Raster Window Example#
This example shows how to create a minimal
QWindow based application using
QPainter for rendering.
Application Entry Point#
if __name__ == "__main__": app = QGuiApplication(argc, argv) window = RasterWindow() window.show() sys.exit(app.exec())
The entry point for a
QWindow based application is the
QGuiApplication class. It manages the GUI application’s control flow and main settings. We pass the command line arguments which can be used to pick up certain system wide options.
From there, we go on to create our window instance and then call the
show() function to tell the windowing system that this window should now be made visible on screen.
Once this is done, we enter the application’s event loop so the application can run.
from PySide6 import QtGui class RasterWindow(QWindow): Q_OBJECT # public RasterWindow = explicit(QWindow parent = None) void = virtual(QPainter painter) # public slots def renderLater(): def renderNow(): # protected event = bool(QEvent event) def resizeEvent(event): def exposeEvent(event): # private m_backingStore = QScopedPointer()
We first start by including the
<QtGui> header. This means we can use all classes in the Qt GUI module. Classes can also be included individually if that is preferred.
The RasterWindow class subclasses
QWindow directly and provides a constructor which allows the window to be a sub-window of another
QWindow . Parent-less QWindows show up in the windowing system as top-level windows.
The class declares a
QBackingStore which is what we use to manage the window’s back buffer for
QPainter based graphics.
The raster window is also reused in a few other examples and adds a few helper functions, like renderLater().
def __init__(self, parent): super().__init__(parent) , m_backingStore(QBackingStore(self)) setGeometry(100, 100, 300, 200)
In the constructor we create the backingstore and pass it the window instance it is supposed to manage. We also set the initial window geometry.
def exposeEvent(self, arg__0): if isExposed(): renderNow()
Shortly after calling
show() on a created window, the virtual function
exposeEvent() will be called to notify us that the window’s exposure in the windowing system has changed. The event contains the exposed sub-region, but since we will anyway draw the entire window every time, we do not make use of that.
isExposed() will tell us if the window is showing or not. We need this as the exposeEvent is called also when the window becomes obscured in the windowing system. If the window is showing, we call renderNow() to draw the window immediately. We want to draw right away so we can present the system with some visual content.
def resizeEvent(self, resizeEvent): m_backingStore.resize(resizeEvent.size())
The resize event is guaranteed to be called prior to the window being shown on screen and will also be called whenever the window is resized while on screen. We use this to resize the back buffer, and defer rendering to the corresponding/following expose event.
def renderNow(self): if not isExposed(): return rect = QRect(0, 0, width(), height()) m_backingStore.beginPaint(rect) device = m_backingStore.paintDevice() painter = QPainter(device) painter.fillRect(0, 0, width(), height(), QGradient.NightFade) render(painter) painter.end() m_backingStore.endPaint() m_backingStore.flush(rect)
The renderNow function sets up what is needed for a
QWindow to render its content using
QPainter . As obscured windows have will not be visible, we abort if the window is not exposed in the windowing system. This can for instance happen when another window fully obscures this window.
We start the drawing by calling
beginPaint() on the region we want to draw. Then we get the
QPaintDevice of the back buffer and create a
QPainter to render to that paint device.
To void leaving traces from the previous rendering and start with a clean buffer, we fill the entire buffer with the color white. Then we call the virtual render() function which does the actual drawing of this window.
After drawing is complete, we call endPaint() to signal that we are done rendering and present the contents in the back buffer using
def render(self, painter): painter.drawText(QRectF(0, 0, width(), height()), Qt.AlignCenter, QStringLiteral("QWindow"))
The render function contains the drawing code for the window. In this minial example, we only draw the string ”
QWindow ” in the center.
def renderLater(self): requestUpdate()
We went through a few places where the window needed to repainted immediately. There are some cases where this is not desirable, but rather let the application return to the event loop and schedule the repaint for later. We achieve this by requesting an update, using
requestUpdate() , which will then be delivered when the system is ready to repaint.
def event(self, QEvent event): if event.type() == QEvent.UpdateRequest: renderNow() return True return QWindow.event(event)
We reimplement the virtual
event() function to handle the update event. When the event comes in we call renderNow() to render the window right away.