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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Qt Toolkit - Signals and Slots</title><style type="text/css"><!--
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<h1 align=center> Signals and Slots</h1><br clear="all">
Signals and slots are used for communication between objects. The
signal/slot mechanism is a central feature of Qt and probably the
part that differs most from other toolkits.
<p>
In most GUI toolkits widgets have a callback for each action they can
trigger. This callback is a pointer to a function. In Qt, signals and
slots have taken over from these messy function pointers.
<p>
Signals and slots can take any number of arguments of any type. They are
completely typesafe: no more callback core dumps!
<p>
All classes that inherit from QObject or one of its subclasses
(e.g. QWidget) can contain signals and slots. Signals are emitted by
objects when they change their state in a way that may be interesting
to the outside world. This is all the object does to communicate. It
does not know if anything is receiving the signal at the other end.
This is true information encapsulation, and ensures that the object
can be used as a software component.
<p>
Slots can be used for receiving signals, but they are normal member
functions. A slot does not know if it has any signal(s) connected to
it. Again, the object does not know about the communication mechanism and
can be used as a true software component.
<p>
You can connect as many signals as you want to a single slot, and a
signal can be connected to as many slots as you desire. It is even
possible to connect a signal directly to another signal. (This will
emit the second signal immediately whenever the first is emitted.)
<p>
Together, signals and slots make up a powerful component programming
mechanism.
<p>
<h2>A Small Example</h2>
<p>
A minimal C++ class declaration might read:
<p>
<pre> class Foo
{
public:
Foo();
int value() const { return val; }
void setValue( int );
private:
int val;
};
</pre>
<p>
A small Qt class might read:
<p>
<pre> class Foo : public QObject
{
Q_OBJECT
public:
Foo();
int value() const { return val; }
public slots:
void setValue( int );
signals:
void valueChanged( int );
private:
int val;
};
</pre>
<p>
This class has the same internal state, and public methods to access the
state, but in addition it has support for component programming using
signals and slots: This class can tell the outside world that its state
has changed by emitting a signal, <code>valueChanged()</code>, and it has
a slot which other objects may send signals to.
<p>
All classes that contain signals and/or slots must mention Q_OBJECT in
their declaration.
<p>
Slots are implemented by the application programmer (that's you).
Here is a possible implementation of Foo::setValue():
<p>
<pre> void Foo::setValue( int v )
{
if ( v != val ) {
val = v;
emit valueChanged(v);
}
}
</pre>
<p>
The line <code>emit valueChanged(v)</code> emits the signal
<code>valueChanged</code> from the object. As you can see, you emit a
signal by using <code>emit signal(arguments)</code>.
<p>
Here is one way to connect two of these objects together:
<p>
<pre> Foo a, b;
connect(&a, SIGNAL(valueChanged(int)), &b, SLOT(setValue(int)));
b.setValue( 11 );
a.setValue( 79 );
b.value(); // this would now be 79, why?
</pre>
<p>
Calling <code>a.setValue(79)</code> will make <code>a</code> emit a
signal, which <code>b</code> will receive,
i.e. <code>b.setValue(79)</code> is invoked. <code>b</code> will in turn
emit the same signal, which nobody receives, since no slot has been
connected to it, so it disappears into hyperspace.
<p>
Note that the <code>setValue()</code> function sets the value and emits
the signal only if <code>v != val</code>. This prevents infinite looping
in the case of cyclic connections (e.g. if <code>b.valueChanged()</code>
were connected to <code>a.setValue()</code>).
<p>
This example illustrates that objects can work together without knowing
each other, as long as there is someone around to set up a connection
between them initially.
<p>
The preprocessor changes or removes the <code>signals</code>,
<code>slots</code> and <code>emit</code> keywords so the compiler won't
see anything it can't digest.
<p>
Run the <a href="moc.html">moc</a> on class definitions that contains
signals or slots. This produces a C++ source file which should be compiled
and linked with the other object files for the application.
<p>
<h2>Signals</h2>
<p>
Signals are emitted by an object when its internal state has changed
in some way that might be interesting to the object's client or owner.
Only the class that defines a signal and its subclasses can emit the
signal.
<p>
A list box, for instance, emits both <code>highlighted()</code> and
<code>activated()</code> signals. Most object will probably only be
interested in <code>activated()</code> but some may want to know about
which item in the list box is currently highlighted. If the signal is
interesting to two different objects you just connect the signal to
slots in both objects.
<p>
When a signal is emitted, the slots connected to it are executed
immediately, just like a normal function call. The signal/slot
mechanism is totally independent of any GUI event loop. The
<code>emit</code> will return when all slots have returned.
<p>
If several slots are connected to one signal, the slots will be
executed one after the other, in an arbitrary order, when the signal
is emitted.
<p>
Signals are automatically generated by the moc and must not be implemented
in the .cpp file. They can never have return types (i.e. use <code>void).</code>
<p>
A word about arguments: Our experience shows that signals and slots
are more reusable if they do <em>not</em> use special types. If <a href="qscrollbar.html#492c92">QScrollBar::valueChanged()</a> were to use a special type such as the
hypothetical QRangeControl::Range, it could only be connected to slots
designed specifically for QRangeControl. Something as simple as the
program in <a href="t5.html">Tutorial 5</a> would be impossible.
<p>
<h2>Slots</h2>
<p>
A slot is called when a signal connected to it is emitted. Slots are
normal C++ functions and can be called normally; their only special
feature is that signals can be connected to them. A slot's arguments
cannot have default values, and as for signals, it is generally a bad
idea to use custom types for slot arguments.
<p>
Since slots are normal member functions with just a little extra
spice, they have access rights like everyone else. A slot's access
right determines who can connect to it:
<p>
A <code>public slots:</code> section contains slots that anyone can
connect signals to. This is very useful for component programming:
You create objects that know nothing about each other, connect their
signals and slots so information is passed correctly, and, like a
model railway, turn it on and leave it running.
<p>
A <code>protected slots:</code> section contains slots that this class
and its subclasses may connect signals to. This is intended for
slots that are part of the class' implementation rather than its
interface towards the rest of the world.
<p>
A <code>private slots:</code> section contains slots that only the
class itself may connect signals to. This is intended for very
tightly connected classes, where even subclasses aren't trusted to get
the connections right.
<p>
Of course, you can also define slots to be virtual. We have found
this to be very useful.
<p>
Signals and slots are fairly efficient. Of course there's some loss of
speed compared to "real" callbacks due to the increased flexibility, but
the loss is fairly small, we measured it to approximately 10 microseconds
on a i586-133 running Linux (less than 1 microsecond when no slot has been
connected) , so the simplicity and flexibility the mechanism affords is
well worth it.
<p>
<h2>Meta Object Information</h2>
<p>
The meta object compiler (moc) parses the class declaration in a C++
file and generates C++ code that initializes the meta object. The meta
object contains names of all signal and slot members, as well as
pointers to these functions.
<p>
The meta object contains additional information such as the object's <a href="qobject.html#fa7716">class name</a>. You can also check if an object
<a href="qobject.html#beb5d8">inherits</a> a specific class, for example:
<p>
<pre> if ( widget->inherits("QButton") ) {
// yes, it is a push button, radio button etc.
}
</pre>
<p>
<h2>A Real Example</h2>
<p>
Here is a simple commented example (code fragments from <a href="qlcdnumber-h.html">qlcdnumber.h</a> ).
<p>
<pre> #include "qframe.h"
#include "qbitarray.h"
class QLCDNumber : public QFrame
</pre>
<p>
QLCDNumber inherits QObject, which has most of the signal/slot
knowledge, via QFrame and QWidget, and #include's the relevant
declarations.
<p>
<pre> {
Q_OBJECT
</pre>
<p>
Q_OBJECT is expanded by the preprocessor to declare several member
functions that are implemented by the moc; if you get compiler errors
along the lines of "virtual function QButton::className not defined"
you have probably forgotten to <a href="moc.html">run the moc</a> or to
include the moc output in the link command.
<p>
<pre> public:
<a href="qlcdnumber.html">QLCDNumber</a>( <a href="qwidget.html">QWidget</a> *parent=0, const char *name=0 );
<a href="qlcdnumber.html">QLCDNumber</a>( uint numDigits, QWidget *parent=0, const char *name=0 );
</pre>
<p>
It's not obviously relevant to the moc, but if you inherit QWidget you
almost certainly want to have <em>parent</em> and <em>name</em>
arguments to your constructors, and pass them to the parent
constructor.
<p>
Some destructors and member functions are omitted here, the moc
ignores member functions.
<p>
<pre> signals:
void overflow();
</pre>
<p>
QLCDNumber emits a signal when it is asked to show an impossible
value.
<p>
"But I don't care about overflow," or "But I know the number won't
overflow." Very well, then you don't connect the signal to any slot,
and everything will be fine.
<p>
"But I want to call two different error functions when the number
overflows." Then you connect the signal to two different slots. Qt
will call both (in arbitrary order).
<p>
<pre> public slots:
void display( int num );
void display( double num );
void display( const char *str );
void setHexMode();
void setDecMode();
void setOctMode();
void setBinMode();
void smallDecimalPoint( bool );
</pre>
<p>
A slot is a receiving function, used to get information about state
changes in other widgets. QLCDNumber uses it, as you can see, to set
the displayed number. Since <code>display()</code> is part of the
class' interface with the rest of the program, the slot is public.
<p>
Several of the example program connect the newValue signal of a
QScrollBar to the display slot, so the LCD number continuously shows
the value of the scroll bar.
<p>
Note that display() is overloaded; Qt will select the appropriate version
when you connect a signal to the slot.With callbacks, you'd have to find
five different names and keep track of the types yourself.
<p>
Some more irrelevant member functions have been omitted from this
example.
<p>
<pre> };
</pre>
<p>
<h2>Moc output</h2>
<p>
This is really internal to Qt, but for the curious, here is the meat
of the resulting mlcdnum.cpp:
<p>
<pre> const char *QLCDNumber::className() const
{
return "QLCDNumber";
}
<a href="qmetaobject.html">QMetaObject</a> *QLCDNumber::metaObj = 0;
void QLCDNumber::initMetaObject()
{
if ( metaObj )
return;
if ( !QFrame::metaObject() )
<a href="qobject.html#9c0b5a">QFrame::initMetaObject</a>();
</pre>
<p>
That last line is because QLCDNumber inherits QFrame. The next part,
which sets up the table/signal structures, has been deleted for
brevity.
<p>
<pre> }
// SIGNAL overflow
void QLCDNumber::overflow()
{
activate_signal( "overflow()" );
}
</pre>
<p>
One function is generated for each signal, and at present it almost always
is a single call to the internal Qt function activate_signal(), which
finds the appropriate slot or slots and passes on the call. It is not
recommended to call activate_signal() directly.
<p><address><hr><div align="center">
<table width="100%" cellspacing="0" border="0"><tr>
<td>Copyright 2001 Trolltech<td><a href="http://www.trolltech.com/trademarks.html">Trademarks</a>
<td align="right"><div align="right">Qt version 2.3.2</div>
</table></div></address></body></html>
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