path: root/doc/signalsandslots.doc

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/*! \page signalsandslots.html

\title Signals and Slots

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.

In GUI programming we often want a change in one widget to be notified
to another widget. More generally, we want objects of any kind to be
able to communicate with one another. For example if we were parsing
an XML file we might want to notify a list view that we're using to
represent the XML file's structure whenever we encounter a new tag. 

Older toolkits achieve this kind of communication using callbacks. A
callback is a pointer to a function, so if you want a processing
function to notify you about some event you pass a pointer to another
function (the callback) to the processing function. The processing
function then calls the callback when appropriate. Callbacks have two
fundamental flaws. Firstly they are not type safe. We can never be
certain that the processing function will call the callback with the
correct arguments. Secondly the callback is strongly coupled to the
processing function since the processing function must know which
callback to call.

\img abstract-connections.png
\caption An abstract view of some signals and slots connections

In Qt we have an alternative to the callback technique. We use signals
and slots. A signal is emitted when a particular event occurs. Qt's
widgets have many pre-defined signals, but we can always subclass to
add our own. A slot is a function that is called in reponse to a
particular signal. Qt's widgets have many pre-defined slots, but it is
common practice to add your own slots so that you can handle the
signals that you are interested in. 

The signals and slots mechanism is type safe: the signature of a
signal must match the signature of the receiving slot. (In fact a slot
may have a shorter signature than the signal it receives because it
can ignore extra arguments.) Since the signatures are compatible, the
compiler can help us detect type mismatches. Signals and slots are
loosely coupled: a class which emits a signal neither knows nor cares
which slots receive the signal. Qt's signals and slots mechanism
ensures that if you connect a signal to a slot, the slot will be
called with the signal's parameters at the right time. Signals and
slots can take any number of arguments of any type. They are
completely typesafe: no more callback core dumps!

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 or care whether anything is receiving the signals it
emits. This is true information encapsulation, and ensures that the
object can be used as a software component.

\img concrete-connections.png
\caption An example of signals and slots connections

Slots can be used for receiving signals, but they are also normal
member functions. Just as an object does not know if anything receives
its signals, a slot does not know if it has any signals connected to
it. This ensures that truly independent components can be created with
Qt.

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.)

Together, signals and slots make up a powerful component programming
mechanism.

\section1 A Small Example

A minimal C++ class declaration might read:

\code
    class Foo
    {
    public:
	Foo();
	int value() const { return val; }
	void setValue( int );
    private:
	int val;
    };
\endcode

A small Qt class might read:

\code
    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;
    };
\endcode

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, \c{valueChanged()}, and it has
a slot which other objects can send signals to.

All classes that contain signals or slots must mention Q_OBJECT in
their declaration.

Slots are implemented by the application programmer.
Here is a possible implementation of Foo::setValue():

\code
    void Foo::setValue( int v )
    {
	if ( v != val ) {
	    val = v;
	    emit valueChanged(v);
	}
    }
\endcode

The line \c{emit valueChanged(v)} emits the signal
\c{valueChanged} from the object. As you can see, you emit a
signal by using \c{emit signal(arguments)}.

Here is one way to connect two of these objects together:

\code
    Foo a, b;
    connect(&a, SIGNAL(valueChanged(int)), &b, SLOT(setValue(int)));
    b.setValue( 11 ); // a == undefined  b == 11
    a.setValue( 79 ); // a == 79         b == 79
    b.value();        // returns 79        
\endcode

Calling \c{a.setValue(79)} will make \c{a} emit a \c{valueChanged()}
signal, which \c{b} will receive in its \c{setValue()} slot,
i.e. \c{b.setValue(79)} is called. \c{b} will then, in turn,
emit the same \c{valueChanged()} signal, but since no slot has been
connected to \c{b}'s \c{valueChanged()} signal, nothing happens (the
signal is ignored). 

Note that the \c{setValue()} function sets the value and emits
the signal only if \c{v != val}. This prevents infinite looping
in the case of cyclic connections (e.g. if \c{b.valueChanged()}
were connected to \c{a.setValue()}).

A signal is emitted for \e{every} connection you make, so if you
duplicate a connection, two signals will be emitted. You can always
break a connection using \c{QObject::disconnect()}.

This example illustrates that objects can work together without knowing
about each other, as long as there is someone around to set up a
connection between them initially.

The preprocessor changes or removes the \c{signals}, \c{slots} and
\c{emit} keywords so that the compiler is presented with standard C++.

Run the \link moc.html moc\endlink on class definitions that contain
signals or slots. This produces a C++ source file which should be compiled
and linked with the other object files for the application. If you use
\link qmake-manual.book qmake\endlink, the makefile rules to
automatically invoke the \link moc.html moc\endlink will be added to
your makefile for you.

\section1 Signals

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.

A list box, for example, emits both \c{clicked()} and
\c{currentChanged()} signals. Most objects will probably only be
interested in \c{currentChanged()} which gives the current list item
whether the user clicked it or used the arrow keys to move to it. But
some objects may only want to know which item was clicked. If the
signal is interesting to two different objects you just connect the
signal to slots in both objects.

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
\c{emit} will return when all slots have returned.

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.

Signals are automatically generated by the \link moc.html moc\endlink
and must not be implemented in the \c .cpp file. They can never have
return types (i.e. use \c void).

A note about arguments. Our experience shows that signals and slots
are more reusable if they do \e not use special types. If \l
QScrollBar::valueChanged() were to use a special type such as the
hypothetical \c QRangeControl::Range, it could only be connected to
slots designed specifically for QRangeControl. Something as simple as
the program in \link tutorial1-05.html Tutorial #1 part 5\endlink
would be impossible.

\section1 Slots

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, like signals, it is rarely wise to
use your own custom types for slot arguments.

Since slots are normal member functions with just a little extra
spice, they have access rights like ordinary member functions. A
slot's access right determines who can connect to it:

A \c{public slots} 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 that information is passed correctly, and, like a model
railway, turn it on and leave it running.

A \c{protected slots} section contains slots that this class and its
subclasses may connect signals to. This is intended for slots that are
part of the class's implementation rather than its interface to the
rest of the world.

A \c{private slots} 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.

You can also define slots to be virtual, which we have found quite
useful in practice.

The signals and slots mechanism is efficient, but not quite as fast as
"real" callbacks. Signals and slots are slightly slower because of the
increased flexibility they provide, although the difference for real
applications is insignificant. In general, emitting a signal that is
connected to some slots, is approximately ten times slower than
calling the receivers directly, with non-virtual function calls. This
is the overhead required to locate the connection object, to safely
iterate over all connections (i.e. checking that subsequent receivers
have not been destroyed during the emission) and to marshall any
parameters in a generic fashion. While ten non-virtual function calls
may sound like a lot, it's much less overhead than any 'new' or
'delete' operation, for example. As soon as you perform a string,
vector or list operation that behind the scene requires 'new' or
'delete', the signals and slots overhead is only responsible for a
very small proportion of the complete function call costs. The same is
true whenever you do a system call in a slot; or indirectly call more
than ten functions. On an i586-500, you can emit around 2,000,000
signals per second connected to one receiver, or around 1,200,000 per
second connected to two receivers. The simplicity and flexibility of
the signals and slots mechanism is well worth the overhead, which your
users won't even notice.


\section1 Meta Object Information

The meta object compiler (\link moc.html moc\endlink) parses the class
declaration in a C++ file and generates C++ code that initializes the
meta object. The meta object contains the names of all the signal and
slot members, as well as pointers to these functions. (For more
information on Qt's Meta Object System, see \link templates.html Why
doesn't Qt use templates for signals and slots?\endlink.)

The meta object contains additional information such as the object's \link
QObject::className() class name\endlink. You can also check if an object
\link QObject::inherits() inherits\endlink a specific class, for example:

\code
  if ( widget->inherits("QButton") ) {
	// yes, it is a push button, radio button etc.
  }
\endcode

\section1 A Real Example

Here is a simple commented example (code fragments from \l qlcdnumber.h ).

\code
    #include "qframe.h"
    #include "qbitarray.h"

    class QLCDNumber : public QFrame
\endcode

QLCDNumber inherits QObject, which has most of the signal/slot
knowledge, via QFrame and QWidget, and #include's the relevant
declarations.

\code
    {
	Q_OBJECT
\endcode

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 \link moc.html run the moc\endlink or to
include the moc output in the link command.

\code
    public:
	QLCDNumber( QWidget *parent=0, const char *name=0 );
	QLCDNumber( uint numDigits, QWidget *parent=0, const char *name=0 );
\endcode

It's not obviously relevant to the moc, but if you inherit QWidget you
almost certainly want to have the \e{parent} and \e{name}
arguments in your constructors, and pass them to the parent
constructor.

Some destructors and member functions are omitted here; the moc
ignores member functions.

\code
    signals:
	void	overflow();
\endcode

QLCDNumber emits a signal when it is asked to show an impossible
value.

If you don't care about overflow, or you know that overflow cannot
occur, you can ignore the overflow() signal, i.e. don't connect it to
any slot. 

If, on the other hand, you want to call two different error functions
when the number overflows, simply connect the signal to two different
slots. Qt will call both (in arbitrary order).

\code
    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 );
\endcode

A slot is a receiving function, used to get information about state
changes in other widgets. QLCDNumber uses it, as the code above
indicates, to set the displayed number. Since \c{display()} is part
of the class's interface with the rest of the program, the slot is
public.

Several of the example programs connect the newValue() signal of a
QScrollBar to the display() slot, so the LCD number continuously shows
the value of the scroll bar.

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.

Some irrelevant member functions have been omitted from this
example.

\code
    };
\endcode

*/