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/*!
\page qtqml-writing-a-module.html
\title Writing QML Modules
\brief How to write a custom QML module.

You can declare a QML module using the \l{qt_add_qml_module}
{CMake QML Module API} to:

\list
\li Generate \l {Module Definition qmldir Files}{qmldir} and
    \l {Type Description Files}{*.qmltypes files}.
\li Register C++ types annotated with \l QML_ELEMENT.
\li Invoke \l {Ahead-of-Time-Compilation}{qmlcachegen}.
\li Provide modules both in the physical and in \l{The Qt Resource System}
    {resource file system}.
\li Use the pre-compiled versions of QML files.
\li Bundle the module's files in the resource file system.
\li Combine QML files and C++-based types in the same module.
\li Create a backing library and an optional plugin. Link the backing library
    into the application to avoid loading the plugin at run time.
\endlist

All the above actions can also be configured separately.
For more information, see \l {qt_add_qml_module}{CMake QML Module API}.

\section1 Multiple QML Modules in One Binary

You can add multiple QML modules into the same binary. Define a CMake target for
each module and then link the targets to the executable.
If the extra targets are all static libraries, the result will be one binary,
which contains multiple QML modules. In short you can create an application
like this:

\badcode
myProject
    | - CMakeLists.txt
    | - main.cpp
    | - main.qml
    | - onething.h
    | - onething.cpp
    | - ExtraModule
        | - CMakeLists.txt
        | - Extra.qml
        | - extrathing.h
        | - extrathing.cpp
\endcode

To begin, let's assume main.qml contains an instantiation of Extra.qml:

    \badcode
    import ExtraModule
    Extra { ... }
    \endcode

The extra module has to be a static library so that you can link it
into the main program. Therefore, state as much in ExtraModule/CMakeLists.txt:

\quotefromfile qml/CMakeLists.txt
\printuntil extrathing.h
\printuntil )

This generates two targets: \c extra_module for the backing library, and
\c extra_moduleplugin for the plugin. Being a static library too, the plugin cannot
be loaded at runtime.

In myProject/CMakeLists.txt you need to specify the QML module that main.qml
and any types declared in onething.h are part of:

\quotefromfile qml/myProject-CMakeLists.txt
\printuntil onething.h
\printuntil )


From there, you add the subdirectory for the extra module:

\quotefromfile qml/CMakeLists.txt
\skipto add_subdirectory
\printuntil )

To ensure that linking the extra module works correctly, you need to:

\list
\li Define a symbol in the extra module.
\li Create a reference to the symbol from the main program.
\endlist

QML plugins contain a symbol you can use for this purpose.
You can use the \l Q_IMPORT_QML_PLUGIN macro to create a reference to this symbol.
Add the following code to the main.cpp:

\badcode
#include <QtQml/QQmlExtensionPlugin>
Q_IMPORT_QML_PLUGIN(ExtraModulePlugin)
\endcode

\c ExtraModulePlugin is the name of the generated plugin class. It's composed
of the module URI with \c Plugin appended to it. Then, in the main program's
CMakeLists.txt, link the plugin, not the backing library, into the main program:

\quotefromfile qml/myProject-CMakeLists.txt
\skipto target_link_libraries
\printuntil )

\section1 Exporting Multiple Major Versions from The Same Module

\l qt_add_qml_module by default considers the major version given in its
URI argument, even if the individual types declare other versions in their
added specific version via \l QT_QML_SOURCE_VERSIONS or \l Q_REVISION.
If a module is available under more than one version, you also need to decide
what versions the individual QML files are available under. To declare further
major versions, you can use the \c PAST_MAJOR_VERSIONS option to
\c qt_add_qml_module as well as the \c {QT_QML_SOURCE_VERSIONS} property on
individual QML files.

\quotefile qml/MajorProject-CMakeLists.txt

\c MyModule is available in major versions 1, 2, and 3. The maximum version
available is 3.2. You can import any version 1.x or 2.x with a positive x. For
Thing.qml and OtherThing.qml we have added explicit version information.
Thing.qml is available from version 1.4, and OtherThing.qml is available from
version 2.2. You have to specify the later versions, too, in each
\c set_source_files_properties() because you may remove QML files
from a module when bumping the major version. There is no explicit version
information for OneMoreThing.qml. This means that OneMoreThing.qml is available
in all major versions, from minor version 0.

With this setup, the generated registration code will register the module
\c versionsqmlRegisterModule() for each of the major versions. This way, all
versions can be imported.


\section1 Custom Directory Layouts

The easiest way to structure QML modules is to keep them in directories named by
their URIs. For example, a module My.Extra.Module would live in a directory
My/Extra/Module relative to the application that uses it. This way, they can
easily be found at run time and by any tools.

In more complex projects, this convention can be too limiting. You might for
instance want to group all QML modules in one place to avoid polluting the
project's root directory. Or you want to reuse a single module in multiple
applications. For those cases, \c QT_QML_OUTPUT_DIRECTORY in combination with
\c RESOURCE_PREFIX  and \l IMPORT_PATH can be used.

To collect QML modules into a specific output directory, for example a
subdirectory "qml" in the build directory \l QT_QML_OUTPUT_DIRECTORY, set the
following in the top-level CMakeLists.txt:

\badcode
set(QT_QML_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/qml)
\endcode

The output directories of QML modules move to the new location.
Likewise, the \c qmllint and \c qmlcachegen invocations are automatically
adapted to use the new output directory as an \l {QML Import Path}{import path}.
Because the new output directory is not part of the default QML import path,
you have to add it explicitly at run time, so that the QML modules can be found.


Now that the physical file system is taken care of, you may still want to move
the QML modules into a different place in the resource file system. This is what
the RESOURCE_PREFIX option is for. You have to specify it separately in
each \l qt_add_qml_module. The QML module will then be placed under the specified
prefix, with a target path generated from the URI appended. For example,
consider the following module:

\code
qt_add_qml_module(
    URI My.Great.Module
    VERSION 1.0
    RESOURCE_PREFIX /example.com/qml
    QML_FILES
        A.qml
        B.qml
)
\endcode

This will add a directory \c example.com/qml/My/Great/Module to the resource file
system and place the QML module defined above in it. You don't strictly need to
add the resource prefix to the QML import path as the module can still be found
in the physical file system. However, it generally is a good idea to add the
resource prefix to the QML import path because loading from the resource file
system is faster than loading from the physical file system for most modules.

If the QML modules are meant to be used in a larger project with multiple import
paths, you'll have to do an additional step: Even if you add the import paths at
run time, tooling like \c qmllint does not have access to it, and might fail to
find the correct dependencies. Use \c IMPORT_PATH to tell tooling about the
additional paths it has to consider. For example:

\badcode
qt_add_qml_module(
    URI My.Dependent.Module
    VERSION 1.0
    QML_FILES
        C.qml
    IMPORT_PATH "/some/where/else"
)
\endcode

\section1 Eliminating Run Time File System Access

If all QML modules are always loaded from the resource
file system, you can deploy the application as a single binary. Let's first
consider the simple case:

\badcode
QQmlEngine qmlEngine;
qmlEngine.addImportPath(QStringLiteral(":/"));
// Use qmlEngine to load the main.qml file.
\endcode

\note ":/" is used for simplicity here. See \l {Custom Directory Layouts}
for more complex cases.

If all the modules are linked into the application and if you're following
the default resource directory structure, do not add any further import paths as
those might override the one you added.

If you have specified a custom \c RESOURCE_PREFIX, you have to add the custom
resource prefix to the import path instead. You can also add multiple resource
prefixes.

The path \c :/qt-project.org/imports/ is part of the default QML import path. If
you use it, you don't have to specially add it. Qt's own QML modules are placed
there, though. You have to be careful not to overwrite them. For modules that are
heavily re-used across different projects \c :/qt-project.org/imports/ is
acceptable. By using it you can avoid forcing all the users to add custom
import paths.

\section1 Integrating custom QML plugins

If you bundle an \l {QQuickImageProvider}{image provider} in the QML module, you
need to implement the \l {QQmlEngineExtensionPlugin::initializeEngine()}
method. This, in turn, makes it necessary to write the own plugin. To support
this use case, \l NO_GENERATE_PLUGIN_SOURCE can be used.

Let's consider a module that provides its own plugin source:

\quotefile qml/myimageprovider.txt

You may declare an image provider in myimageprovider.h, like this:

\badcode
class MyImageProvider : public QQuickImageProvider
{
    [...]
};
\endcode

In plugin.cpp you can then define the QQmlEngineExtensionPlugin:

\quotefile qml/plugin.cpp.txt

This will make the image provider available. The plugin and the backing library
both are in the same CMake target imageproviderplugin. This is done so that the
linker does not drop parts of the module in various scenarios.

As the plugin creates an image provider, it no longer has a trivial
\c initializeEngine function. Therefore, the plugin is no longer optional.

*/
