// Copyright (C) 2016 The Qt Company Ltd. // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only /*! \example widgets/analogclock \examplecategory {Graphics} \meta tags {widgets} \title Analog Clock \ingroup examples-widgets \brief The Analog Clock example shows how to draw the contents of a custom widget. \borderedimage analogclock-example.png \caption Screenshot of the Analog Clock example This example also demonstrates how the transformation and scaling features of QPainter can be used to make drawing custom widgets easier. \section1 AnalogClock Class Definition The \c AnalogClock class provides a clock widget with hour, minute and second hands that is automatically updated every second. We subclass \l QWidget and reimplement the standard \l{QWidget::paintEvent()}{paintEvent()} function to draw the clock face: \snippet widgets/analogclock/analogclock.h 0 \section1 AnalogClock Class Implementation When the widget is constructed, we set up a one-second timer to keep track of the current time, and we connect it to the standard \l{QWidget::update()}{update()} slot so that the clock face is updated when the timer emits the \l{QTimer::timeout()}{timeout()} signal. Finally, we resize the widget so that it is displayed at a reasonable size. \snippet widgets/analogclock/analogclock.cpp 1 The \c paintEvent() function is called whenever the widget's contents need to be updated. This happens when the widget is first shown, and when it is covered then exposed, but it is also executed when the widget's \l{QWidget::update()}{update()} slot is called. Since we connected the timer's \l{QTimer::timeout()}{timeout()} signal to this slot, it will be called at least once per second. Before we set up the painter and draw the clock, we first define three lists of \l {QPoint}s and three \l{QColor}s that will be used for the hour, minute and second hands. We use the \l{QWidget::palette()}{palette()} function to get appropriate colors that fit into the rest of the window, both in light and dark mode. The hour and minute hands are drawn in the foreground color, the second hand is drawn in the accent color. We also determine the length of the widget's shortest side so that we can fit the clock face inside the widget. It is also useful to determine the current time before we start drawing. \snippet widgets/analogclock/analogclock.cpp 8 \snippet widgets/analogclock/analogclock.cpp 10 The contents of custom widgets are drawn with a QPainter. Painters can be used to draw on any QPaintDevice, but they are usually used with widgets, so we pass the widget instance to the painter's constructor. \snippet widgets/analogclock/analogclock.cpp 11 \snippet widgets/analogclock/analogclock.cpp 14 We call QPainter::setRenderHint() with QPainter::Antialiasing to turn on antialiasing. This makes drawing of diagonal lines much smoother. \snippet widgets/analogclock/analogclock.cpp 12 The translation moves the origin to the center of the widget, and the scale operation ensures that the following drawing operations are scaled to fit within the widget. We use a scale factor that let's us use x and y coordinates between -100 and 100, and that ensures that these lie within the length of the widget's shortest side. \snippet widgets/analogclock/analogclock.cpp 13 To make our code simpler, we will draw a fixed size clock face that will be positioned and scaled so that it lies in the center of the widget. The painter takes care of all the transformations made during the paint event, and ensures that everything is drawn correctly. Letting the painter handle transformations is often easier than performing manual calculations just to draw the contents of a custom widget. \image analogclock-viewport.png We set the pen to be Qt::NoPen because we don't want any outline, and we use a solid brush with the color appropriate for displaying hours. Brushes are used when filling in polygons and other geometric shapes. \snippet widgets/analogclock/analogclock.cpp 15 \snippet widgets/analogclock/analogclock.cpp 16 We draw the hour hand first, using a formula that rotates the coordinate system counterclockwise by a number of degrees determined by the current hour and minute. This means that the hand will be shown rotated clockwise by the required amount. We save and restore the transformation matrix before and after the rotation by instantiating a QPainterStateGuard because we want to place the minute hand without having to take into account any previous rotations. \snippet widgets/analogclock/analogclock.cpp 19 We draw markers around the edge of the clock for each hour in the same color as the hour hand. We draw each marker then rotate the coordinate system so that the painter is ready for the next one. \snippet widgets/analogclock/analogclock.cpp 20 The minute hand is rotated and painted in a similar way to the hour hand. \snippet widgets/analogclock/analogclock.cpp 21 For the seconds hand we do the same and add two circles as a visual highlight. \snippet widgets/analogclock/analogclock.cpp 23 \codeline \snippet widgets/analogclock/analogclock.cpp 24 Finally, we draw markers around the edge of the clock, indicating minutes and seconds. This time we draw them as lines and therefore set the pen to the respective color. \snippet widgets/analogclock/analogclock.cpp 25 \codeline \snippet widgets/analogclock/analogclock.cpp 27 */