Add Qt3 development HEAD version

author: Timothy Pearson <[email protected]> 2011-07-10 15:24:15 -0500
committer: Timothy Pearson <[email protected]> 2011-07-10 15:24:15 -0500
commit: bd0f3345a938b35ce6a12f6150373b0955b8dd12 (patch)
tree: 7a520322212d48ebcb9fbe1087e7fca28b76185c /doc/html/signalsandslots.html
download: qt3-bd0f3345a938b35ce6a12f6150373b0955b8dd12.tar.gz
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+<title>Signals and Slots</title>
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+ <a href="index.html">
+<font color="#004faf">Home</font></a>
+ | <a href="classes.html">
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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>Signals and Slots</h1>
+
+ 
+<p> 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 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. 
+<p> 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.
+<p> <center><img src="abstract-connections.png"></center> <blockquote><p align="center"><em> An abstract view of some signals and slots connections
+</em></p>
+</blockquote><p> 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. 
+<p> 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!
+<p> All classes that inherit from <a href="qobject.html">QObject</a> or one of its subclasses
+(e.g. <a href="qwidget.html">QWidget</a>) 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.
+<p> <center><img src="concrete-connections.png"></center> <blockquote><p align="center"><em> An example of signals and slots connections
+</em></p>
+</blockquote><p> 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.
+<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>
+<a name="1"></a><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 <a href="qobject.html">QObject</a>
+    {
+        <a href="metaobjects.html#Q_OBJECT">Q_OBJECT</a>
+    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, <tt>valueChanged()</tt>, and it has
+a slot which other objects can send signals to.
+<p> All classes that contain signals or slots must mention Q_OBJECT in
+their declaration.
+<p> Slots are implemented by the application programmer.
+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 <tt>emit valueChanged(v)</tt> emits the signal
+<tt>valueChanged</tt> from the object. As you can see, you emit a
+signal by using <tt>emit signal(arguments)</tt>.
+<p> Here is one way to connect two of these objects together:
+<p> <pre>
+    Foo a, b;
+    connect(&amp;a, SIGNAL(valueChanged(int)), &amp;b, SLOT(setValue(int)));
+    b.setValue( 11 ); // a == undefined  b == 11
+    a.setValue( 79 ); // a == 79         b == 79
+    b.value();        // returns 79        
+</pre>
+ 
+<p> Calling <tt>a.setValue(79)</tt> will make <tt>a</tt> emit a <tt>valueChanged()</tt>
+signal, which <tt>b</tt> will receive in its <tt>setValue()</tt> slot,
+i.e. <tt>b.setValue(79)</tt> is called. <tt>b</tt> will then, in turn,
+emit the same <tt>valueChanged()</tt> signal, but since no slot has been
+connected to <tt>b</tt>'s <tt>valueChanged()</tt> signal, nothing happens (the
+signal is ignored). 
+<p> Note that the <tt>setValue()</tt> function sets the value and emits
+the signal only if <tt>v != val</tt>. This prevents infinite looping
+in the case of cyclic connections (e.g. if <tt>b.valueChanged()</tt>
+were connected to <tt>a.setValue()</tt>).
+<p> A signal is emitted for <em>every</em> connection you make, so if you
+duplicate a connection, two signals will be emitted. You can always
+break a connection using <a href="qobject.html#disconnect">QObject::disconnect</a>().
+<p> 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.
+<p> The preprocessor changes or removes the <tt>signals</tt>, <tt>slots</tt> and
+<tt>emit</tt> keywords so that the compiler is presented with standard C++.
+<p> Run the <a href="moc.html">moc</a> 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
+<a href="qmake-manual.html">qmake</a>, the makefile rules to
+automatically invoke the <a href="moc.html">moc</a> will be added to
+your makefile for you.
+<p> <h2> Signals
+</h2>
+<a name="2"></a><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 example, emits both <tt>clicked()</tt> and
+<tt>currentChanged()</tt> signals. Most objects will probably only be
+interested in <tt>currentChanged()</tt> 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.
+<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
+<tt>emit</tt> 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 <a href="moc.html">moc</a>
+and must not be implemented in the <tt>.cpp</tt> file. They can never have
+return types (i.e. use <tt>void</tt>).
+<p> A note 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#valueChanged">QScrollBar::valueChanged</a>() were to use a special type such as the
+hypothetical <tt>QRangeControl::Range</tt>, it could only be connected to
+slots designed specifically for <a href="qrangecontrol.html">QRangeControl</a>. Something as simple as
+the program in <a href="tutorial1-05.html">Tutorial #1 part 5</a>
+would be impossible.
+<p> <h2> Slots
+</h2>
+<a name="3"></a><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, like signals, it is rarely wise to
+use your own custom types for slot arguments.
+<p> 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:
+<p> A <tt>public slots</tt> 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.
+<p> A <tt>protected slots</tt> 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.
+<p> A <tt>private slots</tt> 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> You can also define slots to be virtual, which we have found quite
+useful in practice.
+<p> 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.
+<p> <h2> Meta Object Information
+</h2>
+<a name="4"></a><p> The <a href="metaobjects.html#meta-object">meta object</a> compiler (<a href="moc.html"><a href="moc.html#moc">moc</a></a>) 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 <a href="templates.html">Why
+doesn't Qt use templates for signals and slots?</a>.)
+<p> The meta object contains additional information such as the object's <a href="qobject.html#className">class name</a>. You can also check if an object
+<a href="qobject.html#inherits">inherits</a> a specific class, for example:
+<p> <pre>
+  if ( widget-&gt;inherits("QButton") ) {
+        // yes, it is a push button, radio button etc.
+  }
+</pre>
+ 
+<p> <h2> A Real Example
+</h2>
+<a name="5"></a><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 <a href="qframe.html">QFrame</a>
+</pre>
+ 
+<p> <a href="qlcdnumber.html">QLCDNumber</a> inherits <a href="qobject.html">QObject</a>, which has most of the signal/slot
+knowledge, via <a href="qframe.html">QFrame</a> and <a href="qwidget.html">QWidget</a>, 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 );
+        QLCDNumber( 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 the <em>parent</em> and <em>name</em>
+arguments in 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> <a href="qlcdnumber.html">QLCDNumber</a> emits a signal when it is asked to show an impossible
+value.
+<p> 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. 
+<p> 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).
+<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 the code above
+indicates, to set the displayed number. Since <tt>display()</tt> is part
+of the class's interface with the rest of the program, the slot is
+public.
+<p> Several of the example programs connect the newValue() signal of a
+<a href="qscrollbar.html">QScrollBar</a> 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 irrelevant member functions have been omitted from this
+example.
+<p> <pre>
+    };
+</pre>
+ 
+<p> 
+<!-- eof -->
+<p><address><hr><div align=center>
+<table width=100% cellspacing=0 border=0><tr>
+<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>
+</table></div></address></body>
+</html>
author	Timothy Pearson <[email protected]>	2011-07-10 15:24:15 -0500
committer	Timothy Pearson <[email protected]>	2011-07-10 15:24:15 -0500
commit	bd0f3345a938b35ce6a12f6150373b0955b8dd12 (patch)
tree	7a520322212d48ebcb9fbe1087e7fca28b76185c /doc/html/signalsandslots.html
download	qt3-bd0f3345a938b35ce6a12f6150373b0955b8dd12.tar.gz qt3-bd0f3345a938b35ce6a12f6150373b0955b8dd12.zip