diff options
| author | Timothy Pearson <kb9vqf@pearsoncomputing.net> | 2011-07-10 15:24:15 -0500 |
|---|---|---|
| committer | Timothy Pearson <kb9vqf@pearsoncomputing.net> | 2011-07-10 15:24:15 -0500 |
| commit | bd0f3345a938b35ce6a12f6150373b0955b8dd12 (patch) | |
| tree | 7a520322212d48ebcb9fbe1087e7fca28b76185c /doc/html/tutorial1-08.html | |
| download | qt3-bd0f3345a938b35ce6a12f6150373b0955b8dd12.tar.gz qt3-bd0f3345a938b35ce6a12f6150373b0955b8dd12.zip | |
Add Qt3 development HEAD version
Diffstat (limited to 'doc/html/tutorial1-08.html')
| -rw-r--r-- | doc/html/tutorial1-08.html | 297 |
1 files changed, 297 insertions, 0 deletions
diff --git a/doc/html/tutorial1-08.html b/doc/html/tutorial1-08.html new file mode 100644 index 0000000..beb1f66 --- /dev/null +++ b/doc/html/tutorial1-08.html @@ -0,0 +1,297 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> +<!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/doc/tutorial.doc:868 --> +<html> +<head> +<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> +<title>Qt Tutorial - Chapter 8: Preparing for Battle</title> +<style type="text/css"><!-- +fn { margin-left: 1cm; text-indent: -1cm; } +a:link { color: #004faf; text-decoration: none } +a:visited { color: #672967; text-decoration: none } +body { background: #ffffff; color: black; } +--></style> +</head> +<body> + +<table border="0" cellpadding="0" cellspacing="0" width="100%"> +<tr bgcolor="#E5E5E5"> +<td valign=center> + <a href="index.html"> +<font color="#004faf">Home</font></a> + | <a href="classes.html"> +<font color="#004faf">All Classes</font></a> + | <a href="mainclasses.html"> +<font color="#004faf">Main Classes</font></a> + | <a href="annotated.html"> +<font color="#004faf">Annotated</font></a> + | <a href="groups.html"> +<font color="#004faf">Grouped Classes</font></a> + | <a href="functions.html"> +<font color="#004faf">Functions</font></a> +</td> +<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 8: Preparing for Battle</h1> + + +<p> <center><img src="t8.png" alt="Screenshot of tutorial eight"></center> +<p> In this example, we introduce the first custom widget that can paint +itself. We also add a useful keyboard interface (with two lines of +code). +<p> <ul> +<li> <a href="t8-lcdrange-h.html">t8/lcdrange.h</a> contains the LCDRange class definition. +<li> <a href="t8-lcdrange-cpp.html">t8/lcdrange.cpp</a> contains the LCDRange implementation. +<li> <a href="t8-cannon-h.html">t8/cannon.h</a> contains the CannonField class definition. +<li> <a href="t8-cannon-cpp.html">t8/cannon.cpp</a> contains the CannonField implementation. +<li> <a href="t8-main-cpp.html">t8/main.cpp</a> contains MyWidget and main. +</ul> +<p> <h2> Line-by-line Walkthrough +</h2> +<a name="1"></a><p> <h3> <a href="t8-lcdrange-h.html">t8/lcdrange.h</a> +</h3> +<a name="1-1"></a><p> This file is very similar to the lcdrange.h in Chapter 7. We have added +one slot: setRange(). +<p> + +<p> <pre> void setRange( int minVal, int maxVal ); +</pre> +<p> We now add the possibility of setting the range of the LCDRange. +Until now, it has been fixed at 0..99. +<p> <h3> <a href="t8-lcdrange-cpp.html">t8/lcdrange.cpp</a> +</h3> +<a name="1-2"></a><p> + +<p> There is a change to the constructor (we'll discuss that later). +<p> <pre> void LCDRange::setRange( int minVal, int maxVal ) + { + if ( minVal < 0 || maxVal > 99 || minVal > maxVal ) { + <a href="qapplication.html#qWarning">qWarning</a>( "LCDRange::setRange(%d,%d)\n" + "\tRange must be 0..99\n" + "\tand minVal must not be greater than maxVal", + minVal, maxVal ); + return; + } + <a name="x2334"></a> slider-><a href="qrangecontrol.html#setRange">setRange</a>( minVal, maxVal ); + } +</pre> +<p> SetRange() sets the range of the slider in the LCDRange. Because we +have set up the <a href="qlcdnumber.html">QLCDNumber</a> to always display two digits, we want to +limit the possible range of <tt>minVal</tt> and <tt>maxVal</tt> to 0..99 to avoid +overflow of the QLCDNumber. (We could have allowed values down to -9 +but chose not to.) If the arguments are illegal, we use Qt's +<a href="qapplication.html#qWarning">qWarning</a>() function to issue a warning to the user and return +immediately. qWarning() is a printf-like function that by default +sends its output to <tt>stderr</tt>. If you want, you can install your own handler +function using <a href="qapplication.html#qInstallMsgHandler">::qInstallMsgHandler</a>(). +<p> <h3> <a href="t8-cannon-h.html">t8/cannon.h</a> +</h3> +<a name="1-3"></a><p> CannonField is a new custom widget that knows how to display itself. +<p> + +<p> <pre> class CannonField : public <a href="qwidget.html">QWidget</a> + { + <a href="metaobjects.html#Q_OBJECT">Q_OBJECT</a> + public: + CannonField( <a href="qwidget.html">QWidget</a> *parent=0, const char *name=0 ); +</pre> +<p> CannonField inherits <a href="qwidget.html">QWidget</a>, and we use the same idiom as for LCDRange. +<p> <pre> int angle() const { return ang; } + <a href="qsizepolicy.html">QSizePolicy</a> sizePolicy() const; + + public slots: + void setAngle( int degrees ); + + signals: + void angleChanged( int ); +</pre> +<p> For the time being, CannonField only contains an angle value for which we +provide an interface using the same idiom as for value in LCDRange. +<p> <pre> protected: + void paintEvent( <a href="qpaintevent.html">QPaintEvent</a> * ); +</pre> +<p> This is the second of the many event handlers in QWidget that we +encounter. This virtual function is called by Qt whenever a widget needs +to update itself (i.e., paint the widget's surface). +<p> <h3> <a href="t8-cannon-cpp.html">t8/cannon.cpp</a> +</h3> +<a name="1-4"></a><p> + +<p> <pre> CannonField::CannonField( <a href="qwidget.html">QWidget</a> *parent, const char *name ) + : <a href="qwidget.html">QWidget</a>( parent, name ) + { +</pre> +<p> Again, we use the same idiom as for LCDRange in the previous chapter. +<p> <pre> ang = 45; + <a href="qwidget.html#setPalette">setPalette</a>( QPalette( QColor( 250, 250, 200) ) ); + } +</pre> +<p> The constructor initializes the angle value to 45 degrees and sets a +custom palette for this widget. +<p> This palette uses the indicated color as background and picks other +colors suitably. (For this widget only the background and text +colors will actually be used.) +<p> <pre> void CannonField::setAngle( int degrees ) + { + if ( degrees < 5 ) + degrees = 5; + if ( degrees > 70 ) + degrees = 70; + if ( ang == degrees ) + return; + ang = degrees; + <a href="qwidget.html#repaint">repaint</a>(); + emit angleChanged( ang ); + } +</pre> +<p> This function sets the angle value. We have chosen a legal range of +5..70 and adjust the given number of degrees accordingly. We have +chosen not to issue a warning if the new angle is out of range. +<p> If the new angle equals the old one, we return immediately. It is +important to only emit the signal angleChanged() when the angle <em>really</em> has changed. +<p> Then we set the new angle value and repaint our widget. The <a href="qwidget.html#repaint">QWidget::repaint</a>() function clears the widget (usually filling it with +its background color) and sends a paint event to the widget. This +results in a call to the paint event function of the widget. +<p> Finally, we emit the angleChanged() signal to tell the outside world +that the angle has changed. The <tt>emit</tt> keyword is unique to Qt and +not regular C++ syntax. In fact, it is a macro. +<p> <pre> <a name="x2336"></a>void CannonField::<a href="qwidget.html#paintEvent">paintEvent</a>( <a href="qpaintevent.html">QPaintEvent</a> * ) + { + <a href="qstring.html">QString</a> s = "Angle = " + QString::number( ang ); + <a href="qpainter.html">QPainter</a> p( this ); + <a name="x2335"></a> p.<a href="qpainter.html#drawText">drawText</a>( 200, 200, s ); + } +</pre> +<p> This is our first attempt to write a paint event handler. The event +argument contains a description of the paint event. <a href="qpaintevent.html">QPaintEvent</a> +contains the region in the widget that must be updated. For the time +being, we will be lazy and just paint everything. +<p> Our code displays the angle value in the widget at a fixed position. +First we create a <a href="qstring.html">QString</a> with some text and the angle; then we create +a <a href="qpainter.html">QPainter</a> operating on this widget and use it to paint the string. +We'll come back to QPainter later; it can do a great many things. +<p> <h3> <a href="t8-main-cpp.html">t8/main.cpp</a> +</h3> +<a name="1-5"></a><p> + +<p> <pre> #include "cannon.h" +</pre> +<p> We include our new class. +<p> <pre> class MyWidget: public <a href="qwidget.html">QWidget</a> + { + public: + MyWidget( <a href="qwidget.html">QWidget</a> *parent=0, const char *name=0 ); + }; +</pre> +<p> This time we include a single LCDRange and a CannonField in our top-level +widget. +<p> <pre> LCDRange *angle = new LCDRange( this, "angle" ); +</pre> +<p> In the constructor, we create and set up our LCDRange. +<p> <pre> angle->setRange( 5, 70 ); +</pre> +<p> We set the LCDRange to accept ranges from 5 to 70 degrees. +<p> <pre> CannonField *cannonField + = new CannonField( this, "cannonField" ); +</pre> +<p> We create our CannonField. +<p> <pre> <a href="qobject.html#connect">connect</a>( angle, SIGNAL(valueChanged(int)), + cannonField, SLOT(setAngle(int)) ); + <a href="qobject.html#connect">connect</a>( cannonField, SIGNAL(angleChanged(int)), + angle, SLOT(setValue(int)) ); +</pre> +<p> Here we connect the valueChanged() signal of the LCDRange to the +setAngle() slot of the CannonField. This will update CannonField's angle +value whenever the user operates the LCDRange. We also make the reverse +connection so that changing the angle in the CannonField will update the +LCDRange value. In our example we never change the angle of the +CannonField directly; but by doing the last connect() we ensure that no +future changes will disrupt the synchronization between those two values. +<p> This illustrates the power of component programming and proper +encapsulation. +<p> Notice how important it is to emit the angleChanged() signal only when +the angle actually changes. If both the LCDRange and the CannonField +had omitted this check, the program would have entered an infinite +loop upon the first change of one of the values. +<p> <pre> <a href="qgridlayout.html">QGridLayout</a> *grid = new <a href="qgridlayout.html">QGridLayout</a>( this, 2, 2, 10 ); + //2x2, 10 pixel border +</pre> +<p> So far we have used the no-assembly-required <a href="qvbox.html">QVBox</a> and <a href="qgrid.html">QGrid</a> widgets +for geometry management. Now, however, we want to have a little more +control over the layout, and we switch to the more powerful <a href="qgridlayout.html">QGridLayout</a> +class. QGridLayout isn't a widget; it is a different class that can +manage the children of <em>any</em> widget. +<p> As the comment indicates, we create a two-by-two array with ten pixel +borders. (The constructor for <a href="qgridlayout.html">QGridLayout</a> can be a little cryptic, +so it's good to put in such comments.) +<p> <pre> <a name="x2337"></a> grid-><a href="qgridlayout.html#addWidget">addWidget</a>( quit, 0, 0 ); +</pre> +<p> We add the Quit button in the top-left cell of the grid: 0, 0. +<p> <pre> grid-><a href="qgridlayout.html#addWidget">addWidget</a>( angle, 1, 0, Qt::AlignTop ); +</pre> +<p> We put the angle LCDRange in the bottom-left cell, aligned to the top +of its cell. (This alignment is one of the things QGridLayout allows +but QGrid does not allow.) +<p> <pre> grid-><a href="qgridlayout.html#addWidget">addWidget</a>( cannonField, 1, 1 ); +</pre> +<p> We put the CannonField object in the bottom-right cell. (The top- +right cell is empty.) +<p> <pre> <a name="x2338"></a> grid-><a href="qgridlayout.html#setColStretch">setColStretch</a>( 1, 10 ); +</pre> +<p> We tell <a href="qgridlayout.html">QGridLayout</a> that the right column (column 1) is stretchable. +Because the left column isn't (it has <a href="layout.html#stretch-factor">stretch factor</a> 0, the default +value), QGridLayout will try to let the left-hand widgets' sizes be +unchanged and will resize just the CannonField when the MyWidget is +resized. +<p> <pre> angle->setValue( 60 ); +</pre> +<p> We set an initial angle value. Note that this will trigger the +connection from LCDRange to CannonField. +<p> <pre> <a name="x2339"></a> angle-><a href="qwidget.html#setFocus">setFocus</a>(); +</pre> +<p> Our last action is to set <tt>angle</tt> to have <a href="focus.html#keyboard-focus">keyboard focus</a> so that +keyboard input will go to the LCDRange widget by default. +<p> LCDRange does not contain any keyPressEvent(), so that would seem not +to be terribly useful. However, its constructor just got a new line: +<p> + +<pre> <a href="qwidget.html#setFocusProxy">setFocusProxy</a>( slider ); +</pre> +<p> The LCDRange sets the slider to be its focus proxy. That means that +when someone (the program or the user) wants to give the LCDRange +keyboard focus, the slider should take care of it. <a href="qslider.html">QSlider</a> has a decent +keyboard interface, so with just one line of code we've given LCDRange +one. +<p> <h2> Behavior +</h2> +<a name="2"></a><p> The keyboard now does something - the arrow keys, Home, End, PageUp +and PageDown all do something vaguely sensible. +<p> When the slider is operated, the CannonField displays the new angle +value. Upon resizing, CannonField is given as much space as possible. +<p> On Windows machines with an 8-bit display the new background color is +dithered to death. The next chapter works around this. +<p> (See <a href="tutorial1-07.html#compiling">Compiling</a> for how to create a +makefile and build the application.) +<p> <h2> Exercises +</h2> +<a name="3"></a><p> Try to resize the window. What happens if you make it really narrow +or really squat? +<p> If you remove the AlignTop, what happens to the LCDRange's position +and size? Why? +<p> If you give the left-hand column a non-zero stretch factor, what +happens when you resize the window? +<p> Leave out the setFocus() call. Which behavior do you prefer? +<p> Try to change "Quit" to "&Quit" in the <a href="qbutton.html#setText">QButton::setText</a>() call. How +does the button's look change? What happens if you press Alt+Q while +the program's running? (It is Meta+Q on a few keyboards.) +<p> Center the text in the CannonField. +<p> You're now ready for <a href="tutorial1-09.html">Chapter 9.</a> +<p> [<a href="tutorial1-07.html">Previous tutorial</a>] +[<a href="tutorial1-09.html">Next tutorial</a>] +[<a href="tutorial.html">Main tutorial page</a>] +<p> +<!-- eof --> +<p><address><hr><div align=center> +<table width=100% cellspacing=0 border=0><tr> +<td>Copyright © 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> +</table></div></address></body> +</html> |