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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 byte 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 compiled byte 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.
On the Android platform, cache files are always stored in the cache directory.

\section1 Compiling QML Ahead of Time

The automatic caching of compiled QML documents into cache files results in
significantly faster load times of applications. However, the initial creation
of cache files can still take time, especially when the application starts for
the very first time. To avoid that initial step and provide faster start-up
times from the very beginning, Qt's build system allows you to perform the
compilation step for QML files at the same time as the rest of your
application.

If you would like to deploy your application with QML files compiled ahead of
time, then you must organize the files and the build system in a specific way:

\list
\li All QML documents (including JavaScript files) must be included as resources
through \l{The Qt Resource System}{Qt's Resource system}.
\li Your application must load the QML documents via the \c qrc:/// URL scheme.
\li You can enable Ahead-of-Time compilation using the \c CONFIG+=qtquickcompiler directive.
\li If you are using the CMake build system, then you can achieve this inserting a
\c find_package(Qt5QuickCompiler) call into your \c CMakeLists.txt and replace the use of
of \c qt5_add_resources with \c qtquick_compiler_add_resources .
\endlist

One added benefit of this way of developing and deploying the application is
that you will be notified of syntax errors in your QML documents at application
compile time, instead of run-time when loading the file.

\section2 Limitations

Currently this feature will tie your application to the Qt version you are
compiling against, because it will replace the QML document source code in the
resources with the compiled binary version. The source files are not present
anymore. That means that when using the same application against a different
version of Qt without recompiling it, loading the QML documents will fail with
an error message.

The Ahead-of-Time compilation is implemented this way because the feature
originates from an add-on for use in commercial application environments, where
the deployment of source code is not desirable and requiring a recompilation
when changing Qt is usually acceptable.

We plan to implement support for retaining the source documents in a future
version of Qt.

\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.

*/
