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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Qt Tutorial - Chapter 9: With Cannon You Can</title></head><body bgcolor="#ffffff"><p><table width="100%"><tr><td><a href="index.html"><img width="100" height="100" src="qtlogo.png"alt="Home" border="0"><img width="100"height="100" src="face.png" alt="Home" border="0"></a><td valign="top"><div align="right"><img src="dochead.png" width="472" height="27"><br><a href="classes.html"><b>Classes</b></a>- <a href="annotated.html">Annotated</a>- <a href="hierarchy.html">Tree</a>- <a href="functions.html">Functions</a>- <a href="index.html">Home</a>- <a href="topicals.html"><b>Structure</b> <font face="Arial,Helvetica,Geneva,Swiss,SunSans-Regular" align="center" size=32>Qte</font></a></div></table><p><h1 align=center>Chapter 9: With Cannon You Can</h1><br clear="all"><p><center><img src="t9.png" alt="Screenshot of tutorial nine"></center><p>In this example, we become graphic, by drawing a cute little bluecannon. Only cannon.cpp differs from the previous chapter.<p><ul><li><a href="t9-lcdrange-h.html">lcdrange.h</a> contains the LCDRangeclass definition<li><a href="t9-lcdrange-cpp.html">lcdrange.cpp</a> contains the LCDRangeimplementation<li><a href="t9-cannon-h.html">cannon.h</a> contains the CannonField classdefinition<li><a href="t9-cannon-cpp.html">cannon.cpp</a> contains the CannonFieldimplementation<li><a href="t9-main-cpp.html">main.cpp</a> contains MyWidget and main.<li><a href="t9-makefile.html">Makefile</a> contains some rules forgenerating the meta object information necessary for <ahref="signalsandslots.html">signal/slot creation.</a></ul><p><h2>Line by Line Walk-Through</h2><p><h3><a href="t9-cannon-cpp.html">cannon.cpp</a></h3> <pre> void CannonField::paintEvent( <a href="qpaintevent.html">QPaintEvent</a> * ) { <a href="qpainter.html">QPainter</a> p( this );</pre><p>We'll now start to use QPainter in earnest. We create a painter thatoperates on this widget. <pre> p.<a href="qpainter.html#3e0cc8">setBrush</a>( blue );</pre><p>When QPainter fills a rectangle, a circle or something, it fills theshape using its brush. Here, we set it to use a blue brush. (Wecould also use a pattern.) <pre> p.<a href="qpainter.html#0183e4">setPen</a>( NoPen );</pre><p>And the edges of what QPainter draws are drawn using the pen. Here weset it to NoPen, meaning that there will be no special edge when wedraw something. The blue brush will go all the way to the edges ofthe things we draw. <pre> p.<a href="qpainter.html#eb778c">translate</a>( 0, <a href="qwidget.html#75ae71">rect</a>().bottom() );</pre><p>The <a href="qpainter.html#eb778c">QPainter::translate()</a> function translates the coordinate systemof the QPainter, i.e. moves it by an offset. Here we set the (0,0)point to the bottom left corner of the widget. The x and y directionsremain unchanged, i.e. all the y coordinates inside the widget are nownegative (see <a href="coordsys.html">The Coordinate System</a> formore information about Qt's coordinate system). <pre> p.<a href="qpainter.html#3ca7a2">drawPie</a>( <a href="qrect.html">QRect</a>(-35, -35, 70, 70), 0, 90*16 );</pre><p>The drawPie() function draws a pie shape inside the specifiedrectangle using a start angle and an arc length. The angles arespecified in 1/16th of a degree. Zero degrees is at the 3 o'clockposition. The drawing direction is counter-clockwise. Here we draw aquarter of a circle in the bottom left corner of the widget. The pieis filled with blue and has no outline. <pre> p.<a href="qpainter.html#b5205c">rotate</a>( -ang );</pre><p>The QPainter::rotate() function rotates the coordinate system of theQPainter around the origin. The rotation argument is a <code>float</code> givenin degrees (not given in 1/16th of a degree as above) and clockwise.Here we rotate the coordinate system <code>ang</code> degrees counter-clockwise. <pre> p.<a href="qpainter.html#4c0077">drawRect</a>( <a href="qrect.html">QRect</a>(33, -4, 15, 8) );</pre><p>The QPainter::drawRect() function draws the specified rectangle. Herewe draw the barrel of the cannon.<p>It can often be difficult to envision the resulting drawing when thecoordinate system has been transformed (translated, rotated, scaled orsheared) as above.<p>In this case, the coordinate system is first translated, then rotated.If the rectangle QRect(33, -4, 15, 8) had been drawn in the translatedcoordinate system, it would have looked like this:<p><img src="t9_1.png" alt="The cannon, translated but not rotated"><p>Note that the rectangle is clipped by the border of the CannonFieldwidget. When we rotate the coordinate system, for instance 60degrees, the rectangle will be rotated around (0,0), which is thebottom left corner, since we have translated the coordinate system.The result looks like this:<p><img src="t9_2.png" alt="The cannon, translated and rotated"><p>We're done, except that we haven't explained why Windows didn't ditherthis time. <pre> int main( int argc, char **argv ) { <a href="qapplication.html#1ee2d1">QApplication::setColorSpec</a>( QApplication::CustomColor ); <a href="qapplication.html">QApplication</a> a( argc, argv );</pre><p>We tell Qt that we want a different color allocation strategy for thisprogram. There is no single correct color allocation strategy. Sincethis program uses an unusual yellow but not many colors, <code>CustomColor</code> is best. There are several other allocation strategies.You can read about them in the <a href="qapplication.html#1ee2d1">QApplication::setColorSpec()</a>documentation.<p>Mostly, you can ignore this - the default is good. Occasionally someapplications with unusual color use look bad, and often changing theallocation strategy helps.<p><h2>Behavior</h2><p>When the slider is operated, the angle of the drawn cannon changesaccordingly.<p>The Q on the Quit button is now underlined, and Alt-Q does what youthink it does. If you do not know why, you didn't do the exercises inchapter eight.<p>You may notice that the cannon flickers annoyingly, especially on aslow machine. We'll fix this in the next chapter.<p><h2>Exercises</h2><p>Set a different pen instead of NoPen. Set a patterned brush.<p>Try "Q&uit" or "Qu&it" as button text instead of "&Quit" - whathappens?<p>You may now go on to <a href="t10.html">chapter ten.</a><p>[<a href="t8.html">Previous tutorial</a>][<a href="t10.html">Next tutorial</a>][<a href="tutorial.html">Main tutorial page</a>]<p><address><hr><div align="center"><table width="100%" cellspacing="0" border="0"><tr><td>Copyright ⌨️ 快捷键说明
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