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329 lines
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<title>TQt Tutorial - Chapter 12: Hanging in the Air the Way Bricks Don't</title>
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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>TQt Tutorial - Chapter 12: Hanging in the Air the Way Bricks Don't</h1>
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<p> <center><img src="t12.png" alt="Screenshot of tutorial twelve"></center>
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<p> In this example, we extend our LCDRange class to include a text label.
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We also provide something to shoot at.
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<p> <ul>
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<li> <a href="t12-lcdrange-h.html">t12/lcdrange.h</a> contains the LCDRange
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class definition.
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<li> <a href="t12-lcdrange-cpp.html">t12/lcdrange.cpp</a> contains the LCDRange
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implementation.
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<li> <a href="t12-cannon-h.html">t12/cannon.h</a> contains the CannonField class
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definition.
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<li> <a href="t12-cannon-cpp.html">t12/cannon.cpp</a> contains the CannonField
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implementation.
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<li> <a href="t12-main-cpp.html">t12/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="t12-lcdrange-h.html">t12/lcdrange.h</a>
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</h3>
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<a name="1-1"></a><p> The LCDRange now has a text label.
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<p>
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<p> <pre> class TQLabel;
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</pre>
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<p> We name declare <a href="ntqlabel.html">TQLabel</a> because we want to use a pointer to it in the class
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definition.
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<p> <pre> class LCDRange : public <a href="ntqvbox.html">TQVBox</a>
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{
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<a href="metaobjects.html#TQ_OBJECT">TQ_OBJECT</a>
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public:
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LCDRange( <a href="ntqwidget.html">TQWidget</a> *parent=0, const char *name=0 );
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LCDRange( const char *s, TQWidget *parent=0,
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const char *name=0 );
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</pre>
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<p> We have added a new constructor that sets the label text in addition to
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the parent and name.
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<p> <pre> const char *text() const;
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</pre>
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<p> This function returns the label text.
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<p> <pre> void setText( const char * );
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</pre>
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<p> This slot sets the label text.
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<p> <pre> private:
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void init();
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</pre>
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<p> Because we now have two constructors, we have chosen to put the common
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initialization in the private init() function.
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<p> <pre> <a href="ntqlabel.html">TQLabel</a> *label;
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</pre>
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<p> We also have a new private variable: a TQLabel. TQLabel is one of TQt's
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standard widgets and can show a text or a pixmap with or without a
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frame.
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<p> <h3> <a href="t12-lcdrange-cpp.html">t12/lcdrange.cpp</a>
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</h3>
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<a name="1-2"></a><p>
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<p> <pre> #include <<a href="qlabel-h.html">ntqlabel.h</a>>
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</pre>
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<p> Here we include the <a href="ntqlabel.html">TQLabel</a> class definition.
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<p> <pre> LCDRange::LCDRange( <a href="ntqwidget.html">TQWidget</a> *parent, const char *name )
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: <a href="ntqvbox.html">TQVBox</a>( parent, name )
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{
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init();
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}
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</pre>
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<p> This constructor calls the init() function, which contains the common
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initialization code.
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<p> <pre> LCDRange::LCDRange( const char *s, TQWidget *parent,
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const char *name )
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: <a href="ntqvbox.html">TQVBox</a>( parent, name )
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{
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init();
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setText( s );
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}
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</pre>
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<p> This constructor first calls init() and then sets the label text.
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<p> <pre> void LCDRange::init()
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{
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<a href="ntqlcdnumber.html">TQLCDNumber</a> *lcd = new <a href="ntqlcdnumber.html">TQLCDNumber</a>( 2, this, "lcd" );
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slider = new <a href="ntqslider.html">TQSlider</a>( Horizontal, this, "slider" );
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<a name="x2387"></a> slider-><a href="ntqrangecontrol.html#setRange">setRange</a>( 0, 99 );
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<a name="x2388"></a> slider-><a href="ntqslider.html#setValue">setValue</a>( 0 );
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label = new <a href="ntqlabel.html">TQLabel</a>( " ", this, "label" );
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<a name="x2383"></a> label-><a href="ntqlabel.html#setAlignment">setAlignment</a>( AlignCenter );
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<a name="x2389"></a> <a href="ntqobject.html#connect">connect</a>( slider, TQ_SIGNAL(<a href="ntqslider.html#valueChanged">valueChanged</a>(int)),
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<a name="x2386"></a> lcd, TQ_SLOT(<a href="ntqlcdnumber.html#display">display</a>(int)) );
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<a href="ntqobject.html#connect">connect</a>( slider, TQ_SIGNAL(<a href="ntqslider.html#valueChanged">valueChanged</a>(int)),
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TQ_SIGNAL(valueChanged(int)) );
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<a href="ntqwidget.html#setFocusProxy">setFocusProxy</a>( slider );
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}
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</pre>
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<p> The setup of <tt>lcd</tt> and <tt>slider</tt> is the same as in the previous
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chapter. Next we create a <a href="ntqlabel.html">TQLabel</a> and tell it to align the contents
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centered (both vertically and horizontally). The connect() statements
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have also been taken from the previous chapter.
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<p> <pre> const char *LCDRange::text() const
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{
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<a name="x2385"></a> return label-><a href="ntqlabel.html#text">text</a>();
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}
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</pre>
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<p> This function returns the label text.
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<p> <pre> void LCDRange::setText( const char *s )
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{
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<a name="x2384"></a> label-><a href="ntqlabel.html#setText">setText</a>( s );
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}
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</pre>
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<p> This function sets the label text.
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<p> <h3> <a href="t12-cannon-h.html">t12/cannon.h</a>
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</h3>
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<a name="1-3"></a><p> The CannonField now has two new signals: hit() and missed(). In addition
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it contains a target.
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<p>
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<p> <pre> void newTarget();
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</pre>
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<p> This slot creates a target at a new position.
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<p> <pre> signals:
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void hit();
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void missed();
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</pre>
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<p> The hit() signal is emitted when a shot hits the target. The missed()
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signal is emitted when the shot moves beyond the right or bottom edge
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of the widget (i.e., it is certain that it has not and will not
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hit the target).
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<p> <pre> void paintTarget( <a href="ntqpainter.html">TQPainter</a> * );
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</pre>
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<p> This private function paints the target.
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<p> <pre> <a href="ntqrect.html">TQRect</a> targetRect() const;
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</pre>
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<p> This private function returns the enclosing rectangle of the target.
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<p> <pre> <a href="ntqpoint.html">TQPoint</a> target;
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</pre>
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<p> This private variable contains the center point of the target.
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<p> <h3> <a href="t12-cannon-cpp.html">t12/cannon.cpp</a>
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</h3>
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<a name="1-4"></a><p>
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<p> <pre> #include <<a href="qdatetime-h.html">ntqdatetime.h</a>>
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</pre>
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<p> We include the <a href="qdate.html">TQDate</a>, <a href="qtime.html">TQTime</a>, and <a href="ntqdatetime.html">TQDateTime</a> class definitions.
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<p> <pre> #include <stdlib.h>
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</pre>
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<p> We include the stdlib library because we need the rand() function.
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<p> <pre> newTarget();
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</pre>
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<p> This line has been added to the constructor. It creates a "random"
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position for the target. In fact, the newTarget() function will try
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to paint the target. Because we are in a constructor, the CannonField
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widget is invisible. TQt guarantees that no harm is done when calling
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repaint() on a hidden widget.
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<p> <pre> void CannonField::newTarget()
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{
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static bool first_time = TRUE;
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if ( first_time ) {
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first_time = FALSE;
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<a href="qtime.html">TQTime</a> midnight( 0, 0, 0 );
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<a name="x2399"></a><a name="x2398"></a> srand( midnight.<a href="qtime.html#secsTo">secsTo</a>(TQTime::<a href="qtime.html#currentTime">currentTime</a>()) );
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}
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<a href="ntqregion.html">TQRegion</a> r( targetRect() );
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target = TQPoint( 200 + rand() % 190,
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10 + rand() % 255 );
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<a name="x2395"></a> <a href="ntqwidget.html#repaint">repaint</a>( r.<a href="ntqrect.html#unite">unite</a>( targetRect() ) );
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}
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</pre>
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<p> This private function creates a target center point at a new "random"
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position.
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<p> We use the rand() function to fetch random integers. The rand() function
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normally returns the same series of numbers each time you run a program.
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This would make the target appear at the same position every time. To
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avoid this, we must set a random seed the first time this function is
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called. The random seed must also be random in order to avoid equal random
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number series. The solution is to use the number of seconds that have
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passed since midnight as a pseudo-random value.
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<p> First we create a static bool local variable. A static variable like
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this one is guaranteed to keep its value between calls to the function.
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<p> The <tt>if</tt> test will succeed only the first time this function is called
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because we set <tt>first_time</tt> to FALSE inside the <tt>if</tt> block.
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<p> Then we create the <a href="qtime.html">TQTime</a> object <tt>midnight</tt>, which represents the time
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00:00:00. Next we fetch the number of seconds from midnight until
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now and use it as a random seed. See the documentation for <a href="qdate.html">TQDate</a>,
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<a href="qtime.html">TQTime</a>, and <a href="ntqdatetime.html">TQDateTime</a> for more information.
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<p> Finally we calculate the target's center point. We keep it within
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the rectangle (x=200, y=35, width=190, height=255), (i.e., the
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possible x and y values are x = 200..389 and y = 35..289) in a
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coordinate system where we put y position 0 at the bottom edge of the
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widget and let y values increase upwards X is as normal, with 0 at
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the left edge and with x values increasing to the right.
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<p> By experimentation we have found this to always be in reach of the shot.
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<p> Note that rand() returns a random integer >= 0.
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<p> <pre> void CannonField::moveShot()
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{
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<a href="ntqregion.html">TQRegion</a> r( shotRect() );
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timerCount++;
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<a href="ntqrect.html">TQRect</a> shotR = shotRect();
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</pre>
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<p> This part of the timer event has not changed from the previous chapter.
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<p> <pre> if ( shotR.<a href="ntqrect.html#intersects">intersects</a>( targetRect() ) ) {
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<a name="x2400"></a> autoShootTimer-><a href="ntqtimer.html#stop">stop</a>();
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emit hit();
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</pre>
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<p> This <tt>if</tt> statement checks whether the shot rectangle intersects the
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target rectangle. If it does, the shot has hit the target (ouch!).
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We stop the shoot timer and emit the hit() signal to tell the outside
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world that a target was destroyed, and return.
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<p> Note that we could have created a new target on the spot, but because the
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CannonField is a component we leave such decisions to the user of the
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component.
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<p> <pre> <a name="x2397"></a><a name="x2396"></a> } else if ( shotR.<a href="ntqrect.html#x">x</a>() > width() || shotR.<a href="ntqrect.html#y">y</a>() > height() ) {
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autoShootTimer-><a href="ntqtimer.html#stop">stop</a>();
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emit missed();
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</pre>
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<p> This <tt>if</tt> statement is the same as in the previous chapter, except that
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it now emits the missed() signal to tell the outside world about the
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failure.
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<p> <pre> } else {
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</pre>
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<p> And the rest of the function is as before.
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<p> CannonField::paintEvent() is as before, except that this has been
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added:
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<p> <pre> <a name="x2393"></a> if ( updateR.<a href="ntqrect.html#intersects">intersects</a>( targetRect() ) )
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paintTarget( &p );
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</pre>
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<p> These two lines make sure that the target is also painted when necessary.
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<p> <pre> void CannonField::paintTarget( <a href="ntqpainter.html">TQPainter</a> *p )
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{
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p-><a href="ntqpainter.html#setBrush">setBrush</a>( red );
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p-><a href="ntqpainter.html#setPen">setPen</a>( black );
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p-><a href="ntqpainter.html#drawRect">drawRect</a>( targetRect() );
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}
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</pre>
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<p> This private function paints the target; a rectangle filled with red and
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with a black outline.
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<p> <pre> TQRect CannonField::targetRect() const
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{
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<a href="ntqrect.html">TQRect</a> r( 0, 0, 20, 10 );
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<a name="x2394"></a> r.<a href="ntqrect.html#moveCenter">moveCenter</a>( TQPoint(target.x(),height() - 1 - target.y()) );
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return r;
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}
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</pre>
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<p> This private function returns the enclosing rectangle of the target.
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Remember from newTarget() that the <tt>target</tt> point uses y coordinate 0 at
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the bottom of the widget. We calculate the point in widget coordinates
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before we call <a href="ntqrect.html#moveCenter">TQRect::moveCenter</a>().
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<p> The reason we have chosen this coordinate mapping is to fix the distance
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between the target and the bottom of the widget. Remember that the widget
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can be resized by the user or the program at any time.
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<p> <h3> <a href="t12-main-cpp.html">t12/main.cpp</a>
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</h3>
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<a name="1-5"></a><p>
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<p> There are no new members in the MyWidget class, but we have slightly
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changed the constructor to set the new LCDRange text labels.
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<p> <pre> LCDRange *angle = new LCDRange( "ANGLE", this, "angle" );
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</pre>
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<p> We set the angle text label to "ANGLE".
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<p> <pre> LCDRange *force = new LCDRange( "FORCE", this, "force" );
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</pre>
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<p> We set the force text label to "FORCE".
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<p> <h2> Behavior
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</h2>
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<a name="2"></a><p> The LCDRange widgets look a bit strange - the built-in layout
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management in <a href="ntqvbox.html">TQVBox</a> gives the labels too much space and the rest not
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enough. We'll fix that 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> Make a cheat button that, when pressed, makes the CannonField display
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the shot trajectory for five seconds.
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<p> If you did the "round shot" exercise from the previous chapter, try
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changing the shotRect() to a shotRegion() that returns a <a href="ntqregion.html">TQRegion</a> so
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you can have really accurate collision detection.
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<p> Make a moving target.
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<p> Make sure that the target is always created entirely on-screen.
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<p> Make sure that the widget cannot be resized so that the target isn't
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visible. Hint: <a href="ntqwidget.html#setMinimumSize">TQWidget::setMinimumSize</a>() is your friend.
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<p> Not easy; make it possible to have several shots in the air at the
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same time. Hint: make a Shot object.
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<p> You're now ready for <a href="tutorial1-13.html">Chapter 13.</a>
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<p> [<a href="tutorial1-11.html">Previous tutorial</a>]
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[<a href="tutorial1-13.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>TQt 3.3.8</div>
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</table></div></address></body>
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</html>
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