Qt Tutorial - Chapter 2: Calling it QuitsJump
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Having created a window in Chapter
1, we will now go on to make the application quit properly when the
user tells it to.
We will also use a font that is more exciting than the default one.
/****************************************************************
**
** Qt tutorial 2
**
****************************************************************/
#include <qapplication.h>
#include <qpushbutton.h>
#include <qfont.h>
int main( int argc, char **argv )
{
QApplication a( argc, argv );
QPushButton quit( "Quit", 0 );
quit.resize( 75, 30 );
quit.setFont( QFont( "Times", 18, QFont::Bold ) );
QObject::connect( &quit, SIGNAL(clicked()), &a, SLOT(quit()) );
a.setMainWidget( &quit );
quit.show();
return a.exec();
}
Line-by-line Walkthrough
#include <qfont.h>
Since this program uses QFont, it
needs to include qfont.h. Qt's font abstraction is rather different from
the horror provided by X, and loading and using fonts has been highly
optimized.
QPushButton quit( "Quit", 0 );
This time, the button says "Quit" and that's exactly what the program
will do when the user clicks the button. This is not a coincidence. We
still pass 0 as the parent, since the button is a top-level window.
quit.resize( 75, 30 );
We've chosen another size for the button since the text is a bit
shorter than "Hello world!". We could also have used QFontMetrics
to set right size.
quit.setFont( QFont( "Times", 18, QFont::Bold ) );
Here we choose a new font for the button, an 18-point bold font from
the Times family. Note that we create the font on the spot.
It is also possible to change the default font (using QApplication::setFont())
for the whole application.
QObject::connect( &quit, SIGNAL(clicked()), &a, SLOT(quit()) );
connect() is perhaps the most central feature of Qt. Note that
connect() is a static function in QObject. Do
not confuse it with the connect() function in the socket library.
This line establishes a one-way connection between two Qt objects
(objects that inherit QObject, directly or indirectly). Every Qt object
can have both signals (to send messages) and slots (to
receive messages). All widgets are Qt objects. They inherit QWidget
which in turn inherits QObject.
Here, the clicked() signal of quit is connected to
the quit() slot of a, so that when the button is
clicked, the application quits.
The Signals
and Slots documentation describes this topic in detail.
Behavior
When you run this program, you will see an even smaller window than in
Chapter 1, filled with an even smaller button.
(See Compiling
for how to create a makefile and build the application.)
Exercises
Try to resize the window. Press the button. Oops! That connect() would
seem to make some difference.
Are there any other signals in QPushButton
you can connect to quit? Hint: The QPushButton inherits most of its
behavior from QButton.
You're now ready for Chapter
3.
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