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135 lines
5.4 KiB
Plaintext
135 lines
5.4 KiB
Plaintext
// Copyright (C) 2016 The Qt Company Ltd.
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// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
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/*!
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\example widgets/analogclock
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\examplecategory {Graphics & Multimedia}
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\meta tags {widgets}
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\title Analog Clock
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\ingroup examples-widgets
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\brief The Analog Clock example shows how to draw the contents of a
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custom widget.
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\borderedimage analogclock-example.png
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\caption Screenshot of the Analog Clock example
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This example also demonstrates how the transformation and scaling
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features of QPainter can be used to make drawing custom widgets
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easier.
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\section1 AnalogClock Class Definition
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The \c AnalogClock class provides a clock widget with hour and minute
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hands that is automatically updated every few seconds.
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We subclass \l QWidget and reimplement the standard
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\l{QWidget::paintEvent()}{paintEvent()} function to draw the clock face:
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\snippet widgets/analogclock/analogclock.h 0
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\section1 AnalogClock Class Implementation
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\snippet widgets/analogclock/analogclock.cpp 1
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When the widget is constructed, we set up a one-second timer to
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keep track of the current time, and we connect it to the standard
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\l{QWidget::update()}{update()} slot so that the clock face is
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updated when the timer emits the \l{QTimer::timeout()}{timeout()}
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signal.
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Finally, we resize the widget so that it is displayed at a
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reasonable size.
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\snippet widgets/analogclock/analogclock.cpp 8
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\snippet widgets/analogclock/analogclock.cpp 10
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The \c paintEvent() function is called whenever the widget's
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contents need to be updated. This happens when the widget is
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first shown, and when it is covered then exposed, but it is also
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executed when the widget's \l{QWidget::update()}{update()} slot
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is called. Since we connected the timer's
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\l{QTimer::timeout()}{timeout()} signal to this slot, it will be
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called at least once every five seconds.
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Before we set up the painter and draw the clock, we first define
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two lists of \l {QPoint}s and two \l{QColor}s that will be used
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for the hour and minute hands. The minute hand's color has an
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alpha component of 191, meaning that it's 75% opaque.
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We also determine the length of the widget's shortest side so that we
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can fit the clock face inside the widget. It is also useful to determine
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the current time before we start drawing.
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\snippet widgets/analogclock/analogclock.cpp 11
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\snippet widgets/analogclock/analogclock.cpp 12
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\snippet widgets/analogclock/analogclock.cpp 13
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\snippet widgets/analogclock/analogclock.cpp 14
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The contents of custom widgets are drawn with a QPainter.
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Painters can be used to draw on any QPaintDevice, but they are
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usually used with widgets, so we pass the widget instance to the
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painter's constructor.
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We call QPainter::setRenderHint() with QPainter::Antialiasing to
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turn on antialiasing. This makes drawing of diagonal lines much
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smoother.
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The translation moves the origin to the center of the widget, and
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the scale operation ensures that the following drawing operations
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are scaled to fit within the widget. We use a scale factor that
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let's us use x and y coordinates between -100 and 100, and that
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ensures that these lie within the length of the widget's shortest
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side.
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To make our code simpler, we will draw a fixed size clock face that will
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be positioned and scaled so that it lies in the center of the widget.
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The painter takes care of all the transformations made during the
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paint event, and ensures that everything is drawn correctly. Letting
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the painter handle transformations is often easier than performing
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manual calculations just to draw the contents of a custom widget.
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\image analogclock-viewport.png
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We draw the hour hand first, using a formula that rotates the coordinate
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system counterclockwise by a number of degrees determined by the current
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hour and minute. This means that the hand will be shown rotated clockwise
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by the required amount.
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\snippet widgets/analogclock/analogclock.cpp 15
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\snippet widgets/analogclock/analogclock.cpp 16
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We set the pen to be Qt::NoPen because we don't want any outline,
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and we use a solid brush with the color appropriate for
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displaying hours. Brushes are used when filling in polygons and
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other geometric shapes.
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\snippet widgets/analogclock/analogclock.cpp 17
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\snippet widgets/analogclock/analogclock.cpp 19
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We save and restore the transformation matrix before and after the
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rotation because we want to place the minute hand without having to
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take into account any previous rotations.
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\snippet widgets/analogclock/analogclock.cpp 20
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\codeline
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\snippet widgets/analogclock/analogclock.cpp 21
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We draw markers around the edge of the clock for each hour. We
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draw each marker then rotate the coordinate system so that the
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painter is ready for the next one.
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\snippet widgets/analogclock/analogclock.cpp 22
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\snippet widgets/analogclock/analogclock.cpp 23
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The minute hand is rotated in a similar way to the hour hand.
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\snippet widgets/analogclock/analogclock.cpp 25
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\codeline
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\snippet widgets/analogclock/analogclock.cpp 26
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Again, we draw markers around the edge of the clock, but this
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time to indicate minutes. We skip multiples of 5 to avoid drawing
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minute markers on top of hour markers.
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*/
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