Qt 5.9 | Deploying QML Applications

Qt 5.9.9 Reference Documentation

# 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 `.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.

## Deploying Applications with Qt Creator

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:

- Deploying Qt Applications
- Running on Multiple Platforms
- Deploying to Devices

When running applications on the target platform, the application needs to access the location of the QML libraries. When using qmake, the `QT_INSTALL_QML` environment points to the location of the libraries. The Qt Installers install the QML libraries in <version>`/`<compiler>`/qml` directory.

## 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 `mmap()` system call on POSIX compliant operating systems or `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 `.qmlc` for QML documents and `.jsc` for imported JavaScript modules. On the Android platform, cache files are always stored in the cache directory.

## 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 `.pro` file:

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

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

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

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

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

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

### Limitations

Currently this feature has some limitations:

- Only QML and JavaScript documents that are part of a QML module can be compiled ahead of time.
- For cross-compilation, only the ARMv7 and ARMv8 target architectures are supported.
- 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.

## Prototyping with QML Scene

The Declarative UI package includes a QML runtime tool, 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.

## 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:

- Loading the QML file through a QQuickView instance, or
- Creating a QQmlEngine instance and loading QML files with QQmlComponent

### Initializing with QQuickView

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

      import QtQuick 2.3

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

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

      #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();
      }

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

The application's `.pro` project file must specify the `declarative` module for the `QT` variable. For example:

      TEMPLATE += app
      QT += quick
      SOURCES += main.cpp

### Creating a QQmlEngine directly

If `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, `application.qml` is loaded as a QQmlComponent instance rather than placed into a view:

      #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();
      }

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 Qt GUI module.

See 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 Qt's Resource system.

## Managing Resource Files with the Qt Resource System

The 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:

- Create a `.qrc` resource collection file that lists resource files in XML format
- From C++, load the main QML file as a resource using the `:/` prefix or as a URL with the `qrc` scheme

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 not use the `qrc` scheme. This scheme should only be used from C++ code for referring to resource files.

Here is a application packaged using the Qt resource system. The directory structure looks like this:

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

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

      <!DOCTYPE RCC>
      <RCC version="1.0">

      <qresource prefix="/">
          <file>main.qml</file>
          <file>images/background.png</file>
      </qresource>

      </RCC>

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

      // main.qml
      import QtQuick 2.3

      Image { source: "images/background.png" }

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

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

          QQuickView view;
          view.setSource(QUrl("qrc:/main.qml"));
          view.show();

          return app.exec();
      }

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

      QT += qml

      SOURCES += main.cpp
      RESOURCES += example.qrc

See The Qt Resource System for more information.