/usr/share/GNUstep/Documentation/GDL2Intro/Key-Value-Coding.html

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<a name="Key-Value-Coding"></a>
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<p>
Up: <a href="Concepts.html#Concepts" accesskey="u" rel="up">Concepts</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html#Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Key-Value-Coding-1"></a>
<h2 class="chapter">3 Key Value Coding</h2>
<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>
<a name="Setting-values-through-KVC"></a>
<h2 class="chapter">4 Setting values through KVC</h2>
<p>Setting values through key value coding will try to call a method
&rsquo;-setKeyName:&rsquo; 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 &lsquo;-setKeyName:&lsquo; and there is no 
instance variable with the same name as the key, an exception is thrown.
</p>

<div class="example">
<pre class="verbatim">[anObject setValue:@&quot;bar&quot; forKey:@&quot;foo&quot;];
</pre></div>

<p>Will first try to call -setFoo: then attempt to set the instance variable
named &quot;foo&quot; to &quot;bar&quot;.
</p>
<a name="Accessing-values-through-KVC"></a>
<h2 class="chapter">5 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><div class="example">
<pre class="verbatim">[anObject valueForKey:@&quot;foo&quot;];
</pre></div>

<p>Will first try to call -foo, then attempt to return instance variable named foo.
</p>
<a name="Key-Paths"></a>
<h2 class="chapter">6 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><div class="example">
<pre class="verbatim">[anObject valueForKeyPath:@&quot;foo.bar&quot;];
</pre></div>

<p>Will be equivalent to
</p><div class="example">
<pre class="verbatim">[[anObject valueForKey:@&quot;foo&quot;] valueForKey:@&quot;bar&quot;];
</pre></div>

<a name="Type-promotion"></a>
<h2 class="chapter">7 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><div class="example">
<pre class="verbatim">[@&quot;foo&quot; valueForKey:@&quot;length&quot;];
</pre></div>
<p>Returns a NSNumber object containing &rsquo;3&rsquo;.
</p>

<a name="Class-specific-implementation"></a>
<h2 class="chapter">8 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><div class="example">
<pre class="example">(&quot;Example&quot;, &quot;array&quot;, &quot;containing&quot;, &quot;strings&quot;)
</pre></div>

<p>If we get the value for the key length, it will return an NSArray of NSNumbers
</p><div class="example">
<pre class="example">(7, 5, 10, 7).
</pre></div>

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