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
或
QQmlEngine
, as described below. In addition, the Declarative UI package includes the
qml
tool, which loads
.qml
files. This tool is useful for developing and testing QML code without having 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.
When you run your applications on the target platform, your application needs to access the location of the QML libraries. If you use
qmake
，
QT_INSTALL_QML
environment variable 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 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
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 a sub-directory of
QStandardPaths::CacheLocation
with the name "qmlcache". The file extension is
.qmlc
for QML documents and
.jsc
for imported JavaScript modules.
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.
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.
This will happen automatically if you use the CMake QML 模块 API , for qmake see the section below.
When using qmake, in order to deploy your application with QML files compiled ahead of time, you must organize the files and the build system in a specific way:
qrc:///
URL scheme.
CONFIG+=qtquickcompiler
指令。
The Declarative UI package includes a QML runtime tool, qml , 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, your application must invoke the QML runtime. This is done by writing a Qt C++ application that loads the QQmlEngine by either:
QQuickView
是
QWindow
-based class that can load QML files. For example, if there is a QML file,
application.qml
, it will look like this:
import QtQuick 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
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,
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(); }
If you're not using any graphical items from Qt Quick, you can replace QGuiApplication 采用 QCoreApplication in the code above. This way, you can use QML as a language without any dependencies to the Qt GUI 模块。
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.
注意： 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:
.qrc
resource collection file
that lists resource files in XML format.
:/
prefix or as a URL with the
.qrc
方案。
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
not
使用
.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:
project
|- example.qrc
|- main.qml
|- images
|- background.png
|- main.cpp
|- project.pro
main.qml
and
background.png
files are 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 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
使用
RESOURCES
variable to indicate that
example.qrc
should be used to build the application resources:
QT += qml SOURCES += main.cpp RESOURCES += example.qrc