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<h1 align="center">Qt Jambi Tutorial 2 - Calling it Quits<br /><small></small></h1>
<p align="center"><img src="images/tutorial2-example.png" alt="Screenshot of Chapter 2" /></p><p>Having created a window in Chapter 1,</tt> we will now go on to make the application quit properly when the user tells it to.</p>
<p>We will also use a font that is more exciting than the default one. We give the entire source code of the application:</p>
<pre> public class Quit
{
public static void main(String args[])
{
QApplication.initialize(args);
QPushButton quit = new QPushButton("Quit");
quit.resize(80, 40);
quit.setFont(new QFont("Times", 18, QFont.Weight.Bold.value()));
quit.clicked.connect(QApplication.instance(), "quit()");
quit.setWindowTitle("Calling It Quits");
quit.show();
QApplication.exec();
}
}</pre>
<a name="line-by-line-walkthrough"></a>
<h2>Line by Line Walkthrough</h2>
<pre> QPushButton quit = new QPushButton("Quit");</pre>
<p>This time, the button says <b>Quit</b> and that's exactly what the program will do when the user clicks the button.</p>
<pre> quit.resize(80, 40);</pre>
<p>We've chosen another size for the button since the text is a bit shorter than "Hello world!". We could also have used <a href="gui/QFontMetrics.html"><tt>QFontMetrics</tt></a> to set right size, or let <a href="gui/QPushButton.html"><tt>QPushButton</tt></a> choose a reasonable default.</p>
<pre> quit.setFont(new QFont("Times", 18, QFont.Weight.Bold.value()));</pre>
<p>Here we choose a new font for the button, an 18-point bold font from the Times family. It is also possible to change the default font for the entire application, using QApplication::setFont(). We fetch the value from the Weight enum as the weight of the font is given as an int in the constructor.</p>
<pre> quit.clicked.connect(QApplication.instance(), "quit()");</pre>
<p>We connect the clicked signal to the <tt>quit()</tt> slot in <a href="gui/QApplication.html"><tt>QApplication</tt></a> (<a href="porting4.html#qapplication"><tt>QApplication.</tt></a>instance() returns the application's unique <a href="gui/QApplication.html"><tt>QApplication</tt></a> instance). <tt>clicked</tt> is an instance of the Signal0 class and <tt>quit()</tt> is a regular method in <a href="gui/QApplication.html"><tt>QApplication</tt></a> that quits the application. When <tt>connect()</tt> is invoked a one-way connection between the two QtJambiObjects is established. After the slot is connected to the signal the quit() method is invoked when a method on the signal is invoked; this is called emitting the signal. In this case, the application will exit when the user clicks on the button with the mouse.</p>
<p>Every Qt Jambi object can have both <tt>signals</tt> (to send messages) and <tt>slots</tt> (to receive messages). All widgets are Qt Jambi objects, since they inherit <a href="gui/QWidget.html"><tt>QWidget</tt></a>, which indirectly inherits QtJambiObject.</p>
<p>This signal and slot mechanism is perhaps the most central feature of Qt Jambi. The <a href="qtjambi-signalsandslots.html">Signals and Slots</tt></a> documentation describes this topic in detail.</p>
<a name="running-the-application"></a>
<h2>Running the Application</h2>
<p>When you run this program, you will see an even smaller window than in Chapter 1, filled with an even smaller button.</p>
<p>See Chapter 1</tt> for how to compile and run the application.</p>
<a name="exercises"></a>
<h2>Exercises</h2>
<p>Try to resize the window. Press the button to close the application.</p>
<p>Are there any other signals in <a href="gui/QPushButton.html"><tt>QPushButton</tt></a> you can connect to quit? [Hint: The <a href="gui/QPushButton.html"><tt>QPushButton</tt></a> inherits most of its functionality from <a href="gui/QAbstractButton.html"><tt>QAbstractButton</tt></a>.]</p>
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