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authorTimothy Pearson <kb9vqf@pearsoncomputing.net>2011-07-10 15:24:15 -0500
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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 &lt;<a href="qvbox-h.html">qvbox.h</a>&gt;
+</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-&gt;<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-&gt;<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 &lt; 4 ; r++ ) {
+ for( int c = 0 ; c &lt; 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>]
+<p>
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+<td>Copyright &copy; 2007
+<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
+<td align=right><div align=right>Qt 3.3.8</div>
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