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6<title>Qt Tutorial - Chapter 11: Giving It a Shot</title>
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32<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>
33
34
35<p> <center><img src="t11.png" alt="Screenshot of tutorial eleven"></center>
36<p> In this example we introduce a timer to implement animated shooting.
37<p> <ul>
38<li> <a href="t11-lcdrange-h.html">t11/lcdrange.h</a> contains the LCDRange
39class definition.
40<li> <a href="t11-lcdrange-cpp.html">t11/lcdrange.cpp</a> contains the LCDRange
41implementation.
42<li> <a href="t11-cannon-h.html">t11/cannon.h</a> contains the CannonField class
43definition.
44<li> <a href="t11-cannon-cpp.html">t11/cannon.cpp</a> contains the CannonField
45implementation.
46<li> <a href="t11-main-cpp.html">t11/main.cpp</a> contains MyWidget and main.
47</ul>
48<p> <h2> Line-by-line Walkthrough
49</h2>
50<a name="1"></a><p> <h3> <a href="t11-cannon-h.html">t11/cannon.h</a>
51</h3>
52<a name="1-1"></a><p> The CannonField now has shooting capabilities.
53<p>
54
55<p> <pre> void shoot();
56</pre>
57<p> Calling this slot will make the cannon shoot if a shot is not in the air.
58<p> <pre> private slots:
59 void moveShot();
60</pre>
61<p> This private slot is used to move the shot while it is in the air,
62using a <a href="qtimer.html">QTimer</a>.
63<p> <pre> private:
64 void paintShot( <a href="qpainter.html">QPainter</a> * );
65</pre>
66<p> This private function paints the shot.
67<p> <pre> <a href="qrect.html">QRect</a> shotRect() const;
68</pre>
69<p> This private function returns the shot's enclosing rectangle if
70one is in the air; otherwise the returned rectangle is undefined.
71<p> <pre> int timerCount;
72 <a href="qtimer.html">QTimer</a> * autoShootTimer;
73 float shoot_ang;
74 float shoot_f;
75 };
76</pre>
77<p> These private variables contain information that describes the shot. The
78<tt>timerCount</tt> keeps track of the time passed since the shot was fired.
79The <tt>shoot_ang</tt> is the cannon angle and <tt>shoot_f</tt> is the cannon force
80when the shot was fired.
81<p> <h3> <a href="t11-cannon-cpp.html">t11/cannon.cpp</a>
82</h3>
83<a name="1-2"></a><p>
84
85<p> <pre> #include &lt;math.h&gt;
86</pre>
87<p> We include the math library because we need the sin() and cos() functions.
88<p> <pre> CannonField::CannonField( <a href="qwidget.html">QWidget</a> *parent, const char *name )
89 : <a href="qwidget.html">QWidget</a>( parent, name )
90 {
91 ang = 45;
92 f = 0;
93 timerCount = 0;
94 autoShootTimer = new <a href="qtimer.html">QTimer</a>( this, "movement handler" );
95 <a name="x2379"></a> <a href="qobject.html#connect">connect</a>( autoShootTimer, SIGNAL(<a href="qtimer.html#timeout">timeout</a>()),
96 this, SLOT(moveShot()) );
97 shoot_ang = 0;
98 shoot_f = 0;
99 <a href="qwidget.html#setPalette">setPalette</a>( QPalette( QColor( 250, 250, 200) ) );
100 }
101</pre>
102<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
103shot every time the timer times out.
104<p> <pre> void CannonField::shoot()
105 {
106 <a name="x2376"></a> if ( autoShootTimer-&gt;<a href="qtimer.html#isActive">isActive</a>() )
107 return;
108 timerCount = 0;
109 shoot_ang = ang;
110 shoot_f = f;
111 <a name="x2377"></a> autoShootTimer-&gt;<a href="qtimer.html#start">start</a>( 50 );
112 }
113</pre>
114<p> This function shoots a shot unless a shot is in the air. The <tt>timerCount</tt>
115is reset to zero. The <tt>shoot_ang</tt> and <tt>shoot_f</tt> are set to the current
116cannon angle and force. Finally, we start the timer.
117<p> <pre> void CannonField::moveShot()
118 {
119 <a href="qregion.html">QRegion</a> r( shotRect() );
120 timerCount++;
121
122 <a href="qrect.html">QRect</a> shotR = shotRect();
123
124 if ( shotR.<a href="qrect.html#x">x</a>() &gt; width() || shotR.<a href="qrect.html#y">y</a>() &gt; height() )
125 <a name="x2378"></a> autoShootTimer-&gt;<a href="qtimer.html#stop">stop</a>();
126 else
127 <a name="x2373"></a> r = r.<a href="qrect.html#unite">unite</a>( QRegion( shotR ) );
128 <a href="qwidget.html#repaint">repaint</a>( r );
129 }
130</pre>
131<p> moveShot() is the slot that moves the shot, called every 50
132milliseconds when the <a href="qtimer.html">QTimer</a> fires.
133<p> Its tasks are to compute the new position, repaint the screen with the
134shot in the new position, and if necessary, stop the timer.
135<p> First we make a <a href="qregion.html">QRegion</a> that holds the old shotRect(). A <a href="qregion.html">QRegion</a>
136is capable of holding any sort of region, and we'll use it here to
137simplify the painting. ShotRect() returns the rectangle where the
138shot is now - it is explained in detail later.
139<p> Then we increment the <tt>timerCount</tt>, which has the effect of moving the
140shot one step along its trajectory.
141<p> Next we fetch the new shot rectangle.
142<p> If the shot has moved beyond the right or bottom edge of the widget, we
143stop the timer or we add the new shotRect() to the QRegion.
144<p> Finally, we repaint the QRegion. This will send a single paint event
145for just the one or two rectangles that need updating.
146<p> <pre> void CannonField::<a href="qwidget.html#paintEvent">paintEvent</a>( <a href="qpaintevent.html">QPaintEvent</a> *e )
147 {
148 <a name="x2369"></a> <a href="qrect.html">QRect</a> updateR = e-&gt;<a href="qpaintevent.html#rect">rect</a>();
149 <a href="qpainter.html">QPainter</a> p( this );
150
151 <a name="x2370"></a> if ( updateR.<a href="qrect.html#intersects">intersects</a>( cannonRect() ) )
152 paintCannon( &amp;p );
153 if ( autoShootTimer-&gt;<a href="qtimer.html#isActive">isActive</a>() &amp;&amp;
154 updateR.<a href="qrect.html#intersects">intersects</a>( shotRect() ) )
155 paintShot( &amp;p );
156 }
157</pre>
158<p> The paint event function has been split in two since the previous
159chapter. Now we fetch the bounding rectangle of the region that
160needs painting, check whether it intersects either the cannon and/or
161the shot, and if necessary, call paintCannon() and/or paintShot().
162<p> <pre> void CannonField::paintShot( <a href="qpainter.html">QPainter</a> *p )
163 {
164 p-&gt;<a href="qpainter.html#setBrush">setBrush</a>( black );
165 p-&gt;<a href="qpainter.html#setPen">setPen</a>( NoPen );
166 <a name="x2366"></a> p-&gt;<a href="qpainter.html#drawRect">drawRect</a>( shotRect() );
167 }
168</pre>
169<p> This private function paints the shot by drawing a black filled rectangle.
170<p> We leave out the implementation of paintCannon(); it is the same as
171the paintEvent() from the previous chapter.
172<p> <pre> QRect CannonField::shotRect() const
173 {
174 const double gravity = 4;
175
176 double time = timerCount / 4.0;
177 double velocity = shoot_f;
178 double radians = shoot_ang*3.14159265/180;
179
180 double velx = velocity*cos( radians );
181 double vely = velocity*sin( radians );
182 <a name="x2372"></a> double x0 = ( barrelRect.<a href="qrect.html#right">right</a>() + 5 )*cos(radians);
183 double y0 = ( barrelRect.<a href="qrect.html#right">right</a>() + 5 )*sin(radians);
184 double x = x0 + velx*time;
185 double y = y0 + vely*time - 0.5*gravity*time*time;
186
187 <a href="qrect.html">QRect</a> r = QRect( 0, 0, 6, 6 );
188 <a name="x2371"></a> r.<a href="qrect.html#moveCenter">moveCenter</a>( QPoint( qRound(x), height() - 1 - qRound(y) ) );
189 return r;
190 }
191</pre>
192<p> This private function calculates the center point of the shot and returns
193the enclosing rectangle of the shot. It uses the initial cannon force and
194angle in addition to <tt>timerCount</tt>, which increases as time passes.
195<p> The formula used is the classical Newtonian formula for frictionless
196movement in a gravity field. For simplicity, we've chosen to
197disregard any Einsteinian effects.
198<p> We calculate the center point in a coordinate system where y
199coordinates increase upward. After we have calculated the center
200point, we construct a <a href="qrect.html">QRect</a> with size 6x6 and move its center point to
201the point calculated above. In the same operation we convert the
202point into the widget's coordinate system (see <a href="coordsys.html">The
203Coordinate System</a>).
204<p> The qRound() function is an inline function defined in qglobal.h (included
205by all other Qt header files). qRound() rounds a double to the closest
206integer.
207<p> <h3> <a href="t11-main-cpp.html">t11/main.cpp</a>
208</h3>
209<a name="1-3"></a><p>
210
211<p> <pre> class MyWidget: public <a href="qwidget.html">QWidget</a>
212 {
213 public:
214 MyWidget( <a href="qwidget.html">QWidget</a> *parent=0, const char *name=0 );
215 };
216</pre>
217<p> The only addition is the Shoot button.
218<p> <pre> <a href="qpushbutton.html">QPushButton</a> *shoot = new <a href="qpushbutton.html">QPushButton</a>( "&amp;Shoot", this, "shoot" );
219 <a name="x2382"></a> shoot-&gt;<a href="qwidget.html#setFont">setFont</a>( QFont( "Times", 18, QFont::Bold ) );
220</pre>
221<p> In the constructor we create and set up the Shoot button exactly like we
222did with the Quit button. Note that the first argument to the constructor
223is the button text, and the third is the widget's name.
224<p> <pre> <a href="qobject.html#connect">connect</a>( shoot, SIGNAL(<a href="qbutton.html#clicked">clicked</a>()), cannonField, SLOT(shoot()) );
225</pre>
226<p> Connects the clicked() signal of the Shoot button to the shoot() slot
227of the CannonField.
228<p> <h2> Behavior
229</h2>
230<a name="2"></a><p> The cannon can shoot, but there's nothing to shoot at.
231<p> (See <a href="tutorial1-07.html#compiling">Compiling</a> for how to create a
232makefile and build the application.)
233<p> <h2> Exercises
234</h2>
235<a name="3"></a><p> Make the shot a filled circle. Hint: <a href="qpainter.html#drawEllipse">QPainter::drawEllipse</a>() may
236help.
237<p> Change the color of the cannon when a shot is in the air.
238<p> You're now ready for <a href="tutorial1-12.html">Chapter 12.</a>
239<p> [<a href="tutorial1-10.html">Previous tutorial</a>]
240[<a href="tutorial1-12.html">Next tutorial</a>]
241[<a href="tutorial.html">Main tutorial page</a>]
242<p>
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