[paragui-cvs] CVS: paragui/src/libsigc++/doc/manual Makefile.am,NONE,1.1.2.1 README,NONE,1.1.2.1 libsigc_manual.xml,NONE,1.1.2.1

[Alexander Pipelka <address@hidden>]

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<?xml version="1.0"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN" 
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"; >
<book>

<bookinfo>
        <title>LibSigC++</title>
        <author>
          <firstname>Ainsley</firstname>
          <surname>Pereira</surname>
        </author>

        <pubdate>September 2002</pubdate>

        <abstract>
          <para>LibSigC++ is a C++ template library implementing typesafe 
callbacks. This is an intro to LibSigC++.</para>
        </abstract>
</bookinfo>

<chapter id="sec-introduction">
<title>Introduction</title>

<sect1>
<title>Motivation</title>

        <para>There are many situations in which it is desirable to decouple 
code that
        detects an event, and the code that deals with it. This is especially 
common in
        GUI programming, where a toolkit might provide user interface elements 
such as
        clickable buttons but, being a generic toolkit, doesn't know how an 
individual
        application using that toolkit should handle the user clicking on 
it.</para>

        <para>In C the callbacks are generally handled by the application 
calling a
        'register' function and passing a pointer to a function and a <literal 
remap="tt">void *</literal>
        argument, eg.</para>

<programlisting>
void clicked(void *data);

button * okbutton = create_button("ok");
static char somedata[] = "This is some data I want the clicked() function to 
have";

register_click_handler(okbutton, clicked, somedata);
</programlisting>

        <para>When clicked, the toolkit will call <literal 
remap="tt">clicked()</literal> with the data pointer passed
        to the <literal remap="tt">register_click_handler</literal> 
function.</para>

        <para>This works in C, but is not typesafe. There is no compile-time 
way of
        ensuring that <literal remap="tt">clicked()</literal> isn't expecting a 
struct of some sort instead of a
        <literal remap="tt">char *</literal>.</para>

        <para>As C++ programmers, we want type safety. We also want to be able 
to use
        things other than free-standing functions as callbacks.</para>

        <para>LibSigC++ provides the concept of a slot, which holds a reference 
to one of
        the things that can be used as a callback:
        <itemizedlist>
            <listitem>free-standing functions as in the example</listitem>
            <listitem>pointers to objects that define operator()</listitem>
            <listitem>pointer-to-member and instance of object to invoke that 
on (the
                object must inherit from <literal 
remap="tt">SigC::Object</literal>)</listitem>
        </itemizedlist></para>

        <para>All of which can take different numbers and types of 
arguments.</para>

        <para>To make it easier to construct these, an overloaded template 
called <literal remap="tt">slot</literal>
        is provided. slot takes an argument (or, where necessary arguments) and 
returns
        a generic Slot type that can be invoked with <literal 
remap="tt">operator()</literal>.</para>

        <para>For the other side of the fence, LibSigC++ provides <literal 
remap="tt">signal</literal>s, to which the
        client can attach <literal remap="tt">slot</literal>s. When the 
<literal remap="tt">signal</literal> is emitted, all the connected
        <literal remap="tt">slot</literal>s are called back.</para>
</sect1>
</chapter>

<chapter id="sec-connecting">
<title>Connecting your code to signals</title>

<sect1>
<title>A simple example</title>
        <para>The terminology for signals and slots has come under some
        criticism for not being as intuitive as it maybe could have been, but 
with a little
        experience it won't cause a problem. Honest.</para>

        <para>So to get some experience, lets look at a simple example...</para>

        <para>Lets say you and I are writing an application which informs the 
user when
        aliens land in the car park. To keep the design nice and clean, and 
allow for
        maximum portability to different interfaces, we decide to use LibSigC++ 
to
        split the project in two parts.</para>

        <para>I will write the <literal remap="tt">AlienDetector</literal> 
class, and you will write the code to inform
        the user. (Well, OK, I'll write both, but we're pretending, 
remember?)</para>

        <para>Here's my class:</para>

<programlisting>
class AlienDetector
{
public:
    AlienDetector();

    void run();

    SigC::Signal0&lt;void&gt; detected;
};
</programlisting>

                <para>(I'll explain the type of detected later.)</para>

                <para>Here's your code that uses it:</para>

<programlisting>
void warn_people()
{
    cout &lt;&lt; "There are aliens in the carpark!" &lt;&lt; endl;
}

int main()
{
    AlienDetector mydetector;
    mydetector.detected.connect( SigC::slot(warn_people) );

    mydetector.run();

    return 0;
}
</programlisting>

        <para>Pretty simple really - you call the <literal 
remap="tt">connect()</literal> method on the signal to
        connect your function. <literal remap="tt">connect()</literal> takes a 
<literal remap="tt">slot</literal> parameter (remember slots
        are capable of holding any type of callback), so you convert your
        <literal remap="tt">warn_people()</literal> function to a slot using 
the <literal remap="tt">slot()</literal> function.</para>

        <para>To compile this example from the downloadable example code, 
use:</para>
        <programlisting>g++ example1.cc -o eg1 `pkg-config --cflags --libs 
sigc++-1.2`</programlisting>
        <para>Note that those `` characters are backticks, not single quotes. 
Run it with</para>
        <programlisting>./eg1</programlisting>
        <para>(Try not to panic when the aliens land!)</para>

</sect1>

<sect1>
<title>Using a member function</title>

        <para>Suppose you found a more sophisticated alien alerter class on the 
web,
        such as this:</para>

<programlisting>
class AlienAlerter : public SigC::Object
{
public:
    AlienAlerter(char const* servername);
    void alert();
private:
    // ...
};
</programlisting>

        <para>(Handily it derives from <literal 
remap="tt">SigC::Object</literal> already. This isn't quite so
        unlikely as you might think; all appropriate bits of the popular gtkmm 
library do so,
        for example.)</para>

        <para>You could rewrite your code as follows:</para>

<programlisting>
int main()
{
    AlienDetector mydetector;
    AlienAlerter  myalerter("localhost");       // added
    mydetector.detected.connect( SigC::slot(myalerter, 
&amp;AlienAlerter::alert) ); // changed

    mydetector.run();

    return 0;
}
</programlisting>

        <para>Note that only 2 lines are different - one to create an instance 
of the
        class, and the line to connect the method to the signal.</para>

        <para>This code is in example2.cc, which can be compiled in the same 
way as
        example1.cc</para>
</sect1>

<sect1>
<title>Signals with parameters</title>

        <para>Functions taking no parameters and returning void are quite 
useful,
        especially when they're members of classes that can store unlimited 
amounts of
        safely typed data, but they're not sufficient for everything.</para>

        <para>What if aliens don't land in the carpark, but somewhere else? 
Let's modify
        the example so that the callback function takes a <literal 
remap="tt">std::string</literal> with the location
        in which aliens were detected.</para>

        <para>I change my class to:</para>

<programlisting>
class AlienDetector
{
public:
    AlienDetector();

    void run();

    SigC::Signal1&lt;void, std::string&gt; detected;    // changed
};
</programlisting>

        <para>The only line I had to change was the signal line (in <literal 
remap="tt">run()</literal> I need to change
        my code to supply the argument when I emit the signal too, but that's 
not shown
        here).</para>

        <para>The name of the type is '<literal remap="tt">SignalN</literal>', 
where N is the number of arguments that
        the slots should take. The template parameters are the return type, 
then the
        argument types.</para>

        <para>Obviously LibSigC++ doesn't define an infinite number of <literal 
remap="tt">Signal</literal> templates,
        but it does define <literal remap="tt">Signal0</literal>-<literal 
remap="tt">Signal5</literal>, which should be enough for most people. (If
        you know M4, you can tweak it to provide more if you really need 
to.)</para>

        <para>The types in the function signature are in the same order as the 
template
        parameters, eg:</para>

<programlisting>
SigC::Signal1&lt;void,         std::string&gt;
              void function(std::string foo);
</programlisting>

                <para>So now you can update your alerter (for simplicity, lets 
go back to the
                free-standing function version):</para>

<programlisting>
void warn_people(std::string where)
{
    cout &lt;&lt; "There are aliens in " &lt;&lt; where &lt;&lt; "!" &lt;&lt; 
endl;
}

int main()
{
    AlienDetector mydetector;
    mydetector.detected.connect( SigC::slot(warn_people) );

    mydetector.run();

    return 0;
}
</programlisting>

        <para>Easy.</para>
</sect1>

<sect1>
<title>Disconnecting</title>

        <para>If you decide you no longer want your code to be called whenever 
a signal is
        emitted, you must remember the return value of <literal 
remap="tt">connect()</literal>, which we've been
        ignoring until now.</para>

        <para><literal remap="tt">connect()</literal> returns a <literal 
remap="tt">SigC::Connection</literal> object, which has a member
        <literal remap="tt">disconnect()</literal>. This does just what you 
think it does.</para>

</sect1>
</chapter>

<chapter id="sec-writing">
<title>Writing your own signals</title>

<sect1>
<title>Quick recap</title>
        <para>If all you want to do is use gtkmm, and connect your 
functionality to its
        signals, you can probably stop reading here.</para>

        <para>You might benefit from reading on anyway though, as this section 
is going to
        be quite simple, and the 'Rebinding' technique from the next section is
        occasionally useful.</para>

        <para>We've already covered the way the types of signals are made up, 
but lets
        recap:</para>

        <para>A signal is an instance of a template, named <literal 
remap="tt">SigC::SignalN</literal> where
        N is the number of arguments taken, 0-5. The template arguments are the 
types,
        in the order they appear in the function signature that can be 
connected to that
        signal; that is the return type, then the argument types.</para>

        <para>To provide a signal for people to connect to, you must make 
available an
        instance of that <literal remap="tt">SigC::Signal</literal>. In 
<literal remap="tt">AlienDetector</literal> this was done
        with a public data member. That's not considered good practice usually, 
so you
        might want to consider making a member function that returns the signal 
by
        reference. (This is what gtkmm does.)</para>

        <para>Once you've done this, all you have to do is emit the signal when 
you're
        ready. Look at the code for <literal 
remap="tt">AlienDetector::run()</literal>:</para>

<programlisting>
void AlienDetector::run()
{
    sleep(3); // wait for aliens
    detected.emit(); // panic!
}
</programlisting>

        <para>As a shortcut, <literal remap="tt">Signal</literal> defines 
<literal remap="tt">operator()</literal> as a synonym for
        <literal remap="tt">emit()</literal>, so you could just write <literal 
remap="tt">detected();</literal> as in the second
        example version:</para>

<programlisting>
void AlienDetector::run()
{
    sleep(3);                // wait for aliens
    detected("the carpark"); // this is the std::string version, looks like
                             // they landed in the carpark afterall.
}
</programlisting>
</sect1>

<sect1>
<title>What about return values?</title>
        <para>If you only ever have one slot connected to a signal, or if you 
only care
        about the return value of the last registered one, it's quite 
straightforward:</para>

<programlisting>
SigC::Signal0&lt;int&gt; somesignal;
int a_return_value;

a_return_value = somesignal();
</programlisting>

                <para>If you care about every return value things are a little 
more complicated.
                See the section on Marshallers for more info.</para>
</sect1>
</chapter>

<chapter id="sec-advanced">
<title>Advanced topics</title>

<sect1>
<title>Rebinding</title>
        <para>Suppose you already have a function that you want to be called 
back when a
        signal is emitted, but it takes the wrong argument types. For example, 
lets try
        to attach the <literal remap="tt">warn_people(std::string)</literal> 
function to the detected signal
        from the first example, which didn't supply a location string.</para>

        <para>Just trying to connect it with:</para>

<programlisting>
myaliendetector.detected.connect(SigC::slot(warn_people));
</programlisting>

        <para>results in a compile-time error, because the types don't match. 
This is good!
        This is typesafety at work. In the C way of doing things, this would 
have just
        died at runtime after trying to print a random bit of memory as the 
location -
        ick!</para>

        <para>We have to make up a location string, and bind it to the 
function, so that
        when detected is emitted with no arguments, something adds it in before
        <literal remap="tt">warn_people</literal> is actually called.</para>
        <para>We could write it ourselves - it's not hard:</para>

<programlisting>
void warn_people_wrapper() // note this is the signature that 'detected' expects
{
    warn_people("the carpark");
}
</programlisting>

        <para>but after our first million or so we might start looking for a 
better way. As
        it happens, LibSigC++ has one.</para>

<programlisting>
SigC::bind(slot, arg);
</programlisting>

        <para>binds arg as the argument to slot, and returns a new slot of the 
same return
        type, but with one fewer arguments.</para>

        <para>Now we can write:</para>
<programlisting>
myaliendetector.detected.connect(SigC::bind( SigC::slot(warn_people), "the 
carpark" ) );
</programlisting>

        <para>If the input slot has multiple args, the rightmost one is 
bound.</para>

        <para>The return type can also be bound with <literal 
remap="tt">bind_return(slot, returnvalue);</literal> though
        this is not so commonly useful.</para>

        <para>So if we can attach the new <literal 
remap="tt">warn_people()</literal> to the old detector, can we attach
        the old <literal remap="tt">warn_people</literal> (the one that didn't 
take an argument) to the new detector?</para>

        <para>Of course, we just need to hide the extra argument. This can be 
done with
        <literal remap="tt">SigC::hide</literal>, eg.</para>

<programlisting>
myaliendetector.detected.connect( SigC::hide&lt;std::string&gt;( 
SigC::slot(warn_people) ) );
</programlisting>

        <para>The template arguments are the types to hide (from the right only 
- you can't
        hide the first argument of 3, for example, only the last).</para>

        <para><literal remap="tt">hide_return</literal> effectively makes the 
return type void.</para>
</sect1>

<sect1>
<title>Retyping</title>
        <para>A similar topic is retyping. Perhaps you have a signal that takes 
an <literal remap="tt">int</literal>, but
        you want to connect a function that takes a <literal 
remap="tt">double</literal>.</para>

        <para>This can be achieved with the <literal 
remap="tt">retype</literal> template. <literal remap="tt">retype</literal> has 
template arguments
        just like <literal remap="tt">Signal</literal> - return value, signal 
types.</para>

        <para>It's a function template that takes a <literal 
remap="tt">slot</literal>, and returns a <literal remap="tt">slot</literal>. 
eg.</para>

<programlisting>
void dostuff(double foo)
{
}

SigC::Signal1&lt;void,int&gt; asignal;

asignal.connect(  retype&lt;void,int&gt;( slot(&amp;dostuff) )  );
</programlisting>

        <para>If you only want to change the return type, you can use <literal 
remap="tt">retype_return</literal>.
        <literal remap="tt">retype_return</literal> needs only one template 
argument.</para>
</sect1>

<sect1>
<title>Marshallers</title>
        <para>When I first mentioned return values, I said that more advanced 
handling of
        multiple return values was possible with <literal 
remap="tt">Marshallers</literal>.</para>

        <para>A Marshaller is a class that gets fed all the return values as 
they're
        returned. It can do a couple of things:
        <itemizedlist>
            <listitem>It can stop the emit process at any point, causing no 
further slots
                to be called</listitem>
            <listitem>It can return a value, of any type</listitem>
        </itemizedlist></para>

        <para>For example, if each <literal remap="tt">slot</literal> returned 
an <literal remap="tt">int</literal>, we could use a marshaller return
        the average value as a <literal remap="tt">double</literal>. Or we 
could return all values in a
        <literal remap="tt">std::vector&lt;int&gt;</literal>, or maybe stop as 
soon as the first slot returns 5.</para>

        <para>As an example, here's the averaging marshaller:</para>

<programlisting>
class Averager
{
public:
    // we must typedef InType and OutType for the SigC library
    typedef double OutType;
    typedef int InType;

    Averager()
      : total_(0), number_(0)
      {}


    OutType value() { return (double)total_/(double)number_; } // avoid integer 
division

    static OutType default_value() { return 0; }

    // This is the function called for each return value.
    // If it returns 'true' it stops here.
    bool marshal(InType newval)
    {
        total_ += newval; // total of values
        ++number_;        // count of values
        return false;     // continue emittion process
    };

private:
    int   total_;
    int   number_;
};
</programlisting>

        <para>To use this, we pass the type as an extra template argument when 
defining
        the <literal remap="tt">Signal</literal>, eg.</para>

<programlisting>
SigC::Signal0&lt;int,Averager&gt; mysignal;
</programlisting>

        <para>Now we can do:</para>
<programlisting>
double average_of_all_connected_slots = mysignal();
</programlisting>

        <para>Each connected <literal remap="tt">slot</literal> will be called, 
its value passed to an instance of
        <literal remap="tt">Averager</literal> and that <literal 
remap="tt">Averager</literal>'s <literal remap="tt">value()</literal> will be 
returned.</para>

        <para>In the downloadable examples, this is example6.cc.</para>
</sect1>
</chapter>

<chapter id="sec-reference">
<title>Reference</title>
        <para>See the reference documentation <ulink 
url="http://libsigc.sourceforge.net/libsigc1_2/reference/";>online</ulink></para>
</chapter>
</book>