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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 7: One Thing Leads to Another</h1>
<p> <center><img src="t7.png" alt="Screenshot of tutorial seven"></center>
<p> This example shows how to create custom widgets with signals and
slots, and how to connect them together in more complex ways. For the
first time, the source is split among several files which we've placed
in the <tt>t7</tt> subdirectory.
<p> <ul>
<li> <a href="t7-lcdrange-h.html">t7/lcdrange.h</a> contains the LCDRange class definition.
<li> <a href="t7-lcdrange-cpp.html">t7/lcdrange.cpp</a> contains the LCDRange implementation.
<li> <a href="t7-main-cpp.html">t7/main.cpp</a> contains MyWidget and main.
</ul>
<p> <h2> Line-by-line Walkthrough
</h2>
<a name="1"></a><p> <h3> <a href="t7-lcdrange-h.html">t7/lcdrange.h</a>
</h3>
<a name="1-1"></a><p> This file is mainly lifted from <a href="tutorial1-06.html#main">main.cpp</a> in
Chapter 6; only the changes are noted here.
<p>
<p> <pre> #ifndef LCDRANGE_H
#define LCDRANGE_H
</pre>
<p> This is the classic C construction to avoid errors if a header file
happens to be included more than once. If you don't use it already,
it is a very good habit to develop. The #ifndef should enclose <em>all</em> of the
header file.
<p> <pre> #include <<a href="qvbox-h.html">qvbox.h</a>>
</pre>
<p> <a href="qvbox-h.html">qvbox.h</a> is included. LCDRange inherits <a href="qvbox.html">QVBox</a>, and the header file
of a parent class must always be included. We cheated a bit in the
previous chapters, and we let <a href="qwidget-h.html">qwidget.h</a> be included indirectly via
other header files such as <a href="qpushbutton-h.html">qpushbutton.h</a>.
<p> <pre> class QSlider;
</pre>
<p> This is another classic trick, but one that's much less used often. Because
we don't need <a href="qslider.html">QSlider</a> in the <em>interface</em> of the class, only in the
implementation, we use a forward declaration of the class in the
header file and include the header file for QSlider in the .cpp
file.
<p> This makes the compilation of big projects much faster, because when a
header file has changed, fewer files need to be recompiled. It can
often speed up big compilations by a factor of two or more.
<p> <pre> class LCDRange : public <a href="qvbox.html">QVBox</a>
{
<a href="metaobjects.html#Q_OBJECT">Q_OBJECT</a>
public:
LCDRange( <a href="qwidget.html">QWidget</a> *parent=0, const char *name=0 );
</pre>
<p> Note the Q_OBJECT. This macro must be included in <em>all</em> classes that
contain signals and/or slots. If you are curious, it defines the
functions that are implemented in the
<a href="metaobjects.html">meta object file</a>.
<p> <pre> int value() const;
public slots:
void setValue( int );
signals:
void valueChanged( int );
</pre>
<p> These three members make up an interface between this widget and other
components in a program. Until now, LCDRange didn't really have an
interface at all.
<p> value() is a public function for accessing the value of the LCDRange.
setValue() is our first custom slot and valueChanged() is our first
custom signal.
<p> Slots must be implemented in the normal way (remember that a slot is also
a C++ member function). Signals are automatically implemented in the
<a href="signalsandslots.html">meta object</a> file. Signals follow the
access rules of protected C++ functions (i.e., they can be emitted only
by the class they are defined in or by classes inheriting from it).
<p> The signal valueChanged() is used when the LCDRange's value has
changed - just as you guessed from the name. This is not the last
signal you'll see called <i>something</i>Changed().
<p> <h3> <a href="t7-lcdrange-cpp.html">t7/lcdrange.cpp</a>
</h3>
<a name="1-2"></a><p>
<p> This file is mainly lifted from <a href="tutorial1-06.html#main">t6/main.cpp</a>, and
only the changes are noted here.
<p> <pre> <a name="x2333"></a> <a href="qobject.html#connect">connect</a>( slider, SIGNAL(<a href="qslider.html#valueChanged">valueChanged</a>(int)),
<a name="x2330"></a> lcd, SLOT(<a href="qlcdnumber.html#display">display</a>(int)) );
<a href="qobject.html#connect">connect</a>( slider, SIGNAL(<a href="qslider.html#valueChanged">valueChanged</a>(int)),
SIGNAL(valueChanged(int)) );
</pre>
<p> This code is from the LCDRange constructor.
<p> The first connect is the same that you have seen in the previous chapter.
The second is new; it connects slider's valueChanged() signal to this
object's valueChanged <em>signal</em>. Connect() with 3 arguments always
connects to signals or slots in <tt>this</tt> object.
<p> Yes, that's right. Signals can be connected to other signals. When
the first is emitted, the second signal is also emitted.
<p> Let's look at what happens when the user operates the slider. The
slider sees that its value has changed and emits the valueChanged()
signal. That signal is connected both to the display() slot of the
<a href="qlcdnumber.html">QLCDNumber</a> and to the valueChanged() signal of the LCDRange.
<p> Thus, when the signal is emitted, LCDRange emits its own
valueChanged() signal. In addition, <a href="qlcdnumber.html#display">QLCDNumber::display</a>() is called
and shows the new number.
<p> Note that you're not guaranteed any particular order of execution -
LCDRange::valueChanged() may be emitted before or after
QLCDNumber::display()and is entirely arbitrary.
<p> <pre> int LCDRange::value() const
{
<a name="x2332"></a> return slider-><a href="qslider.html#value">value</a>();
}
</pre>
<p> The implementation of value() is straightforward; it simply returns
the slider's value.
<p> <pre> void LCDRange::setValue( int value )
{
<a name="x2331"></a> slider-><a href="qslider.html#setValue">setValue</a>( value );
}
</pre>
<p> The implementation of setValue() is equally straightforward. Note
that because the slider and LCD number are connected, setting the
slider's value automatically updates the LCD number as well. In
addition, the slider will automatically adjust the value if it is
outside its legal range.
<p> <h3> <a href="t7-main-cpp.html">t7/main.cpp</a>
</h3>
<a name="1-3"></a><p>
<p> <pre> LCDRange *previous = 0;
for( int r = 0 ; r < 4 ; r++ ) {
for( int c = 0 ; c < 4 ; c++ ) {
LCDRange* lr = new LCDRange( grid );
if ( previous )
<a href="qobject.html#connect">connect</a>( lr, SIGNAL(valueChanged(int)),
previous, SLOT(setValue(int)) );
previous = lr;
}
}
</pre>
<p> All of main.cpp is copied from the previous chapter except in
the constructor for MyWidget. When we create the 16 LCDRange object, we
now connect them using the <a href="signalsandslots.html">signal/slot</a> mechanism. Each has its valueChanged() signal
connected to the setValue() slot in the previous one. Because LCDRange
emits the signal valueChanged() when its value changes (surprise!), we
are here creating a "chain" of signals and slots.
<p> <a name="compiling"></a>
<h2> Compiling
</h2>
<a name="2"></a><p> Creating a makefile for a multi-file application is no different from
creating one for a single-file application. If you've saved all the
files in this example in their own directory, all you have to do is:
<pre>
qmake -project
qmake
</pre>
<p> The first command tells <a href="qmake-manual.html">qmake</a> to
create a <tt>.pro</tt> (project) file. The second command tells it to create
a (platform-specific) makefile based on the project file. You should
now be able to type <tt>make</tt> (or <tt>nmake</tt> if you're using Visual
Studio) to build your application.
<p> <h2> Behavior
</h2>
<a name="3"></a><p> On startup, the program's appearance is identical to the previous one.
Try operating the slider to the bottom right...
<p> <h2> Exercises
</h2>
<a name="4"></a><p> Use the bottom right slider to set all LCDs to 50. Then set the top
half to 40 by clicking once to the left of the slider handle. Now,
use the one to the left of the last one operated to set the first
seven LCDs back to 50.
<p> Click to the left of the handle on the bottom right slider. What
happens? Why is this the correct behavior?
<p> You're now ready for <a href="tutorial1-08.html">Chapter 8.</a>
<p> [<a href="tutorial1-06.html">Previous tutorial</a>]
[<a href="tutorial1-08.html">Next tutorial</a>]
[<a href="tutorial.html">Main tutorial page</a>]
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