This example shows how to create a plugin that extends Qt with a new GUI look and feel.
A plugin in Qt is a class stored in a shared library that can be loaded by a QPluginLoader at run-time. When you create plugins in Qt, they either extend a Qt application or Qt itself. Writing a plugin that extends Qt itself is achieved by inheriting one of the plugin base classes , reimplementing functions from that class, and adding a macro. In this example we extend Qt by adding a new GUI look and feel (i.e., making a new QStyle available). A high-level introduction to plugins is given in the plugin 概述文档 .
Plugins that provide new styles inherit the
QStylePlugin
base class. Style plugins are loaded by Qt and made available through
QStyleFactory
; we will look at this later. We have implemented
SimpleStylePlugin
, which provides
SimpleStyle
. The new style contributes to widget styling by drawing button backgrounds in red - not a major contribution, but it still makes a new style.
The new style is platform agnostic in the sense that it is not based on any specific style implementation, but uses QProxyStyle to merely tweak the looks in the current application style that defaults to the native system style.
注意: On some platforms, the native style will prevent the button from having a red background. In this case, try to run the example in another style (e.g., fusion).
We test the plugin with
StyleWindow
, in which we display a
QPushButton
。
SimpleStyle
and
StyleWindow
classes do not contain any plugin specific functionality and their implementations are trivial; we will therefore leap past them and head on to the
SimpleStylePlugin
和
main()
function. After we have looked at that, we examine the plugin's profile.
SimpleStylePlugin
继承
QStylePlugin
and is the plugin class.
class SimpleStylePlugin : public QStylePlugin { Q_OBJECT Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QStyleFactoryInterface" FILE "simplestyle.json") public: SimpleStylePlugin() = default; QStringList keys() const; QStyle *create(const QString &key) override; };
keys()
returns a list of style names that this plugin can create, while
create()
takes such a string and returns the
QStyle
corresponding to the key. Both functions are pure virtual functions reimplemented from
QStylePlugin
. When an application requests an instance of the
SimpleStyle
style, which this plugin creates, Qt will create it with this plugin.
这里是实现为
keys()
:
QStringList SimpleStylePlugin::keys() const { return {"SimpleStyle"}; }
Since this plugin only supports one style, we return a QStringList with the class name of that style.
这里是
create()
函数:
QStyle *SimpleStylePlugin::create(const QString &key) { if (key.toLower() == "simplestyle") return new SimpleStyle; return nullptr; }
Note that the key for style plugins are case insensitive. The case sensitivity varies from plugin to plugin, so you need to check this when implementing new plugins.
main()
function
int main(int argv, char *args[]) { QApplication app(argv, args); QApplication::setStyle(QStyleFactory::create("simplestyle")); StyleWindow window; window.resize(200, 50); window.show(); return app.exec(); }
Qt loads the available style plugins when the QApplication object is initialized. The QStyleFactory class knows about all styles and produces them with create() (it is a wrapper around all the style plugins).
The
SimpleStylePlugin
lives in its own directory and have its own profile:
TEMPLATE = lib CONFIG += plugin QT += widgets HEADERS = simplestyle.h \ simplestyleplugin.h SOURCES = simplestyle.cpp \ simplestyleplugin.cpp TARGET = simplestyleplugin
In the plugin profile we need to set the lib template as we are building a shared library instead of an executable. We must also set the config to plugin. We set the library to be stored in the styles folder under stylewindow because this is a path in which Qt will search for style plugins.
In addition to the plugin 概述文档 , we have other examples and articles that concern plugins.
在 echo plugin example we show how to implement plugins that extends Qt applications rather than Qt itself, which is the case with the style plugin of this example. The plug & paint example shows how to implement a static plugin as well as being a more involved example on plugins that extend applications.