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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html> <head> <title> Attaching Data to Objects </title> <meta name="GENERATOR" content= "Modular DocBook HTML Stylesheet Version 1.45"> <link rel="HOME" title="GTK+ / Gnome Application Development" href="ggad.html"> <link rel="UP" title="The GTK+ Object and Type System" href= "cha-objects.html"> <link rel="PREVIOUS" title="Object Finalization" href= "sec-finalization.html"> <link rel="NEXT" title="GDK Basics" href="cha-gdk.html"> </head> <body bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink= "#840084" alink="#0000FF"> <div class="NAVHEADER"> <table width="100%" border="0" bgcolor="#ffffff" cellpadding= "1" cellspacing="0"> <tr> <th colspan="4" align="center"> <font color="#000000" size="2">GTK+ / Gnome Application Development</font> </th> </tr> <tr> <td width="25%" bgcolor="#ffffff" align="left"> <a href="sec-finalization.html"><font color="#0000ff" size="2"><b><<< Previous</b></font></a> </td> <td width="25%" colspan="2" bgcolor="#ffffff" align= "center"> <font color="#0000ff" size="2"><b><a href="ggad.html"> <font color="#0000ff" size="2"><b> Home</b></font></a></b></font> </td> <td width="25%" bgcolor="#ffffff" align="right"> <a href="cha-gdk.html"><font color="#0000ff" size="2"> <b>Next >>></b></font></a> </td> </tr> </table> </div> <div class="SECT1"> <h1 class="SECT1"> <a name="SEC-OBJECTDATA">Attaching Data to Objects</a> </h1> <p> You can "attach" arbitrary string-pointer pairs to a <span class="STRUCTNAME">GtkObject</span> instance, in effect adding a new data member. GTK+ uses this some internally, but it can also be a convenient way to pass data around in your application. In particular, it's a nice way to pass information to callbacks. </p> <p> Here's a simple example: </p> <table border="0" bgcolor="#E0E0E0" width="100%"> <tr> <td><pre class="PROGRAMLISTING"> GtkWidget* button = gtk_button_new(); GtkWidget* label = gtk_label_new(_("Foo")); gtk_object_set_data(GTK_OBJECT(button), "my_label_key", label); </pre> </td> </tr> </table> <p> Later, when you have a pointer to the button but not the label (perhaps in a callback connected to the button's <span class="SYMBOL">"clicked"</span> signal), you can do this: </p> <table border="0" bgcolor="#E0E0E0" width="100%"> <tr> <td><pre class="PROGRAMLISTING"> GtkWidget* label = gtk_object_get_data(GTK_OBJECT(button), "my_label_key"); /* If no data is found for the key, NULL is returned. */ if (label == NULL) { g_warning("No data was associated with 'my_label_key'!"); } </pre> </td> </tr> </table> <p> A pair of convenience functions use a predetermined key and thus save typing (and remembering) the object data key. These are <tt class="FUNCTION"> gtk_object_set_user_data()</tt> and <tt class="FUNCTION"> gtk_object_get_user_data()</tt>. You can also register a function to free the data when the data is removed or replaced, or the <span class="STRUCTNAME">GtkObject</span> is destroyed; This function should be of type <span class= "STRUCTNAME">GtkDestroyNotify</span>: </p> <table border="0" bgcolor="#E0E0E0" width="100%"> <tr> <td><pre class="PROGRAMLISTING"> typedef void (*GtkDestroyNotify) (gpointer data); </pre> </td> </tr> </table> <p> Conveniently, <tt class="FUNCTION">g_free()</tt> and <tt class="FUNCTION">gtk_object_unref()</tt> will work here. You register a "destroy notification" function when you set the data, using <tt class="FUNCTION"> gtk_object_set_data_full()</tt>. You can remove data before the object is destroyed with <tt class="FUNCTION"> gtk_object_remove_data()</tt>, or remove it without calling the destroy function with <tt class="FUNCTION"> gtk_object_remove_no_notify()</tt>. Setting the data to <span class="STRUCTNAME">NULL</span> is equivalent to removing it with <tt class="FUNCTION"> gtk_object_remove_data()</tt>, and will also call the destroy function if you registered one. <a href= "sec-objectdata.html#FL-OBJECTDATA">Figure 6</a> summarizes the object data functions. </p> <p> It's worth pointing out that the object data system is a thin wrapper around the <span class="STRUCTNAME"> GData</span> facility in glib, which can be used standalone. </p> <div class="FIGURE"> <a name="FL-OBJECTDATA"></a> <div class="FUNCSYNOPSIS"> <a name="FL-OBJECTDATA.SYNOPSIS"></a> <table border="0" bgcolor="#E0E0E0" width="100%"> <tr> <td><pre class="FUNCSYNOPSISINFO">#include <gtk/gtkobject.h></pre> </td> </tr> </table> <p> <code><code class="FUNCDEF">void <tt class="FUNCTION"> gtk_object_set_data</tt></code>(GtkObject* <tt class= "PARAMETER"><i>object</i></tt>, const gchar* <tt class= "PARAMETER"><i>key</i></tt>, gpointer <tt class= "PARAMETER"><i>data</i></tt>);</code> </p> <p> <code><code class="FUNCDEF">void <tt class="FUNCTION"> gtk_object_set_data_full</tt></code>(GtkObject* <tt class="PARAMETER"><i>object</i></tt>, const gchar* <tt class="PARAMETER"><i>key</i></tt>, gpointer <tt class= "PARAMETER"><i>data</i></tt>, GtkDestroyNotify <tt class="PARAMETER"><i>destroy</i></tt>);</code> </p> <p> <code><code class="FUNCDEF">void <tt class="FUNCTION"> gtk_object_remove_data</tt></code>(GtkObject* <tt class="PARAMETER"><i>object</i></tt>, const gchar* <tt class="PARAMETER"><i>key</i></tt>);</code> </p> <p> <code><code class="FUNCDEF">gpointer <tt class= "FUNCTION">gtk_object_get_data</tt></code>(GtkObject* <tt class="PARAMETER"><i>object</i></tt>, const gchar* <tt class="PARAMETER"><i>key</i></tt>);</code> </p> <p> <code><code class="FUNCDEF">void <tt class="FUNCTION"> gtk_object_remove_no_notify</tt></code>(GtkObject* <tt class="PARAMETER"><i>object</i></tt>, const gchar* <tt class="PARAMETER"><i>key</i></tt>);</code> </p> <p> <code><code class="FUNCDEF">void <tt class="FUNCTION"> gtk_object_set_user_data</tt></code>(GtkObject* <tt class="PARAMETER"><i>object</i></tt>, gpointer <tt class="PARAMETER"><i>data</i></tt>);</code> </p> <p> <code><code class="FUNCDEF">gpointer <tt class= "FUNCTION"> gtk_object_get_user_data</tt></code>(GtkObject* <tt class="PARAMETER"><i>object</i></tt>);</code> </p> </div> <p> <b>Figure 6. Attaching key-value pairs to a <span class= "STRUCTNAME">GtkObject</span></b> </p> </div> </div> <div class="NAVFOOTER"> <br> <br> <table width="100%" border="0" bgcolor="#ffffff" cellpadding= "1" cellspacing="0"> <tr> <td width="25%" bgcolor="#ffffff" align="left"> <a href="sec-finalization.html"><font color="#0000ff" size="2"><b><<< Previous</b></font></a> </td> <td width="25%" colspan="2" bgcolor="#ffffff" align= "center"> <font color="#0000ff" size="2"><b><a href="ggad.html"> <font color="#0000ff" size="2"><b> Home</b></font></a></b></font> </td> <td width="25%" bgcolor="#ffffff" align="right"> <a href="cha-gdk.html"><font color="#0000ff" size="2"> <b>Next >>></b></font></a> </td> </tr> <tr> <td colspan="2" align="left"> <font color="#000000" size="2"><b>Object Finalization</b></font> </td> <td colspan="2" align="right"> <font color="#000000" size="2"><b>GDK Basics</b></font> </td> </tr> </table> </div> </body></html>?? 快捷鍵說明
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