<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Appendix B. Signals</title> <link rel="stylesheet" href="style.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.1"> <link rel="home" href="index.html" title="Programming with gtkmm"> <link rel="up" href="index.html" title="Programming with gtkmm"> <link rel="prev" href="sec-refptr-constness.html" title="Constness"> <link rel="next" href="sec-writing-signal-handlers.html" title="Writing signal handlers"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <div class="navheader"> <table width="100%" summary="Navigation header"> <tr><th colspan="3" align="center">Appendix B. Signals</th></tr> <tr> <td width="20%" align="left"> <a accesskey="p" href="sec-refptr-constness.html"><img src="icons/prev.png" alt="Prev"></a> </td> <th width="60%" align="center"> </th> <td width="20%" align="right"> <a accesskey="n" href="sec-writing-signal-handlers.html"><img src="icons/next.png" alt="Next"></a> </td> </tr> </table> <hr> </div> <div class="appendix" title="Appendix B. Signals"> <div class="titlepage"><div><div><h2 class="title"> <a name="chapter-signals"></a>Appendix B. Signals</h2></div></div></div> <div class="toc"> <p><b>Table of Contents</b></p> <ul> <li><span class="sect1"><a href="chapter-signals.html#sec-connecting-signal-handlers">Connecting signal handlers</a></span></li> <li><span class="sect1"><a href="sec-writing-signal-handlers.html">Writing signal handlers</a></span></li> <li><span class="sect1"><a href="sec-disconnecting-signal-handlers.html">Disconnecting signal handlers</a></span></li> <li><span class="sect1"><a href="sec-overriding-default-signal-handlers.html">Overriding default signal handlers</a></span></li> <li><span class="sect1"><a href="sec-binding-extra-arguments.html">Binding extra arguments</a></span></li> <li><span class="sect1"><a href="sec-xeventsignals.html">X Event signals</a></span></li> </ul> </div> <div class="sect1" title="Connecting signal handlers"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="sec-connecting-signal-handlers"></a>Connecting signal handlers</h2></div></div></div> <p> <span class="application">gtkmm</span> widget classes have signal accessor methods, such as <code class="methodname">Gtk::Button::signal_clicked()</code>, which allow you to connect your signal handler. Thanks to the flexibility of <span class="application">libsigc++</span>, the callback library used by <span class="application">gtkmm</span>, the signal handler can be almost any kind of function, but you will probably want to use a class method. Among <span class="application">GTK+</span> C coders, these signal handlers are often named callbacks. </p> <p> Here's an example of a signal handler being connected to a signal: </p> <p> </p> <pre class="programlisting"> #include <gtkmm/button.h> void on_button_clicked() { std::cout << "Hello World" << std::endl; } main() { Gtk::Button button("Hello World"); button.signal_clicked().connect(sigc::ptr_fun(&on_button_clicked)); } </pre> <p> </p> <p> There's rather a lot to think about in this (non-functional) code. First let's identify the parties involved: </p> <div class="itemizedlist"><ul class="itemizedlist" type="disc"> <li class="listitem"><p> The signal handler is <code class="methodname">on_button_clicked()</code>. </p></li> <li class="listitem"><p> We're hooking it up to the <code class="classname">Gtk::Button</code> object called <code class="varname">button</code>. </p></li> <li class="listitem"><p> When the Button emits its <code class="literal">clicked</code> signal, <code class="methodname">on_button_clicked()</code> will be called. </p></li> </ul></div> <p> Now let's look at the connection again: </p> <p> </p> <pre class="programlisting"> ... button.signal_clicked().connect(sigc::ptr_fun(&on_button_clicked)); ... </pre> <p> </p> <p> Note that we don't pass a pointer to <code class="methodname">on_button_clicked()</code> directly to the signal's <code class="methodname">connect()</code> method. Instead, we call <code class="function">sigc::ptr_fun()</code>, and pass the result to <code class="methodname">connect()</code>. </p> <p> <code class="function">sigc::ptr_fun()</code> generates a <code class="classname">sigc::slot</code>. A slot is an object which looks and feels like a function, but is actually an object. These are also known as function objects, or functors. <code class="function">sigc::ptr_fun()</code> generates a slot for a standalone function or static method. <code class="function">sigc::mem_fun()</code> generates a slot for a member method of a particular instance. </p> <p> Here's a slightly larger example of slots in action: </p> <p> </p> <pre class="programlisting"> void on_button_clicked(); class some_class { void on_button_clicked(); }; some_class some_object; main() { Gtk::Button button; button.signal_clicked().connect( sigc::ptr_fun(&on_button_clicked) ); button.signal_clicked().connect( sigc::mem_fun(some_object, &some_class::on_button_clicked) ); } </pre> <p> </p> <p> The first call to <code class="methodname">connect()</code> is just like the one we saw last time; nothing new here.</p> <p>The next is more interesting. <code class="function">sigc::mem_fun()</code> is called with two arguments. The first argument is <em class="parameter"><code>some_object</code></em>, which is the object that our new slot will be pointing at. The second argument is a pointer to one of its methods. This particular version of <code class="function">sigc::mem_fun()</code> creates a slot which will, when "called", call the pointed-to method of the specified object, in this case <code class="methodname">some_object.on_button_clicked()</code>. </p> <p> Another thing to note about this example is that we made the call to <code class="methodname">connect()</code> twice for the same signal object. This is perfectly fine - when the button is clicked, both signal handlers will be called. </p> <p> We just told you that the button's <code class="literal">clicked</code> signal is expecting to call a method with no arguments. All signals have requirements like this - you can't hook a function with two arguments to a signal expecting none (unless you use an adapter, such as <code class="function">sigc::bind()</code>, of course). Therefore, it's important to know what type of signal handler you'll be expected to connect to a given signal. </p> </div> </div> <div class="navfooter"> <hr> <table width="100%" summary="Navigation footer"> <tr> <td width="40%" align="left"> <a accesskey="p" href="sec-refptr-constness.html"><img src="icons/prev.png" alt="Prev"></a> </td> <td width="20%" align="center"> </td> <td width="40%" align="right"> <a accesskey="n" href="sec-writing-signal-handlers.html"><img src="icons/next.png" alt="Next"></a> </td> </tr> <tr> <td width="40%" align="left" valign="top">Constness </td> <td width="20%" align="center"><a accesskey="h" href="index.html"><img src="icons/home.png" alt="Home"></a></td> <td width="40%" align="right" valign="top"> Writing signal handlers</td> </tr> </table> </div> </body> </html>