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156 lines
7.6 KiB
156 lines
7.6 KiB
14 years ago
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/doc/tutorial.doc:1153 -->
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>Qt Tutorial - Chapter 9: With Cannon You Can</title>
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<font color="#004faf">Home</font></a>
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<font color="#004faf">All Classes</font></a>
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<font color="#004faf">Functions</font></a>
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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Qt Tutorial - Chapter 9: With Cannon You Can</h1>
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<p> <center><img src="t9.png" alt="Screenshot of tutorial nine"></center>
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<p> In this example we become graphic by drawing a cute little blue
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cannon. Only cannon.cpp differs from the previous chapter.
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<p> <ul>
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<li> <a href="t9-lcdrange-h.html">t9/lcdrange.h</a> contains the LCDRange
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class definition.
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<li> <a href="t9-lcdrange-cpp.html">t9/lcdrange.cpp</a> contains the LCDRange
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implementation.
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<li> <a href="t9-cannon-h.html">t9/cannon.h</a> contains the CannonField class
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definition.
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<li> <a href="t9-cannon-cpp.html">t9/cannon.cpp</a> contains the CannonField
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implementation.
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<li> <a href="t9-main-cpp.html">t9/main.cpp</a> contains MyWidget and main.
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</ul>
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<p> <h2> Line-by-line Walkthrough
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</h2>
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<a name="1"></a><p> <h3> <a href="t9-cannon-cpp.html">t9/cannon.cpp</a>
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</h3>
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<a name="1-1"></a><p>
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<p> <pre> <a name="x2346"></a>void CannonField::<a href="qwidget.html#paintEvent">paintEvent</a>( <a href="qpaintevent.html">QPaintEvent</a> * )
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{
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<a href="qpainter.html">QPainter</a> p( this );
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</pre>
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<p> We'll now start to use <a href="qpainter.html">QPainter</a> in earnest. We create a painter that
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operates on this widget.
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<p> <pre> <a name="x2343"></a> p.<a href="qpainter.html#setBrush">setBrush</a>( blue );
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</pre>
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<p> When QPainter fills a rectangle, a circle, or whatever, it fills the
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shape using its brush. Here we set it to use a blue brush. (We
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could also use a pattern.)
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<p> <pre> <a name="x2344"></a> p.<a href="qpainter.html#setPen">setPen</a>( NoPen );
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</pre>
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<p> And the edges of what QPainter draws are drawn using the pen. Here we
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set it to NoPen, meaning that there will be no special edge when we
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draw something; the blue brush will go all the way to the edges of
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the things we draw.
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<p> <pre> <a name="x2345"></a> p.<a href="qpainter.html#translate">translate</a>( 0, rect().bottom() );
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</pre>
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<p> The <a href="qpainter.html#translate">QPainter::translate</a>() function translates the coordinate
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system of the QPainter; i.e., it moves it by an offset. Here we set
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the (0, 0) point to the bottom-left corner of the widget. The x and
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y directions remain unchanged, i.e., all the y coordinates inside the
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widget are now negative (see <a href="coordsys.html">The Coordinate
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System</a> for more information about Qt's coordinate system).
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<p> <pre> <a name="x2340"></a> p.<a href="qpainter.html#drawPie">drawPie</a>( QRect(-35, -35, 70, 70), 0, 90*16 );
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</pre>
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<p> The drawPie() function draws a pie shape inside the specified
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rectangle using a start angle and an arc length. The angles are
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specified in 1/16th of a degree. Zero degrees is at the 3 o'clock
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position. The drawing direction is counter-clockwise. Here we draw a
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quarter of a circle in the bottom-left corner of the widget. The pie
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is filled with blue and has no outline.
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<p> <pre> <a name="x2342"></a> p.<a href="qpainter.html#rotate">rotate</a>( -ang );
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</pre>
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<p> The <a href="qpainter.html#rotate">QPainter::rotate</a>() function rotates the coordinate system of the
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<a href="qpainter.html">QPainter</a> around the origin. The rotation argument is a <tt>float</tt> given
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in degrees (not given in 1/16th of a degree as above) and clockwise.
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Here we rotate the coordinate system <tt>ang</tt> degrees counter-clockwise.
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<p> <pre> <a name="x2341"></a> p.<a href="qpainter.html#drawRect">drawRect</a>( QRect(33, -4, 15, 8) );
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</pre>
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<p> The <a href="qpainter.html#drawRect">QPainter::drawRect</a>() function draws the specified rectangle. Here
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we draw the barrel of the cannon.
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<p> It can often be difficult to envision the resulting drawing when the
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coordinate system has been transformed (translated, rotated, scaled, or
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sheared) as above.
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<p> In this case the coordinate system is first translated and then rotated.
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If the rectangle <a href="qrect.html">QRect</a>(33, -4, 15, 8) had been drawn in the translated
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coordinate system, it would have looked like this:
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<p> <center><img src="t9_1.png" alt="The cannon translated but not rotated"></center>
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<p> Note that the rectangle is clipped by the border of the CannonField
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widget. When we rotate the coordinate system, for instance 60
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degrees, the rectangle will be rotated around (0, 0), which is the
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bottom-left corner because we have translated the coordinate system.
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The result looks like this:
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<p> <center><img src="t9_2.png" alt="The cannon translated and rotated"></center>
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<p> We're done, except that we haven't explained why Windows didn't dither
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this time.
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<p>
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<pre> int main( int argc, char **argv )
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{
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<a name="x2347"></a> QApplication::<a href="qapplication.html#setColorSpec">setColorSpec</a>( QApplication::CustomColor );
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<a href="qapplication.html">QApplication</a> a( argc, argv );
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</pre>
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<p> We tell Qt that we want a different color-allocation strategy for this
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program. There is no single correct color-allocation strategy. Because
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this program uses an unusual yellow but not many colors, <tt>CustomColor</tt> is best. There are several other allocation strategies; you can read about them in the <a href="qapplication.html#setColorSpec">QApplication::setColorSpec</a>()
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documentation.
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<p> Mostly you can ignore this, since the default is good. Occasionally
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some applications with unusual color use look bad; changing the
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allocation strategy often helps then.
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<p> <h2> Behavior
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</h2>
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<a name="2"></a><p> When the slider is operated the angle of the drawn cannon changes
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accordingly.
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<p> The Q on the Quit button is now underlined, and Alt+Q does what you
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think it does. If you do not know why, you didn't do the exercises in
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Chapter 8.
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<p> You may notice that the cannon flickers annoyingly, especially on a
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slow machine. We'll fix this in the next chapter.
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<p> (See <a href="tutorial1-07.html#compiling">Compiling</a> for how to create a
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makefile and build the application.)
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<p> <h2> Exercises
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</h2>
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<a name="3"></a><p> Set a different pen instead of NoPen. Set a patterned brush.
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<p> Try "Q&uit" or "Qu&it" as button text instead of "&Quit". What
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happens?
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<p> You're now ready for <a href="tutorial1-10.html">Chapter 10.</a>
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<p> [<a href="tutorial1-08.html">Previous tutorial</a>]
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[<a href="tutorial1-10.html">Next tutorial</a>]
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[<a href="tutorial.html">Main tutorial page</a>]
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<p>
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<!-- eof -->
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<p><address><hr><div align=center>
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<table width=100% cellspacing=0 border=0><tr>
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<td>Copyright © 2007
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<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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<td align=right><div align=right>Qt 3.3.8</div>
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</table></div></address></body>
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</html>
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