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/*!
\page qtquick-deployment.html
\title Deploying QML Applications
\brief Deploying QML applications


QML documents are loaded and executed by the QML runtime. This includes the
Declarative UI engine along with the built-in QML types and plugin modules,
and it also provides access to third-party QML types and modules.

Applications that use QML need to invoke the QML runtime in order to
execute QML documents. This can be done by creating a QQuickView
or a QQmlEngine, as described below. In addition, the Declarative UI
package includes the qmlscene tool, which loads \c .qml files. This tool is
useful for developing and testing QML code without the need to write
a C++ application to load the QML runtime.

\section1 Deploying Applications with Qt Creator

\l{Qt Creator Manual}{Qt Creator} deploys and packages QML applications to
various platforms. For mobile devices, Qt Creator can directly bundle
applications to the respective platform package formats such as APK.

For more information, visit:
\list
\li \l{Deploying Qt Applications}
\li \l{Qt Creator: Running on Multiple Platforms}{Running on Multiple Platforms}
\li \l{Qt Creator: Deploying to Devices}{Deploying to Devices}
\endlist

When running applications on the target platform, the application needs to
access the location of the QML libraries. When using \l{qmake Manual}{qmake},
the \c QT_INSTALL_QML environment points to the location of the libraries.
The \l{Downloads}{Qt Installers} install the QML libraries in
\e{<version>}\c{/}\e{<compiler>}\c{/qml} directory.

\section1 QML Caching

The QML runtime loads QML documents by parsing them and generating native code.
Most of the time the document hasn't changed since the last time it was loaded.
In order to speed up this loading process, the QML runtime maintains a cache
file for each qml document. This cache file contains the native code and a
binary representation of the QML document structure. In addition, when multiple
applications use the same QML document, the memory needed for the code is
shared between application processes. The cache files are loaded via the \c
mmap() system call on POSIX compliant operating systems or \c
CreateFileMapping() on Windows, resulting in significant memory savings.

Each time you load a changed QML document, the cache is automatically
re-created. Cache files are located in the same directory as the source code,
if the directory is writable. Otherwise they will be placed in a sub-directory
of QStandardPaths::CacheLocation with the name "qmlcache". The file extension
is \c .qmlc for QML documents and \c .jsc for imported JavaScript modules.

\section1 QML Caching for Deployment (Preview)

The automatic caching of QML documents into cache files result in significantly
faster load times of applications. However, the initial creation of cache files
can still take time, especially when the application starts first. To avoid
that initial step and provide faster start-up times from the very beginning,
Qt's build system allows you to create these cache files in advance.

If you would like to deploy your application with cache files generated ahead
of time, you must satisfy four conditions in your \c .pro file:

\list
\li All QML documents (including JavaScript files) must be added to the
\c QML_FILES variable.
\li Your .pro file must use the \c load(qml_module) or \c load(qml_plugin)
directive at the end, to activate the processing of \c QML_FILES and generation
of install rules.
\li The \c TARGETPATH variable must contain the import name of your QML
module with forward slashes as separators.
\li You must enable Ahead-of-Time caching using the \c CONFIG+=qmlcache directive.
\endlist

For example if you are developing the module \c MyCompany.CommonComponents,
then your \c .pro file could look like this:

\code
TARGETPATH = MyCompany/CommonComponents
QML_FILES = BlueButton.qml RedSlider.qml qmldir
CONFIG += qmlcache
load(qml_module)
\endcode

Similarly, if your module contains a C++ plugin then you use \c qml_plugin:

\code
TARGETPATH = MyCompany/CommonComponents
QML_FILES = BlueButton.qml RedSlider.qml qmldir
CONFIG += qmlcache
SOURCES = plugin.cpp
QT += quick
load(qml_plugin)
\endcode

In these examples the QML module consisting of the QML documents, the \c qmldir
file, and optionally the C++ plugin, will be installed into the
MyCompany/CommonComponents sub-directory of \c $$[QT_INSTALL_QML]. By enabling
the \c qmlcache configuration, the cache files will be created at build time
and also installed into the same directory for deployment.

\section2 Limitations

Currently this feature has some limitations:

\list
\li Only QML and JavaScript documents that are part of a QML module can be
compiled ahead of time.
\li For cross-compilation, only the ARMv7 and ARMv8 target architectures are
supported.
\li For native compilation, Ahead-of-Time caching is limited to architectures
where the QML runtime supports Just-in-Time compilation. This includes x86, x86-64,
ARMv7, ARMv8 and MIPS32.
\endlist

\section1 Prototyping with QML Scene

The Declarative UI package includes a QML runtime tool,
\l{qtquick-qmlscene.html}{qmlscene}, which loads and displays QML documents.
This is useful during the application development phase for prototyping
QML-based applications without writing your own C++ applications to invoke
the QML runtime.

\section1 Initializing the QML Runtime in Applications

To run an application that uses QML, the QML runtime must be invoked by
the application. This is done by writing a Qt C++ application that loads the
QQmlEngine by either:

\list
\li Loading the QML file through a QQuickView instance, or
\li Creating a QQmlEngine instance and loading QML files with QQmlComponent
\endlist


\section2 Initializing with QQuickView

QQuickView is a QWindow-based class that is able to load QML files.
For example, if there is a QML file, \c application.qml, it will
look like this:

\qml
    import QtQuick 2.3

    Rectangle { width: 100; height: 100; color: "red" }
\endqml

It can be loaded in a Qt application's \c main.cpp file like this:

\code
    #include <QGuiApplication>
    #include <QQuickView>

    int main(int argc, char *argv[])
    {
        QGuiApplication app(argc, argv);

        QQuickView view;
        view.setSource(QUrl::fromLocalFile("application.qml"));
        view.show();

        return app.exec();
    }
\endcode

This creates a QWindow-based view that displays the contents of
\c application.qml.

The application's \c .pro \l{Creating Project Files}{project file} must specify
the \c declarative module for the \c QT variable. For example:

\code
    TEMPLATE += app
    QT += quick
    SOURCES += main.cpp
\endcode


\section2 Creating a QQmlEngine directly

If \c application.qml does not have any graphical components, or if it is
preferred to avoid QQuickView for other reasons, the QQmlEngine
can be constructed directly instead. In this case, \c application.qml is
loaded as a QQmlComponent instance rather than placed into a view:

\code
    #include <QGuiApplication>
    #include <QQmlEngine>
    #include <QQmlContext>
    #include <QQmlComponent>

    int main(int argc, char *argv[])
    {
        QGuiApplication app(argc, argv);

        QQmlEngine engine;
        QQmlContext *objectContext = new QQmlContext(engine.rootContext());

        QQmlComponent component(&engine, "application.qml");
        QObject *object = component.create(objectContext);

        // ... delete object and objectContext when necessary

        return app.exec();
    }
\endcode

QGuiApplication can be replaced by a QCoreApplication in the code above in case you are not
using any graphical items from Qt Quick. This allows using QML as a language without any
dependencies to the \l {Qt GUI} module.

See \l{qtqml-cppintegration-data.html}{qtqml-cppintegration-exposecppattributes.html}{Exposing Attributes of C++ Types to QML}
for more information about using QQmlEngine, QQmlContext and QQmlComponent, as well
as details on including QML files through \l{The Qt Resource System}{Qt's Resource system}.

\section1 Managing Resource Files with the Qt Resource System

The \l {The Qt Resource System}{Qt resource system} allows resource files to be stored as
binary files in an application executable. This can be useful when building a mixed
QML/C++ application as it enables QML files (as well as other resources such as images
and sound files) to be referred to through the resource system URI scheme rather than
relative or absolute paths to filesystem resources. Note, however, that if you use the resource
system, the application executable must be re-compiled whenever a QML source file is changed
in order to update the resources in the package.

To use the resource system in a mixed QML/C++ application:

\list
\li Create a \c .qrc \l {The Qt Resource System}{resource collection file} that lists resource
   files in XML format
\li From C++, load the main QML file as a resource using the \c :/ prefix or as a URL with the
   \c qrc scheme
\endlist

Once this is done, all files specified by relative paths in QML will be loaded from
the resource system instead. Use of the resource system is completely transparent to
the QML layer; this means all QML code should refer to resource files using relative
paths and should \e not use the \c qrc scheme. This scheme should only be used from
C++ code for referring to resource files.

Here is a application packaged using the \l {The Qt Resource System}{Qt resource system}.
The directory structure looks like this:

\code
project
    |- example.qrc
    |- main.qml
    |- images
        |- background.png
    |- main.cpp
    |- project.pro
\endcode

The \c main.qml and \c background.png files will be packaged as resource files. This is
done in the \c example.qrc resource collection file:

\quotefile qmlapp/qtbinding/resources/example.qrc

Since \c background.png is a resource file, \c main.qml can refer to it using the relative
path specified in \c example.qrc:

\snippet qmlapp/qtbinding/resources/main.qml 0

To allow QML to locate resource files correctly, the \c main.cpp loads the main QML
file, \c main.qml, as a resource file using the \c qrc scheme:

\snippet qmlapp/qtbinding/resources/main.cpp 0

Finally, \c project.pro uses the RESOURCES variable to indicate that \c example.qrc should
be used to build the application resources:

\quotefile qmlapp/qtbinding/resources/resources.pro

See \l {The Qt Resource System} for more information.

*/
