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<h4>Nat!</h4>
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<article>
<h1>Category</h1>
<div>
<h3 id="syntax-topic">Syntax topic</h3>
<div class="language-objective-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code> <span class="k">@interface</span> <span class="nc">SomeClass</span><span class="p">(</span> <span class="nl">Additions</span><span class="p">)</span>
<span class="k">+</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">additionalClassMethod</span><span class="p">;</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="nf">instanceMethod</span><span class="p">:(</span><span class="n">id</span><span class="p">)</span> <span class="n">argument</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">SomeClass</span><span class="p">(</span> <span class="nl">Additions</span><span class="p">)</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="nf">instanceMethod</span><span class="p">:(</span><span class="n">id</span><span class="p">)</span> <span class="n">argument</span>
<span class="p">{</span>
<span class="n">IMP</span> <span class="n">imp</span><span class="p">;</span>
<span class="n">imp</span> <span class="o">=</span> <span class="n">MulleObjCOverriddenIMP</span><span class="p">(</span> <span class="n">self</span><span class="p">,</span> <span class="n">_cmd</span><span class="p">);</span>
<span class="p">(</span><span class="o">*</span><span class="n">imp</span><span class="p">)(</span> <span class="n">self</span><span class="p">,</span> <span class="n">_cmd</span><span class="p">,</span> <span class="n">argument</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">@end</span>
</code></pre></div></div>
<h2 id="category">Category</h2>
<p>A category adds methodlists to an existing class:</p>
<div class="language-objective-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@interface</span> <span class="nc">CGPoint</span><span class="p">(</span> <span class="nl">Additions</span><span class="p">)</span>
<span class="k">+</span> <span class="p">(</span><span class="n">instancetype</span><span class="p">)</span> <span class="nf">newWithX</span><span class="p">:(</span><span class="kt">float</span><span class="p">)</span> <span class="n">dx</span>
<span class="n">y</span><span class="o">:</span><span class="p">(</span><span class="kt">float</span><span class="p">)</span> <span class="n">dy</span><span class="p">;</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="nf">rotateWithRadians</span><span class="p">:(</span><span class="kt">float</span><span class="p">)</span> <span class="n">radians</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">CGPoint</span><span class="p">(</span> <span class="nl">Additions</span><span class="p">)</span>
<span class="k">+</span> <span class="p">(</span><span class="n">instancetype</span><span class="p">)</span> <span class="nf">newWithX</span><span class="p">:(</span><span class="kt">float</span><span class="p">)</span> <span class="n">x</span>
<span class="n">y</span><span class="o">:</span><span class="p">(</span><span class="kt">float</span><span class="p">)</span> <span class="n">y</span>
<span class="p">{</span>
<span class="n">CGPoint</span> <span class="o">*</span><span class="n">obj</span><span class="p">;</span>
<span class="n">obj</span> <span class="o">=</span> <span class="n">mulle_objc_infraclass_alloc_instance</span><span class="p">(</span> <span class="n">self</span><span class="p">);</span>
<span class="n">obj</span><span class="o">-></span><span class="n">x</span> <span class="o">=</span> <span class="n">x</span><span class="p">;</span>
<span class="n">obj</span><span class="o">-></span><span class="n">y</span> <span class="o">=</span> <span class="n">y</span><span class="p">;</span>
<span class="k">return</span><span class="p">(</span> <span class="n">obj</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="nf">rotateWithRadians</span><span class="p">:(</span><span class="kt">float</span><span class="p">)</span> <span class="n">radians</span>
<span class="p">{</span>
<span class="kt">float</span> <span class="n">cos_rad</span><span class="p">;</span>
<span class="kt">float</span> <span class="n">sin_rad</span><span class="p">;</span>
<span class="kt">float</span> <span class="n">rotated_x</span><span class="p">;</span>
<span class="kt">float</span> <span class="n">rotated_y</span><span class="p">;</span>
<span class="n">cos_rad</span> <span class="o">=</span> <span class="n">cosf</span><span class="p">(</span> <span class="n">radians</span><span class="p">);</span>
<span class="n">sin_rad</span> <span class="o">=</span> <span class="n">sinf</span><span class="p">(</span> <span class="n">radians</span><span class="p">);</span>
<span class="n">rotated_x</span> <span class="o">=</span> <span class="n">self</span><span class="o">-></span><span class="n">x</span> <span class="o">*</span> <span class="n">cos_rad</span> <span class="o">-</span> <span class="n">self</span><span class="o">-></span><span class="n">y</span> <span class="o">*</span> <span class="n">sin_rad</span><span class="p">;</span>
<span class="n">rotated_y</span> <span class="o">=</span> <span class="n">self</span><span class="o">-></span><span class="n">x</span> <span class="o">*</span> <span class="n">sin_rad</span> <span class="o">+</span> <span class="n">self</span><span class="o">-></span><span class="n">y</span> <span class="o">*</span> <span class="n">cos_rad</span><span class="p">;</span>
<span class="n">self</span><span class="o">-></span><span class="n">x</span> <span class="o">=</span> <span class="n">rotated_x</span><span class="p">;</span>
<span class="n">self</span><span class="o">-></span><span class="n">y</span> <span class="o">=</span> <span class="n">rotated_y</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">@end</span>
</code></pre></div></div>
<p>The category can be in the same file as the class, or in a separate file, and
even in a different library.. The category is attached to the class at
runtime.</p>
<p><a href="images/category.svg"><img src="images/category.svg" alt="" /></a></p>
<p>The order of the methodlists is important. The search direction for a method
is from the back to the front. So a category has the option to override methods
in the class.</p>
<p>The order of the categories is therefore also important. You specify them with one of the <a href="https://github.com/mulle-objc/MulleObjC/blob/master/src/protocol/MulleObjCRuntimeObject.h#L71C1-L71C67"><code class="language-plaintext highlighter-rouge">MULLE_OBJC_DEPENDS_...</code> macros</a>..</p>
<p>To retrieve a overridden method, you can use
<code class="language-plaintext highlighter-rouge">MulleObjCOverriddenIMP</code>or <code class="language-plaintext highlighter-rouge">MulleObjCClobberedIMP</code> in your category
implementation. (<code class="language-plaintext highlighter-rouge">MulleObjCClobberedIMP</code> stops at the class boundary).</p>
<blockquote>
<h4 id="caveat">Caveat</h4>
<p>You need some knowledge about the <a href="https://www.mulle-kybernetik.com/weblog/2015/mulle_objc_meta_call_convention.html">metaABI</a>
though to pass more than one argument or arguments that are not <code class="language-plaintext highlighter-rouge">void *</code>
compatible.</p>
</blockquote>
<p>Category ids are stored separately and are available from the classpair of
infraclass and metaclass. They are very rarely used by the API programmer.</p>
<hr />
<h2 id="api">API</h2>
<table>
<thead>
<tr>
<th>Function/Method Name</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code class="language-plaintext highlighter-rouge">-mulleInterposeBeforeClass:</code></td>
<td>Insert category methods before another class (method version)</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCObjectSearchOverriddenIMP</code></td>
<td>Find method implementation overridden by category</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCObjectSearchClobberedIMP</code></td>
<td>Find method implementation clobbered by category (stops at class boundary)</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCClassInterposeBeforeClass</code></td>
<td>Insert category methods before another class</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MULLE_OBJC_DEPENDS_ON_CLASS</code></td>
<td>Declare category dependency on a class</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MULLE_OBJC_DEPENDS_ON_CATEGORY</code></td>
<td>Declare category dependency on another category</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MULLE_OBJC_DEPENDS_ON_LIBRARY</code></td>
<td>Declare category dependency on a library</td>
</tr>
</tbody>
</table>
</div>
</article>
</div>
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