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251 lines
11 KiB
251 lines
11 KiB
14 years ago
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<!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/doc/tutorial.doc:1506 -->
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>Qt Tutorial - Chapter 11: Giving It a Shot</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 11: Giving It a Shot</h1>
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<p> <center><img src="t11.png" alt="Screenshot of tutorial eleven"></center>
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<p> In this example we introduce a timer to implement animated shooting.
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<p> <ul>
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<li> <a href="t11-lcdrange-h.html">t11/lcdrange.h</a> contains the LCDRange
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class definition.
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<li> <a href="t11-lcdrange-cpp.html">t11/lcdrange.cpp</a> contains the LCDRange
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implementation.
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<li> <a href="t11-cannon-h.html">t11/cannon.h</a> contains the CannonField class
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definition.
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<li> <a href="t11-cannon-cpp.html">t11/cannon.cpp</a> contains the CannonField
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implementation.
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<li> <a href="t11-main-cpp.html">t11/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="t11-cannon-h.html">t11/cannon.h</a>
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</h3>
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<a name="1-1"></a><p> The CannonField now has shooting capabilities.
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<p>
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<p> <pre> void shoot();
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</pre>
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<p> Calling this slot will make the cannon shoot if a shot is not in the air.
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<p> <pre> private slots:
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void moveShot();
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</pre>
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<p> This private slot is used to move the shot while it is in the air,
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using a <a href="qtimer.html">QTimer</a>.
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<p> <pre> private:
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void paintShot( <a href="qpainter.html">QPainter</a> * );
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</pre>
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<p> This private function paints the shot.
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<p> <pre> <a href="qrect.html">QRect</a> shotRect() const;
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</pre>
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<p> This private function returns the shot's enclosing rectangle if
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one is in the air; otherwise the returned rectangle is undefined.
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<p> <pre> int timerCount;
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<a href="qtimer.html">QTimer</a> * autoShootTimer;
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float shoot_ang;
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float shoot_f;
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};
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</pre>
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<p> These private variables contain information that describes the shot. The
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<tt>timerCount</tt> keeps track of the time passed since the shot was fired.
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The <tt>shoot_ang</tt> is the cannon angle and <tt>shoot_f</tt> is the cannon force
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when the shot was fired.
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<p> <h3> <a href="t11-cannon-cpp.html">t11/cannon.cpp</a>
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</h3>
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<a name="1-2"></a><p>
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<p> <pre> #include <math.h>
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</pre>
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<p> We include the math library because we need the sin() and cos() functions.
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<p> <pre> CannonField::CannonField( <a href="qwidget.html">QWidget</a> *parent, const char *name )
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: <a href="qwidget.html">QWidget</a>( parent, name )
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{
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ang = 45;
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f = 0;
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timerCount = 0;
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autoShootTimer = new <a href="qtimer.html">QTimer</a>( this, "movement handler" );
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<a name="x2379"></a> <a href="qobject.html#connect">connect</a>( autoShootTimer, SIGNAL(<a href="qtimer.html#timeout">timeout</a>()),
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this, SLOT(moveShot()) );
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shoot_ang = 0;
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shoot_f = 0;
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<a href="qwidget.html#setPalette">setPalette</a>( QPalette( QColor( 250, 250, 200) ) );
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}
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</pre>
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<p> We initialize our new private variables and connect the <a href="qtimer.html#timeout">QTimer::timeout</a>() signal to our moveShot() slot. We'll move the
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shot every time the timer times out.
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<p> <pre> void CannonField::shoot()
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{
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<a name="x2376"></a> if ( autoShootTimer-><a href="qtimer.html#isActive">isActive</a>() )
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return;
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timerCount = 0;
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shoot_ang = ang;
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shoot_f = f;
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<a name="x2377"></a> autoShootTimer-><a href="qtimer.html#start">start</a>( 50 );
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}
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</pre>
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<p> This function shoots a shot unless a shot is in the air. The <tt>timerCount</tt>
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is reset to zero. The <tt>shoot_ang</tt> and <tt>shoot_f</tt> are set to the current
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cannon angle and force. Finally, we start the timer.
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<p> <pre> void CannonField::moveShot()
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{
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<a href="qregion.html">QRegion</a> r( shotRect() );
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timerCount++;
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<a href="qrect.html">QRect</a> shotR = shotRect();
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if ( shotR.<a href="qrect.html#x">x</a>() > width() || shotR.<a href="qrect.html#y">y</a>() > height() )
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<a name="x2378"></a> autoShootTimer-><a href="qtimer.html#stop">stop</a>();
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else
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<a name="x2373"></a> r = r.<a href="qrect.html#unite">unite</a>( QRegion( shotR ) );
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<a href="qwidget.html#repaint">repaint</a>( r );
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}
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</pre>
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<p> moveShot() is the slot that moves the shot, called every 50
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milliseconds when the <a href="qtimer.html">QTimer</a> fires.
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<p> Its tasks are to compute the new position, repaint the screen with the
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shot in the new position, and if necessary, stop the timer.
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<p> First we make a <a href="qregion.html">QRegion</a> that holds the old shotRect(). A <a href="qregion.html">QRegion</a>
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is capable of holding any sort of region, and we'll use it here to
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simplify the painting. ShotRect() returns the rectangle where the
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shot is now - it is explained in detail later.
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<p> Then we increment the <tt>timerCount</tt>, which has the effect of moving the
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shot one step along its trajectory.
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<p> Next we fetch the new shot rectangle.
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<p> If the shot has moved beyond the right or bottom edge of the widget, we
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stop the timer or we add the new shotRect() to the QRegion.
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<p> Finally, we repaint the QRegion. This will send a single paint event
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for just the one or two rectangles that need updating.
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<p> <pre> void CannonField::<a href="qwidget.html#paintEvent">paintEvent</a>( <a href="qpaintevent.html">QPaintEvent</a> *e )
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{
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<a name="x2369"></a> <a href="qrect.html">QRect</a> updateR = e-><a href="qpaintevent.html#rect">rect</a>();
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<a href="qpainter.html">QPainter</a> p( this );
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<a name="x2370"></a> if ( updateR.<a href="qrect.html#intersects">intersects</a>( cannonRect() ) )
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paintCannon( &p );
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if ( autoShootTimer-><a href="qtimer.html#isActive">isActive</a>() &&
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updateR.<a href="qrect.html#intersects">intersects</a>( shotRect() ) )
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paintShot( &p );
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}
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</pre>
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<p> The paint event function has been split in two since the previous
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chapter. Now we fetch the bounding rectangle of the region that
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needs painting, check whether it intersects either the cannon and/or
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the shot, and if necessary, call paintCannon() and/or paintShot().
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<p> <pre> void CannonField::paintShot( <a href="qpainter.html">QPainter</a> *p )
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{
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p-><a href="qpainter.html#setBrush">setBrush</a>( black );
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p-><a href="qpainter.html#setPen">setPen</a>( NoPen );
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<a name="x2366"></a> p-><a href="qpainter.html#drawRect">drawRect</a>( shotRect() );
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}
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</pre>
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<p> This private function paints the shot by drawing a black filled rectangle.
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<p> We leave out the implementation of paintCannon(); it is the same as
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the paintEvent() from the previous chapter.
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<p> <pre> QRect CannonField::shotRect() const
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{
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const double gravity = 4;
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double time = timerCount / 4.0;
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double velocity = shoot_f;
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double radians = shoot_ang*3.14159265/180;
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double velx = velocity*cos( radians );
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double vely = velocity*sin( radians );
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<a name="x2372"></a> double x0 = ( barrelRect.<a href="qrect.html#right">right</a>() + 5 )*cos(radians);
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double y0 = ( barrelRect.<a href="qrect.html#right">right</a>() + 5 )*sin(radians);
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double x = x0 + velx*time;
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double y = y0 + vely*time - 0.5*gravity*time*time;
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<a href="qrect.html">QRect</a> r = QRect( 0, 0, 6, 6 );
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<a name="x2371"></a> r.<a href="qrect.html#moveCenter">moveCenter</a>( QPoint( qRound(x), height() - 1 - qRound(y) ) );
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return r;
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}
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</pre>
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<p> This private function calculates the center point of the shot and returns
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the enclosing rectangle of the shot. It uses the initial cannon force and
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angle in addition to <tt>timerCount</tt>, which increases as time passes.
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<p> The formula used is the classical Newtonian formula for frictionless
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movement in a gravity field. For simplicity, we've chosen to
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disregard any Einsteinian effects.
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<p> We calculate the center point in a coordinate system where y
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coordinates increase upward. After we have calculated the center
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point, we construct a <a href="qrect.html">QRect</a> with size 6x6 and move its center point to
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the point calculated above. In the same operation we convert the
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point into the widget's coordinate system (see <a href="coordsys.html">The
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Coordinate System</a>).
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<p> The qRound() function is an inline function defined in qglobal.h (included
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by all other Qt header files). qRound() rounds a double to the closest
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integer.
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<p> <h3> <a href="t11-main-cpp.html">t11/main.cpp</a>
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</h3>
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<a name="1-3"></a><p>
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<p> <pre> class MyWidget: public <a href="qwidget.html">QWidget</a>
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{
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public:
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MyWidget( <a href="qwidget.html">QWidget</a> *parent=0, const char *name=0 );
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};
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</pre>
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<p> The only addition is the Shoot button.
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<p> <pre> <a href="qpushbutton.html">QPushButton</a> *shoot = new <a href="qpushbutton.html">QPushButton</a>( "&Shoot", this, "shoot" );
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<a name="x2382"></a> shoot-><a href="qwidget.html#setFont">setFont</a>( QFont( "Times", 18, QFont::Bold ) );
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</pre>
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<p> In the constructor we create and set up the Shoot button exactly like we
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did with the Quit button. Note that the first argument to the constructor
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is the button text, and the third is the widget's name.
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<p> <pre> <a href="qobject.html#connect">connect</a>( shoot, SIGNAL(<a href="qbutton.html#clicked">clicked</a>()), cannonField, SLOT(shoot()) );
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</pre>
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<p> Connects the clicked() signal of the Shoot button to the shoot() slot
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of the CannonField.
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<p> <h2> Behavior
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</h2>
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<a name="2"></a><p> The cannon can shoot, but there's nothing to shoot at.
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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> Make the shot a filled circle. Hint: <a href="qpainter.html#drawEllipse">QPainter::drawEllipse</a>() may
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help.
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<p> Change the color of the cannon when a shot is in the air.
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<p> You're now ready for <a href="tutorial1-12.html">Chapter 12.</a>
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<p> [<a href="tutorial1-10.html">Previous tutorial</a>]
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[<a href="tutorial1-12.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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