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
或
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.
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:
When running applications on the target platform, the application needs to access the location of the QML libraries. When using
qmake
，
QT_INSTALL_QML
environment points to the location of the libraries. The
Qt 安装程序
install the QML libraries in
<version>
/
<compiler>
/qml
目录。
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.
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
文件：
QML_FILES
变量。
load(qml_module)
or
load(qml_plugin)
directive at the end, to activate the processing of
QML_FILES
and generation of install rules.
TARGETPATH
variable must contain the import name of your QML module with forward slashes as separators.
CONFIG+=qmlcache
指令。
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.
Currently this feature has some limitations:
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.
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:
QQuickView
是
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
文件像这样：
#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
若
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 模块。
见 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 .
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:
.qrc
resource collection file
that lists resource files in XML format
:/
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
使用
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>
由于
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(); }
最后，
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
见 Qt 资源系统 了解更多信息。