Plot Example

The Plot example shows how to display a graph from data using an opaque container.

It draws an sine graph using QPainter.drawPolyline() from a list of points. The list of points is continuously updated, as is the case for a example for a graph of an oscilloscope or medical patient monitor. In this case, it makes sense from a performance point of view to avoid the conversion of a Python list of data to a C++ list (QList<QPoint>) for each call to the plot function QPainter.drawPolyline(). This is where opaque containers come into play.

Instead of Python list of points, a QPointList is instantiated to store the data. QPointList is an opaque container wrapping a QList<QPoint>. It can be passed to QPainter.drawPolyline() instead of a Python list of points.

The type is declared in the entry for the QList container type in the type system file of the QtCore library:

<container-type name="QList" type="list"
                opaque-containers="int:QIntList;QPoint:QPointList;QPointF:QPointFList">
    ...
</container-type>

In the shift() member function, new data are appended to the list while old data moving out of the visible window are removed from the front of the list.

plot.py

# Copyright (C) 2022 The Qt Company Ltd.
# SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause

import math
import sys

from PySide6.QtWidgets import QWidget, QApplication
from PySide6.QtCore import QPoint, QRect, QTimer, Qt
from PySide6.QtGui import QPainter, QPointList


WIDTH = 680
HEIGHT = 480


class PlotWidget(QWidget):
    """Illustrates the use of opaque containers. QPointList
       wraps a C++ QList<QPoint> directly, removing the need to convert
       a Python list in each call to QPainter.drawPolyline()."""

    def __init__(self, parent=None):
        super().__init__(parent)
        self._timer = QTimer(self)
        self._timer.setInterval(20)
        self._timer.timeout.connect(self.shift)

        self._points = QPointList()
        self._points.reserve(WIDTH)
        self._x = 0
        self._delta_x = 0.05
        self._half_height = HEIGHT / 2
        self._factor = 0.8 * self._half_height

        for i in range(WIDTH):
            self._points.append(QPoint(i, self.next_point()))

        self.setFixedSize(WIDTH, HEIGHT)

        self._timer.start()

    def next_point(self):
        result = self._half_height - self._factor * math.sin(self._x)
        self._x += self._delta_x
        return result

    def shift(self):
        last_x = self._points[WIDTH - 1].x()
        self._points.pop_front()
        self._points.append(QPoint(last_x + 1, self.next_point()))
        self.update()

    def paintEvent(self, event):
        with QPainter(self) as painter:
            rect = QRect(QPoint(0, 0), self.size())
            painter.fillRect(rect, Qt.white)
            painter.translate(-self._points[0].x(), 0)
            painter.drawPolyline(self._points)


if __name__ == "__main__":

    app = QApplication(sys.argv)

    w = PlotWidget()
    w.show()
    sys.exit(app.exec())