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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 5: Building Blocks</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 5: Building Blocks</h1><br clear="all"><p><center><img src="t5.png" alt="Screenshot of tutorial five"></center><p>This example shows how to create and connect together several widgetsusing signals and slots, and how to handle resize events. <pre>/******************************************************************** Qt tutorial 5******************************************************************/#include <<a href="qapplication-h.html">qapplication.h</a>>#include <<a href="qpushbutton-h.html">qpushbutton.h</a>>#include <<a href="qslider-h.html">qslider.h</a>>#include <<a href="qlcdnumber-h.html">qlcdnumber.h</a>>#include <<a href="qfont-h.html">qfont.h</a>>#include <<a href="qvbox-h.html">qvbox.h</a>>class MyWidget : public QVBox{public: MyWidget( <a href="qwidget.html">QWidget</a> *parent=0, const char *name=0 );};MyWidget::MyWidget( <a href="qwidget.html">QWidget</a> *parent, const char *name ) : <a href="qvbox.html">QVBox</a>( parent, name ){ <a href="qpushbutton.html">QPushButton</a> *quit = new <a href="qpushbutton.html">QPushButton</a>( "Quit", this, "quit" ); quit-><a href="qwidget.html#090d60">setFont</a>( <a href="qfont.html">QFont</a>( "Times", 18, QFont::Bold ) ); <a href="qobject.html#fbde73">connect</a>( quit, SIGNAL(clicked()), qApp, SLOT(quit()) ); <a href="qlcdnumber.html">QLCDNumber</a> *lcd = new <a href="qlcdnumber.html">QLCDNumber</a>( 2, this, "lcd" ); <a href="qslider.html">QSlider</a> * slider = new <a href="qslider.html">QSlider</a>( Horizontal, this, "slider" ); slider-><a href="qrangecontrol.html#f2409c">setRange</a>( 0, 99 ); slider-><a href="qslider.html#a03234">setValue</a>( 0 ); <a href="qobject.html#fbde73">connect</a>( slider, SIGNAL(valueChanged(int)), lcd, SLOT(display(int)) );}int main( int argc, char **argv ){ <a href="qapplication.html">QApplication</a> a( argc, argv ); MyWidget w; a.<a href="qapplication.html#7ad759">setMainWidget</a>( &w ); w.<a href="qwidget.html#200ee5">show</a>(); return a.<a href="qapplication.html#84c7bf">exec</a>();}</pre><p><h2>Line by Line Walk-Through</h2> <pre> #include <<a href="qapplication-h.html">qapplication.h</a>> #include <<a href="qpushbutton-h.html">qpushbutton.h</a>> #include <<a href="qslider-h.html">qslider.h</a>> #include <<a href="qlcdnumber-h.html">qlcdnumber.h</a>> #include <<a href="qfont-h.html">qfont.h</a>> #include <<a href="qvbox-h.html">qvbox.h</a>></pre><p>Three new include files here. qslider.h and qlcdnumber.h are therebecause we use two new widgets; QSlider and QLCDNumber. qvbox.h isthere because we use Qt's automatic layout support. <pre> class MyWidget : public QVBox { public: MyWidget( <a href="qwidget.html">QWidget</a> *parent=0, const char *name=0 ); };</pre><p><a name=constructor></a> <pre> MyWidget::MyWidget( <a href="qwidget.html">QWidget</a> *parent, const char *name ) : <a href="qvbox.html">QVBox</a>( parent, name ) {</pre><p>MyWidget is now derived from QVBox instead of QWidget. That way we usethe layout of the QVBox (which places all of its children verticallyinside itself). Resizes are handled automatically by the QVBox andtherefore by MyWidget too, now. <pre> <a href="qlcdnumber.html">QLCDNumber</a> *lcd = new <a href="qlcdnumber.html">QLCDNumber</a>( 2, this, "lcd" );</pre><p><code>lcd</code> is a QLCDNumber, a widget which displays numbers in an LCD-likefashion. This instance is set up to display two digits, be a child of<em>this</em> and is named "lcd". <pre> <a href="qslider.html">QSlider</a> * slider = new <a href="qslider.html">QSlider</a>( Horizontal, this, "slider" ); slider-><a href="qrangecontrol.html#f2409c">setRange</a>( 0, 99 ); slider-><a href="qslider.html#a03234">setValue</a>( 0 );</pre><p>QSlider is a classical slider; a widget where the user can dragsomething to adjust an integer value in a range. Here, we create ahorizontal one, set its range to 0-99 (inclusive, see the <a href="qrangecontrol.html#f2409c">QSlider::setRange()</a> documentation) and its initial value to 0. <pre> <a href="qobject.html#fbde73">connect</a>( slider, SIGNAL(valueChanged(int)), lcd, SLOT(display(int)) );</pre><p>Here we use the <a href="signalsandslots.html">signal/slot mechanism</a>to connect the slider's valueChanged() signal to the LCD number'sdisplay() slot.<p>Whenever the slider's value changes, it broadcasts the new value byemitting the valueChanged() signal. Since that signal is connected tothe LCD number's display() slot, the slot is called when the signal isbroadcast. Neither of the objects know about the other. This isessential in component programming.<p>Slots are otherwise normal C++ member functions and follow the normalC++ access rules.<p><h2>Behavior</h2><p>The LCD number reflects everything you do to the slider, and thewidget handles resizing well. Notice how the LCD number widgetchanges in size when the window is resized (because it can), but theothers stay about the same (because they would look stupid otherwise).<p><h2>Exercises</h2><p>Try changing the LCD number to add more digits or <a href="qlcdnumber.html#4521f9">change mode.</a> You can even add four pushbuttons to set the number base.<p>You can also change the slider's range.<p>Perhaps it would have been better to use <a href="qspinbox.html">QSpinBox</a> than a slider?<p>Try to make the application quit when the LCD number overflows.<p>You may now go on to <a href="t6.html">chapter six.</a><p>[<a href="t4.html">Previous tutorial</a>][<a href="t6.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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