/usr/share/GNUstep/Documentation/GDL2Intro_3.html is in gnustep-dl2 0.12.0-10.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html401/loose.dtd">
<html>
<!-- C2006 Free Software Foundation
Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions.
-->
<!-- Created on May 20, 2013 by texi2html 1.82
texi2html was written by:
Lionel Cons <Lionel.Cons@cern.ch> (original author)
Karl Berry <karl@freefriends.org>
Olaf Bachmann <obachman@mathematik.uni-kl.de>
and many others.
Maintained by: Many creative people.
Send bugs and suggestions to <texi2html-bug@nongnu.org>
-->
<head>
<title>GNUstep Database Library Introduction 0.1: 3. Key Value Coding</title>
<meta name="description" content="GNUstep Database Library Introduction 0.1: 3. Key Value Coding">
<meta name="keywords" content="GNUstep Database Library Introduction 0.1: 3. Key Value Coding">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="texi2html 1.82">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.smallquotation {font-size: smaller}
pre.display {font-family: serif}
pre.format {font-family: serif}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: serif; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: serif; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.roman {font-family:serif; font-weight:normal;}
span.sansserif {font-family:sans-serif; font-weight:normal;}
ul.toc {list-style: none}
-->
</style>
</head>
<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
<a name="Key-Value-Coding"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="GDL2Intro_2.html#Concepts" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Setting-values-through-KVC" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="GDL2Intro_2.html#Concepts" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro.html#Top" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_4.html#Classes" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="GDL2Intro.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_11.html#Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<a name="Key-Value-Coding-1"></a>
<h1 class="chapter">3. Key Value Coding</h1>
<a name="index-KVC_002c-Key-Value-Coding"></a>
<p>Key Value Coding is a concept used widely throughout GDL2,
it provides a mechanism by where you can access and modify an objects
set/accessor methods or even instance variables directly, through a named key.
</p>
<p>Additionally some classes may implement KVC in a way specific to the class.
</p>
<hr size="6">
<a name="Setting-values-through-KVC"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Key-Value-Coding" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Accessing-values-through-KVC" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Key-Value-Coding" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Key-Value-Coding" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_4.html#Classes" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="GDL2Intro.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_11.html#Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<h2 class="section">3.1 Setting values through KVC</h2>
<p>Setting values through key value coding will try to call a method
’-setKeyName:’ with the value as the parameter to -setKeyName:
as a parameter failing that, if anObject had an instance variable
with the same name as the key that would be modified directly.
</p>
<p>If anObject does not respond to ‘-setKeyName:‘ and there is no
instance variable with the same name as the key, an exception is thrown.
</p>
<table><tr><td> </td><td><pre class="verbatim">[anObject setValue:@"bar" forKey:@"foo"];
</pre></td></tr></table>
<p>Will first try to call -setFoo: then attempt to set the instance variable
named "foo" to "bar".
</p>
<hr size="6">
<a name="Accessing-values-through-KVC"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Setting-values-through-KVC" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Key-Paths" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Key-Value-Coding" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Key-Value-Coding" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_4.html#Classes" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="GDL2Intro.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_11.html#Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<h2 class="section">3.2 Accessing values through KVC</h2>
<p>Accessing values through Key Value Coding first attempts to
call the -keyName method on anObject if it responds.
If the object does not respond then it will try to access an instance
variable with the name of the key.
</p>
<p>If there is no method or instance variable with the name of the key an
exception will be thrown.
</p>
<p>For example,
</p><table><tr><td> </td><td><pre class="verbatim">[anObject valueForKey:@"foo"];
</pre></td></tr></table>
<p>Will first try to call -foo, then attempt to return instance variable named foo.
</p>
<hr size="6">
<a name="Key-Paths"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Accessing-values-through-KVC" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Type-promotion" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Key-Value-Coding" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Key-Value-Coding" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_4.html#Classes" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="GDL2Intro.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_11.html#Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<h2 class="section">3.3 Key Paths</h2>
<p>Key paths are a list of keys separated by a dot.
</p>
<p>The first key accesses the key on the target object through normal KVC,
and each subsequent key is sent to the object returned through the previous
key in the list.
</p>
<p>For example,
</p><table><tr><td> </td><td><pre class="verbatim">[anObject valueForKeyPath:@"foo.bar"];
</pre></td></tr></table>
<p>Will be equivalent to
</p><table><tr><td> </td><td><pre class="verbatim">[[anObject valueForKey:@"foo"] valueForKey:@"bar"];
</pre></td></tr></table>
<hr size="6">
<a name="Type-promotion"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Key-Paths" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="#Class-specific-implementation" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Key-Value-Coding" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Key-Value-Coding" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_4.html#Classes" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="GDL2Intro.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_11.html#Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<h2 class="section">3.4 Type promotion</h2>
<p>When a accessing a key, you may access keys for things such as standard c numerical
types, and they will be automatically promoted to their object equivalent
</p>
<p>For example:
</p><table><tr><td> </td><td><pre class="verbatim">[@"foo" valueForKey:@"length"];
</pre></td></tr></table>
<p>Returns a NSNumber object containing ’3’.
</p>
<hr size="6">
<a name="Class-specific-implementation"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Type-promotion" title="Previous section in reading order"> < </a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_4.html#Classes" title="Next section in reading order"> > </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="#Key-Value-Coding" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="#Key-Value-Coding" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_4.html#Classes" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="GDL2Intro.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_11.html#Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<h2 class="section">3.5 Class specific implementation</h2>
<p>By implementing valueForKey: and setValueForKey: classes can implement
functionality to contain keys in an instance variable such as a dictionary,
but they can also implement something to work on a collection of objects.
</p>
<p>For instance NSArray implements KVC to forward key value coding to all objects
in the array.
</p>
<p>Suppose we have an array contain a few string objects.
</p><table><tr><td> </td><td><pre class="example">("Example", "array", "containing", "strings")
</pre></td></tr></table>
<p>If we get the value for the key length, it will return an NSArray of NSNumbers
</p><table><tr><td> </td><td><pre class="example">(7, 5, 10, 7).
</pre></td></tr></table>
<hr size="6">
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="#Key-Value-Coding" title="Beginning of this chapter or previous chapter"> << </a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_4.html#Classes" title="Next chapter"> >> </a>]</td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left"> </td>
<td valign="middle" align="left">[<a href="GDL2Intro.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_11.html#Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="GDL2Intro_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
<p>
<font size="-1">
This document was generated by <em>root</em> on <em>May 20, 2013</em> using <a href="http://www.nongnu.org/texi2html/"><em>texi2html 1.82</em></a>.
</font>
<br>
</p>
</body>
</html>
|