Plug & Paint Example#
Demonstrates how to extend Qt applications using plugins.
A plugin is a dynamic library that can be loaded at run-time to extend an application. Qt makes it possible to create custom plugins and to load them using
QPluginLoader . To ensure that plugins don’t get lost, it is also possible to link them statically to the executable. The Plug & Paint example uses plugins to support custom brushes, shapes, and image filters. A single plugin can provide multiple brushes, shapes, and/or filters.
If you want to learn how to make your own application extensible through plugins, we recommend that you start by reading this overview, which explains how to make an application use plugins. Afterwards, you can read the Basic Tools and Extra Filters overviews, which show how to implement static and dynamic plugins, respectively.
Plug & Paint consists of the following classes:
QMainWindowsubclass that provides the menu system and that contains a
PaintAreaas the central widget.
QWidgetthat allows the user to draw using a brush and to insert shapes.
PluginDialogis a dialog that shows information about the plugins detected by the application.
FilterInterfaceare abstract base classes that can be implemented by plugins to provide custom brushes, shapes, and image filters.
The Plugin Interfaces#
We will start by reviewing the interfaces defined in
interfaces.h. These interfaces are used by the Plug & Paint application to access extra functionality. They are implemented in the plugins.
class BrushInterface(): # public virtual ~BrushInterface() = default virtual QStringList brushes() = 0 QRect = virtual() QPoint pos) = 0 QRect = virtual() QPoint oldPos, QPoint newPos) = 0 QRect = virtual() QPoint pos) = 0
BrushInterface class declares four pure virtual functions. The first pure virtual function,
brushes(), returns a list of strings that identify the brushes provided by the plugin. By returning a
QStringList instead of a
QString , we make it possible for a single plugin to provide multiple brushes. The other functions have a
brush parameter to identify which brush (among those returned by
brushes()) is used.
mouseRelease() take a
QPainter and one or two
QPoint s, and return a
QRect identifying which portion of the image was altered by the brush.
The class also has a virtual destructor. Interface classes usually don’t need such a destructor (because it would make little sense to
delete the object that implements the interface through a pointer to the interface), but some compilers emit a warning for classes that declare virtual functions but no virtual destructor. We provide the destructor to keep these compilers happy.
class ShapeInterface(): # public virtual ~ShapeInterface() = default virtual QStringList shapes() = 0 QPainterPath = virtual() QWidget parent) = 0
ShapeInterface class declares a
shapes() function that works the same as
brushes() function, and a
generateShape() function that has a
shape parameter. Shapes are represented by a
QPainterPath , a data type that can represent arbitrary 2D shapes or combinations of shapes. The
parent parameter can be used by the plugin to pop up a dialog asking the user to specify more information.
class FilterInterface(): # public virtual ~FilterInterface() = default virtual QStringList filters() = 0 QImage = virtual() QWidget parent) = 0
FilterInterface class declares a
filters() function that returns a list of filter names, and a
filterImage() function that applies a filter to an image.
#define BrushInterface_iid "org.qt-project.Qt.Examples.PlugAndPaint.BrushInterface/1.0" Q_DECLARE_INTERFACE(BrushInterface, BrushInterface_iid)
To make it possible to query at run-time whether a plugin implements a given interface, we must use the
Q_DECLARE_INTERFACE() macro. The first argument is the name of the interface. The second argument is a string identifying the interface in a unique way. By convention, we use a “Java package name” syntax to identify interfaces. If we later change the interfaces, we must use a different string to identify the new interface; otherwise, the application might crash. It is therefore a good idea to include a version number in the string, as we did above.
A note on naming: It might have been tempting to give the
filters() functions a more generic name, such as
features(). However, that would have made multiple inheritance impractical. When creating interfaces, we should always try to give unique names to the pure virtual functions.
The MainWindow Class#
MainWindow class is a standard
QMainWindow subclass, as found in many of the other examples (e.g., Application ). Here, we’ll concentrate on the parts of the code that are related to plugins.
def loadPlugins(self): staticInstances = QPluginLoader.staticInstances() for plugin in staticInstances: populateMenus(plugin)
loadPlugins() function is called from the
MainWindow constructor to detect plugins and update the Brush, Shapes, and Filters menus. We start by handling static plugins (available through
To the application that uses the plugin, a Qt plugin is simply a
QObject . That
QObject implements plugin interfaces using multiple inheritance.
The next step is to load dynamic plugins. We initialize the
pluginsDir member variable to refer to the
plugins subdirectory of the Plug & Paint example. On Unix, this is just a matter of initializing the
QDir variable with
applicationDirPath() , the path of the executable file, and to do a
cd() . On Windows and macOS, this file is usually located in a subdirectory, so we need to take this into account.
entryList = pluginsDir.entryList(QDir.Files) for fileName in entryList: loader = QPluginLoader(pluginsDir.absoluteFilePath(fileName)) plugin = loader.instance() if plugin: populateMenus(plugin) pluginFileNames += fileName
entryList() to get a list of all files in that directory. Then we iterate over the result using a range-based for loop and try to load the plugin using
QObject provided by the plugin is accessible through
instance() . If the dynamic library isn’t a Qt plugin, or if it was compiled against an incompatible version of the Qt library,
instance() returns a null pointer.
instance() is non-null, we add it to the menus.
brushMenu.setEnabled(not brushActionGroup.actions().isEmpty()) shapesMenu.setEnabled(not shapesMenu.actions().isEmpty()) filterMenu.setEnabled(not filterMenu.actions().isEmpty())
At the end, we enable or disable the Brush, Shapes, and Filters menus based on whether they contain any items.
def populateMenus(self, plugin): iBrush = BrushInterface(plugin) if iBrush: addToMenu(plugin, iBrush->brushes(), brushMenu.changeBrush, brushActionGroup) iShape = ShapeInterface(plugin) if iShape: addToMenu(plugin, iShape->shapes(), shapesMenu.insertShape) iFilter = FilterInterface(plugin) if iFilter: addToMenu(plugin, iFilter->filters(), filterMenu.applyFilter)
For each plugin (static or dynamic), we check which interfaces it implements using
qobject_cast() . First, we try to cast the plugin instance to a
BrushInterface; if it works, we call the private function
addToMenu() with the list of brushes returned by
brushes(). Then we do the same with the
ShapeInterface and the
def aboutPlugins(self): dialog = PluginDialog(pluginsDir.path(), pluginFileNames, self) dialog.exec()
aboutPlugins() slot is called on startup and can be invoked at any time through the About Plugins action. It pops up a
PluginDialog, providing information about the loaded plugins.
addToMenu() function is called from
loadPlugin() to create
QAction s for custom brushes, shapes, or filters and add them to the relevant menu. The
QAction is created with the plugin from which it comes from as the parent; this makes it convenient to get access to the plugin later.
def changeBrush(self): action = QAction(sender()) if not action: return iBrush = BrushInterface(action.parent()) if not iBrush: return brush = action.text() paintArea.setBrush(iBrush, brush)
changeBrush() slot is invoked when the user chooses one of the brushes from the Brush menu. We start by finding out which action invoked the slot using
sender() . Then we get the
BrushInterface out of the plugin (which we conveniently passed as the
QAction ‘s parent) and we call
PaintArea::setBrush() with the
BrushInterface and the string identifying the brush. Next time the user draws on the paint area,
PaintArea will use this brush.
def insertShape(self): action = QAction(sender()) if not action: return iShape = ShapeInterface(action.parent()) if not iShape: return path = iShape.generateShape(action.text(), self) if not path.isEmpty(): paintArea.insertShape(path)
insertShape() is invoked when the use chooses one of the shapes from the Shapes menu. We retrieve the
QAction that invoked the slot, then the
ShapeInterface associated with that
QAction , and finally we call
ShapeInterface::generateShape() to obtain a
def applyFilter(self): action = QAction(sender()) if not action: return iFilter = FilterInterface(action.parent()) if not iFilter: return image = iFilter.filterImage(action.text(), paintArea.image(), self) paintArea.setImage(image)
applyFilter() slot is similar: We retrieve the
QAction that invoked the slot, then the
FilterInterface associated to that
QAction , and finally we call
FilterInterface::filterImage() to apply the filter onto the current image.
The PaintArea Class#
PaintArea class contains some code that deals with
BrushInterface, so we’ll review it briefly.
def setBrush(self, brushInterface, brush): self.brushInterface = brushInterface self.brush = brush
setBrush(), we simply store the
BrushInterface and the brush that are given to us by
def mouseMoveEvent(self, event): if (event.buttons() Qt.LeftButton) and lastPos != QPoint(-1, -1): if brushInterface: painter = QPainter(theImage) setupPainter(painter) rect = brushInterface.mouseMove(brush, painter, lastPos, event.position().toPoint()) update(rect) lastPos = event.position().toPoint()
mouse move event handler , we call the
BrushInterface::mouseMove() function on the current
BrushInterface, with the current brush. The mouse press and mouse release handlers are very similar.
The PluginDialog Class#
PluginDialog class provides information about the loaded plugins to the user. Its constructor takes a path to the plugins and a list of plugin file names. It calls
findPlugins() to fill the QTreeWdiget with information about the plugins:
def findPlugins(self, path,): fileNames) = QStringList() label.setText(tr("Plug Paint found the following plugins\n" "(looked in %1):") .arg(QDir.toNativeSeparators(path))) dir = QDir(path) staticInstances = QPluginLoader.staticInstances() for plugin in staticInstances: populateTreeWidget(plugin, tr("%1 (Static Plugin)") .arg(plugin.metaObject().className())) for fileName in fileNames: loader = QPluginLoader(dir.absoluteFilePath(fileName)) plugin = loader.instance() if plugin: populateTreeWidget(plugin, fileName)
findPlugins() is very similar to
MainWindow::loadPlugins(). It uses
QPluginLoader to access the static and dynamic plugins. Its helper function
qobject_cast() to find out which interfaces are implemented by the plugins:
def populateTreeWidget(self, plugin, text): pluginItem = QTreeWidgetItem(treeWidget) pluginItem.setText(0, text) pluginItem.setExpanded(True) boldFont = pluginItem.font(0) boldFont.setBold(True) pluginItem.setFont(0, boldFont) if plugin: iBrush = BrushInterface(plugin) if iBrush: addItems(pluginItem, "BrushInterface", iBrush.brushes()) iShape = ShapeInterface(plugin) if iShape: addItems(pluginItem, "ShapeInterface", iShape.shapes()) iFilter = FilterInterface(plugin) if iFilter: addItems(pluginItem, "FilterInterface", iFilter.filters())
Importing Static Plugins#
The Basic Tools plugin is built as a static plugin, to ensure that it is always available to the application. This requires using the
Q_IMPORT_PLUGIN() macro somewhere in the application (in a
.cpp file) and specifying the plugin in the
For Plug & Paint, we have chosen to put
from mainwindow import * from PySide6.QtWidgets import QApplication Q_IMPORT_PLUGIN(BasicToolsPlugin) if __name__ == "__main__": app = QApplication() window = MainWindow() window.show() sys.exit(app.exec())
The argument to
Q_IMPORT_PLUGIN() is the plugin name, which corresponds with the name of the class that declares metadata for the plugin with
.pro file, we need to specify the static library. Here’s the project file for building Plug & Paint:
<Code snippet "tools/plugandpaint/app/app.pro:0" not found>
LIBS line variable specifies the library
pnp_basictools located in the
../plugandpaint/plugins/basictools directory. (Although the
LIBS syntax has a distinct Unix flavor,
qmake supports it on all platforms.)
CONFIG() code at the end is necessary for this example because the example is part of the Qt distribution and Qt can be configured to be built simultaneously in debug and in release modes. You don’t need to for your own plugin applications.
This completes our review of the Plug & Paint application. At this point, you might want to take a look at the Basic Tools example plugin.