_NAME option can be left out.
*/