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183 lines
8.5 KiB
183 lines
8.5 KiB
14 years ago
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/doc/tutorial.doc:378 -->
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>Qt Tutorial - Chapter 4: Let There Be Widgets</title>
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<style type="text/css"><!--
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fn { margin-left: 1cm; text-indent: -1cm; }
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a:link { color: #004faf; text-decoration: none }
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a:visited { color: #672967; text-decoration: none }
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body { background: #ffffff; color: black; }
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--></style>
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</head>
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<body>
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<table border="0" cellpadding="0" cellspacing="0" width="100%">
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<tr bgcolor="#E5E5E5">
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<td valign=center>
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<a href="index.html">
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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">Main Classes</font></a>
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<font color="#004faf">Annotated</font></a>
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<font color="#004faf">Grouped Classes</font></a>
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<font color="#004faf">Functions</font></a>
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</td>
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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 4: Let There Be Widgets</h1>
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<p> <center><img src="t4.png" alt="Screenshot of tutorial four"></center>
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<p> This example shows how to create your own widget, describes how to control the
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minimum and maximum sizes of a widget, and introduces widget names.
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<p> <pre>/****************************************************************
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**
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** Qt tutorial 4
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**
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****************************************************************/
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#include <<a href="qapplication-h.html">qapplication.h</a>>
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#include <<a href="qpushbutton-h.html">qpushbutton.h</a>>
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#include <<a href="qfont-h.html">qfont.h</a>>
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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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<a name="f552"></a>MyWidget::MyWidget( <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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<a href="qwidget.html#setMinimumSize">setMinimumSize</a>( 200, 120 );
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<a href="qwidget.html#setMaximumSize">setMaximumSize</a>( 200, 120 );
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<a href="qpushbutton.html">QPushButton</a> *quit = new <a href="qpushbutton.html">QPushButton</a>( "Quit", this, "quit" );
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quit-><a href="qwidget.html#setGeometry">setGeometry</a>( 62, 40, 75, 30 );
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quit-><a href="qwidget.html#setFont">setFont</a>( QFont( "Times", 18, QFont::Bold ) );
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<a href="qobject.html#connect">connect</a>( quit, SIGNAL(<a href="qbutton.html#clicked">clicked</a>()), qApp, SLOT(<a href="qapplication.html#quit">quit</a>()) );
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}
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int main( int argc, char **argv )
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{
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<a href="qapplication.html">QApplication</a> a( argc, argv );
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MyWidget w;
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w.<a href="qwidget.html#setGeometry">setGeometry</a>( 100, 100, 200, 120 );
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a.<a href="qapplication.html#setMainWidget">setMainWidget</a>( &w );
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w.<a href="qwidget.html#show">show</a>();
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return a.<a href="qapplication.html#exec">exec</a>();
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}
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</pre>
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<p> <h2> Line-by-line Walkthrough
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</h2>
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<a name="1"></a><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> Here we create a new class. Because this class inherits from <a href="qwidget.html">QWidget</a>,
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the new class is a widget and may be a top level window or a child
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widget (like the push button in Chapter 3).
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<p> This class has only one member, a constructor (in addition to the
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members it inherits from QWidget). The constructor is a standard Qt
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widget constructor; you should always include a similar constructor
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when you create widgets.
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<p> The first argument is its parent widget. To create a top-level window
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you specify a null pointer as the parent. As you can see, this widget
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defaults to be a top-level window.
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<p> The second argument is the widget's name. This is <em>not</em> the text
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that appears in the window's title bar or in the button. It is a name
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associated with a widget to make it possible to <a href="qobject.html#queryList">look up</a> this widget later, and there is
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also a <a href="qobject.html#dumpObjectTree">handy debugging function</a> that will list a complete widget hierarchy.
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<p> <pre> MyWidget::MyWidget( <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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</pre>
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<p> The implementation of the constructor starts here. Like most widgets,
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it just passes on the <tt>parent</tt> and <tt>name</tt> to the <a href="qwidget.html">QWidget</a>
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constructor.
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<p> <pre> {
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<a href="qwidget.html#setMinimumSize">setMinimumSize</a>( 200, 120 );
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<a href="qwidget.html#setMaximumSize">setMaximumSize</a>( 200, 120 );
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</pre>
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<p> Because this widget doesn't know how to handle resizing, we fix its size
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by setting the minimum and maximum to be equal. In the next chapter
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we will show how a widget can respond to resize event from the user.
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<p> <pre> <a href="qpushbutton.html">QPushButton</a> *quit = new <a href="qpushbutton.html">QPushButton</a>( "Quit", this, "quit" );
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<a name="x2308"></a> quit-><a href="qwidget.html#setGeometry">setGeometry</a>( 62, 40, 75, 30 );
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<a name="x2307"></a> quit-><a href="qwidget.html#setFont">setFont</a>( QFont( "Times", 18, QFont::Bold ) );
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</pre>
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<p> Here we create and set up a child widget of this widget (the new widget's
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parent is <tt>this</tt>) which has the widget name "quit". The widget
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name has nothing to do with the button text; it just happens to be
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similar in this case.
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<p> Note that <tt>quit</tt> is a local variable in the constructor. MyWidget
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does not keep track of it, but Qt does, and will by default delete it
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when MyWidget is deleted. This is why MyWidget doesn't need a
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destructor. (On the other hand, there is no harm in deleting a child
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when you choose to, the child will automatically tell Qt about its
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imminent death.)
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<p> The setGeometry() call does the same as move() and resize() did in the
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previous chapters.
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<p> <pre> <a name="x2306"></a><a name="x2304"></a> <a href="qobject.html#connect">connect</a>( quit, SIGNAL(<a href="qbutton.html#clicked">clicked</a>()), qApp, SLOT(<a href="qapplication.html#quit">quit</a>()) );
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}
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</pre>
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<p> Because the MyWidget class doesn't know about the application object, it
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has to connect to Qt's pointer to it, <tt>qApp</tt>.
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<p> A widget is a software component and should know as little as possible
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about its environment in order to be as general and reusable as
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possible.
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<p> Knowing the name of the application object would break this principle,
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so Qt offers an alias, qApp, for the cases in which a component such as
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MyWidget needs to talk to the application object.
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<p> <pre> int main( int argc, char **argv )
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{
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<a href="qapplication.html">QApplication</a> a( argc, argv );
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MyWidget w;
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w.<a href="qwidget.html#setGeometry">setGeometry</a>( 100, 100, 200, 120 );
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<a name="x2305"></a> a.<a href="qapplication.html#setMainWidget">setMainWidget</a>( &w );
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<a name="x2309"></a> w.<a href="qwidget.html#show">show</a>();
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<a name="x2303"></a> return a.<a href="qapplication.html#exec">exec</a>();
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}
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</pre>
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<p> Here we instantiate our new child, set it to be the main widget, and
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execute the application.
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<p> <h2> Behavior
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</h2>
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<a name="2"></a><p> This program is very similar in behavior to the previous one. The
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difference lies in the way we have implemented it. It does behave
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slightly differently, however. Just try to resize it to see.
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<p> (See <a href="tutorial1-01.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> Try to create another MyWidget object in main(). What happens?
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<p> Try to add more buttons or put in widgets other than <a href="qpushbutton.html">QPushButton</a>.
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<p> You're now ready for <a href="tutorial1-05.html">Chapter 5.</a>
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<p> [<a href="tutorial1-03.html">Previous tutorial</a>]
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[<a href="tutorial1-05.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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