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<h2>De Re mulle-objc <div style="font-size:10px; display: inline-block;">0.50.0</div> <a href="feed.xml"><img class="inline-img" src='images/feed-icon-28x28.png'></a></h2>
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<li class="no-bullet"><a class="internal-link" href="/de-re-mulle-objc/intro.html" title="De Re mulle-objc">De Re mulle-objc</a>
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<li class="no-bullet"><a class="internal-link" href="/de-re-mulle-objc/modern.html" title="Project Setup">Project Setup</a>
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<li class="no-bullet"><a class="internal-link" href="/de-re-mulle-objc/pnp-dependency.html" title="Dependencies">Dependencies</a>
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<li class="no-bullet"><a class="internal-link" href="/de-re-mulle-objc/modern-complex.html" title="Projects with Multiple Targets">Projects with Multiple Targets</a>
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<li class="no-bullet"><a class="internal-link" href="/de-re-mulle-objc/good.html" title="Good mulle-objc Code">Good mulle-objc Code</a>
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<article>
<li class="no-bullet"><a class="internal-link" href="/de-re-mulle-objc/basics.html" title="A complete class">A complete class</a>
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<li class="no-bullet"><a class="internal-link" href="/de-re-mulle-objc/objects.html" title="Objects and Properties">Objects and Properties</a>
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<li class="no-bullet"><a class="internal-link" href="/de-re-mulle-objc/pnp-class.html" title="Plug And Play Class">Plug And Play Class</a>
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<li class="no-bullet"><a class="internal-link" href="/de-re-mulle-objc/classcluster.html" title="Advanced: Classcluster Classes">Advanced: Classcluster Classes</a>
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<h5> Or read it like a <a href="book.html"><b>book</b></a>.</h6>
</div> <!-- contents -->
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<div id="post-second" class="eight columns">
<div class="title">
<div>
<h2>De Re mulle-objc <div style="font-size:10px; display: inline-block;">0.50.0</div> <a href="feed.xml"><img class="inline-img" src='images/feed-icon-28x28.png'></a></h2>
</div>
<h6>© 2025 by Nat!, Mulle kybernetiK. All rights reserved.</h6>
<p>
</div> <!-- title -->
<p>
<div id="main" role="main">
<article>
<h1>Plug And Play Class</h1>
<div>
<p>A category might need to do a little more to install itself into the
class system, we’ll show how in the next example. Here we will be extending
<code class="language-plaintext highlighter-rouge">NSDateFormatter</code>.</p>
<p>A <code class="language-plaintext highlighter-rouge">NSDateFormatter</code> has several build-in behaviors that are specified by <code class="language-plaintext highlighter-rouge">NSDateFormatterBehavior10_0</code> or <code class="language-plaintext highlighter-rouge">NSDateFormatterBehavior10_4</code>.</p>
<p>We’d like to extend this with a custom behavior <code class="language-plaintext highlighter-rouge">MyDateFormatterBehavior</code> and
here is how to do it.</p>
<h2 id="create-a-subclass-of-nsdateformatter">Create a subclass of NSDateFormatter</h2>
<p>This will implement the desired behavior. How this is implemented is not
shown. Lets just assume it’s a class called <code class="language-plaintext highlighter-rouge">MyDateFormatter</code>.</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@interface</span> <span class="nc">MyDateFormatter</span> <span class="p">:</span> <span class="nc">NSDateFormatter</span>
<span class="p">...</span>
</code></pre></div></div>
<h2 id="patch-class-into-nsdateformatter">Patch class into NSDateFormatter</h2>
<p>NSDateFormatter has an extension mechanism in place called <code class="language-plaintext highlighter-rouge">+mulleSetClass:forFormatterBehavior:</code>,
where we associate a custom class with the behavior.</p>
<p>We would like to do this ASAP, which is when our custom class gets loaded into
the runtime. <code class="language-plaintext highlighter-rouge">+load</code> will have to be used, since no one will be calling our
class directly.</p>
<p>The only thing we need to advertise to the user is our new constant value
<code class="language-plaintext highlighter-rouge">MyDateFormatterBehavior</code> in a header somewhere for the new behavior:</p>
<p>Now comes the bad news, which is: it can be tricky.
The following implementation is correct, but you have to know why.</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@implementation</span> <span class="nc">MyDateFormatter</span>
<span class="k">+</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">load</span>
<span class="p">{</span>
<span class="p">[</span><span class="n">NSDateFormatter</span> <span class="nf">mulleSetClass</span><span class="p">:</span><span class="n">self</span>
<span class="nl">forFormatterBehavior:</span><span class="n">MyDateFormatterBehavior</span><span class="p">];</span>
<span class="p">}</span>
<span class="k">@end</span>
</code></pre></div></div>
<p>As long as you are not using any other Objective-C class except your own class
and its superclasses, your code is safe. But as soon you are using another
class, even just <code class="language-plaintext highlighter-rouge">@"some constant string"</code> then you have to declare its usage
beforehand. This can be tricky, when class-clusters come into play, where you
may not even know the class (<code class="language-plaintext highlighter-rouge">NSConstantString</code> in this case for instance).
Even worse, if you are messaging another class, this in turn may message other
classes. And that may be hard to predict and maintain.</p>
<p>Fortunately you can declare a dependency on libraries, which in most cases
should be <strong>Foundation</strong> or some equivalent.</p>
<p>Adding</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">MULLE_OBJC_DEPENDS_ON_LIBRARY</span><span class="p">(</span> <span class="n">Foundation</span><span class="p">);</span>
</code></pre></div></div>
<p>before the <code class="language-plaintext highlighter-rouge">+load</code> method definition will be sufficient in most cases.</p>
<blockquote>
<p>See bla bla for in depth</p>
</blockquote>
</div>
</article>
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var u="//www.mulle-kybernetik.com/matomo/";
_paq.push(['setTrackerUrl', u+'matomo.php']);
_paq.push(['setSiteId', '1']);
var d=document, g=d.createElement('script'), s=d.getElementsByTagName('script')[0];
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<noscript><p><img src="//www.mulle-kybernetik.com/matomo/matomo.php?idsite=1&rec=1" style="border:0;" alt="" /></p></noscript>
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</html>