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/*!
\page qtquick-deployment.html
\title Deploying QML Applications
\brief Provides information on how to use deploy QML applications.

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

Applications that use QML must invoke the QML runtime to run QML documents. You can do this 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 having 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.

When you run your applications on the target platform, your application needs to access
the location of the QML libraries. If you use \l{qmake Manual}{qmake}, the
\c QT_INSTALL_QML environment variable points to the location of the libraries. The
\l{Downloads}{Qt Installers} install the QML libraries in:
\c{<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. 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 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.

\target Compiling Ahead of Time
\section1 Ahead-of-Time Compilation

The automatic caching of compiled QML documents into cache files results in significantly faster
application load time. 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
startup times from the very beginning, Qt's build system allows you to perform the compilation step
for QML files ahead of time, when compiling the C++ parts of your application.

To deploy your application with QML files compiled ahead of time, 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 via
    \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're using the CMake build system, then you can achieve this by inserting a
        \c find_package(Qt5QuickCompiler) call into your \c CMakeLists.txt and replacing the use
        of \c qt5_add_resources with \c qtquick_compiler_add_resources.
\endlist

One benefit of compiling ahead of time is that, in the event of syntax errors in your QML
documents, you are notified at application compile-time instead of at run-time, when the file is
loaded.

If you have \c .qml or \c .js files which should not be compiled but just bundled by the resource
system, such as \c .js files used with \l{Qt WebEngine}, you can omit them from the compilation
via the \c QTQUICK_COMPILER_SKIPPED_RESOURCES variable. In your project file, specify the resource
files to omit, as follows:

\code
QTQUICK_COMPILER_SKIPPED_RESOURCES += bundle_only.qrc
\endcode

By default, this feature ties your application to the Qt version you are compiling against, because
it replaces the QML document source code in the resources with the compiled binary version. The
source files are not present anymore. Consequently, when you use the same application against a
different version of Qt without recompiling it, loading the QML documents will fail with an error
message.

Ahead-of-Time compilation is implemented this way because the feature originates from an add-on for
use in commercial application environments, where deploying source code is not desirable but it's
usually acceptable to require a recompilation when changing Qt.

You can retain the QML and JavaScript documents in the resources by passing a list of resource
\c{(*.qrc)} files in the \c QTQUICK_COMPILER_RETAINED_RESOURCES qmake variable. These resource
files will then not be filtered, and any QML and JavaScript files specified in them will be
readable in full source by your application. Then, if you run the application with a different
Qt version, the QML and JavaScript files will be recompiled at runtime, slowing down the
initial start of the application.

\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, your application must invoke the QML runtime. 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.
    \li Creating a QQmlEngine instance and loading QML files with QQmlComponent.
\endlist


\section2 Initializing with QQuickView

QQuickView is a QWindow-based class that can 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 doesn't have any graphical components, or if it's 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

If you're not using any graphical items from Qt Quick, you can replace QGuiApplication with a
QCoreApplication in the code above. This way, you can use QML as a language without any
dependencies to the \l{Qt GUI} module.

\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 and 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 If you use the resource system, the application executable must be re-compiled whenever a
QML source file is changed, 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 are 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 \b not use the \c .qrc
scheme. This scheme should only be used from C++ code to refer to resource files.

Here's an application packaged using the Qt resource system; its directory structure is as follows:

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

The \c main.qml and \c background.png files are 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 \c RESOURCES variable to indicate that \c example.qrc should
be used to build the application resources:

\quotefile qmlapp/qtbinding/resources/resources.pro

\section1 Related Information
\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}
    \li \l{qtqml-cppintegration-data.html}{qtqml-cppintegration-exposecppattributes.html}{Exposing Attributes of C++ Types to QML}
    \li \l{The Qt Resource System}
\endlist

*/