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<?xml version="1.0" encoding="utf-8"?><feed xmlns="http://www.w3.org/2005/Atom" ><generator uri="https://jekyllrb.com/" version="4.3.4">Jekyll</generator><link href="https://www.mulle-kybernetik.com/objc-runtime-in-pictures/feed.xml" rel="self" type="application/atom+xml" /><link href="https://www.mulle-kybernetik.com/objc-runtime-in-pictures/" rel="alternate" type="text/html" /><updated>2025-01-26T22:58:03+01:00</updated><id>https://www.mulle-kybernetik.com/objc-runtime-in-pictures/feed.xml</id><title type="html">Objective-C Runtime in Pictures</title><subtitle>How your sourcecode ends up in the Objective-C runtime - The Mulle Way.</subtitle><entry><title type="html">Appendix</title><link href="https://www.mulle-kybernetik.com/objc-runtime-in-pictures/appendix.html" rel="alternate" type="text/html" title="Appendix" /><published>2024-12-01T00:00:00+01:00</published><updated>2024-12-01T00:00:00+01:00</updated><id>https://www.mulle-kybernetik.com/objc-runtime-in-pictures/appendix</id><content type="html" xml:base="https://www.mulle-kybernetik.com/objc-runtime-in-pictures/appendix.html"><![CDATA[<h2 id="syntax-topic">Syntax topic</h2>
<div class="language-objective-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="p">[</span><span class="n">obj</span> <span class="nf">methodWithArgument</span><span class="p">:</span><span class="n">argument</span> <span class="nf">additionalArgument</span><span class="p">:];</span>
<span class="p">[</span><span class="n">super</span> <span class="nf">method</span><span class="p">:</span><span class="n">argument</span><span class="p">];</span>
</code></pre></div></div>
<h2 id="method-call"><code class="language-plaintext highlighter-rouge">[method call]</code></h2>
<p>When you invoke or call an Objective-C method on an object, the object is
the <strong>receiver</strong> of a message. This is just nomenclature.</p>
<p>The receiver appears first in the method call, followed by the selector
interspersed with arguments in square brackets. So a call to method <code class="language-plaintext highlighter-rouge">- (void) insertObject:(id) obj atIndex:(NSUInteger) index</code> will look like this:</p>
<p><a href="images/method-call.svg"><img src="images/method-call.svg" alt="Method call deconstructed" /></a></p>
<h3 id="method-search">Method search</h3>
<p>So what does a method call do ? First it looks at the <a href="https://alwaysprocessing.blog/2023/01/19/objc-class-isa">isa</a>
pointer of the object. This will be a pointer to a <a href="class.html">class</a> object.
The class has
at least one methodlist. It can have multiple methodlists, if some were added
by <a href="category.html">categories</a>. The search order goes from back to front, so the most
recently added category methodlist comes first.</p>
<p>If no method has been found, the class may inherit methodlists from its
<a href="protocolclass.html">protocolclasses</a> and its <a href="inheritance.html">superclass</a>.
Again, the search order is back to first, the latest defined protocolclass is
searched first, then the others and the superclass last.</p>
<p>The superclass itself can be the same mixture of categories, protocolclasses
and a possible superclass.</p>
<p>Finally after the rootclass has been reached, and no method has been found yet,
the runtime will turn to forwarding (not part of this pamphlet) and if that
fails, the runtime will crash.</p>
<h3 id="super-method-search">Super method search</h3>
<p>The special super call, starts the message search from the “place” of
implementation. It will ignore all sibling methodlists of its implementation
class and then start a regular method search in the protocolclasses
and superclass.</p>
<h3 id="clobber-method-search">Clobber method search</h3>
<p>Only available with MulleObjC C runtime functions. This search stops at the
implementation class boundary. It searches backwards from the current
methodlist.</p>
<h3 id="override-method-search">Override method search</h3>
<p>Like the Clobber method search, but does not stop at the class boundary.</p>
<h2 id="case-studies">Case studies</h2>
<p><em>Only instance methodlists are shown for simplicity.</em></p>
<h3 id="class-a-the-simplest-case">Class A, the simplest case</h3>
<p><strong>A</strong> contains a method <code class="language-plaintext highlighter-rouge">instanceMethod</code>, its <code class="language-plaintext highlighter-rouge">@implementation</code> is not shown,
as it’s boring:</p>
<div class="language-objective-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@interface</span> <span class="nc">A</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">instanceMethod</span><span class="p">;</span>
<span class="k">@end</span>
</code></pre></div></div>
<p><a href="images/A.svg"><img src="images/A.svg" alt="A" /></a></p>
<p>This is just for warming up. There is no <strong>super</strong> method in a root class,
and nothing has been overridden. So its <code class="language-plaintext highlighter-rouge">-instanceMethod</code> only.</p>
<h3 id="subclass-b-of-a-and-categories-on-b">Subclass B of A and categories on B</h3>
<p>B overrides the superclass method <code class="language-plaintext highlighter-rouge">instanceMethod</code>. And so do the two
categories P and Q on B. Again, implementations are not shown:</p>
<div class="language-objective-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@interface</span> <span class="nc">B</span> <span class="p">:</span> <span class="nc">A</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">instanceMethod</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">B</span><span class="p">(</span> <span class="nl">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="n">instanceMethod</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">B</span><span class="p">(</span> <span class="nl">Q</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">instanceMethod</span><span class="p">;</span>
<span class="k">@end</span>
</code></pre></div></div>
<p>this will result in more complex layout:</p>
<p><a href="images/B.svg"><img src="images/B.svg" alt="B" /></a></p>
<p>As explained before, the search order will be from back to front.
So <em>methodlist #2</em> of B comes first, then <em>methodlist #1</em> of B, then
<em>methodlist #0</em> of B. The final search is through <em>methodlist #0</em> of A.</p>
<p>In short notation the various searches result in:</p>
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code> super chain: B(Q) -> A
clobber chain: B(Q) -> B(P) -> B
override chain: B(Q) -> B(P) -> B -> A
</code></pre></div></div>
<h3 id="subclass-c-of-b-and-multiple-inheritance-with-protocolclasses">Subclass C of B and multiple inheritance with protocolclasses</h3>
<p>The next class <code class="language-plaintext highlighter-rouge">C</code> builds on <code class="language-plaintext highlighter-rouge">B</code> and has two protocolclasses added to the mix:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@protocol</span> <span class="nc">X</span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">X</span> <span class="o"><</span><span class="n">X</span><span class="o">></span>
<span class="k">@end</span>
<span class="k">@protocol</span> <span class="nc">Y</span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">Y</span> <span class="o"><</span><span class="n">Y</span><span class="o">></span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">C</span> <span class="p">:</span> <span class="nc">B</span> <span class="o"><</span> <span class="n">X</span><span class="p">,</span> <span class="n">Y</span> <span class="o">></span>
<span class="k">@end</span>
</code></pre></div></div>
<p>The result looks like this:</p>
<p><a href="images/C.svg"><img src="images/C.svg" alt="C" /></a></p>
<p>Now with methodlist of <code class="language-plaintext highlighter-rouge">C</code> as the starting point, we get:</p>
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code> super chain: C -> Y -> X -> B(Q) -> A
clobber chain: C
override chain: C -> Y -> X -> B(Q) -> B(P) -> B -> A
</code></pre></div></div>
<h3 id="class-d-inheritance-with-duplicate-protocolclasses">Class D, inheritance with duplicate protocolclasses</h3>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@interface</span> <span class="nc">D</span> <span class="p">:</span> <span class="nc">C</span> <span class="o"><</span> <span class="n">Y</span><span class="p">,</span> <span class="n">X</span> <span class="o">></span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">D</span><span class="p">(</span> <span class="nl">P</span><span class="p">)</span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">D</span><span class="p">(</span> <span class="nl">Q</span><span class="p">)</span>
<span class="k">@end</span>
</code></pre></div></div>
<p><a href="images/D.svg"><img src="images/D.svg" alt="D" /></a></p>
<p>Before we go on, lets reconsider, that a protocolclass is a root class and
has no business calling “super” by itself. But if you were, here is what
would happen:</p>
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code> super chain: D(Q) -> X -> Y -> C -> Y -> C -> ... -> INFINITE LOOP CRASH
clobber chain: D(Q) -> D(P) -> D
override chain: D(Q) -> D(P) -> D -> X -> Y -> C -> Y -> C -> ... -> INFINITE LOOP CRASH
</code></pre></div></div>
<hr />
<h2 id="api">API</h2>
<table>
<thead>
<tr>
<th>Method Name</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code class="language-plaintext highlighter-rouge">-performSelector:</code></td>
<td>Perform method taking no arguments</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">-performSelector:withObject:</code></td>
<td>Perform method taking one argument</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">-performSelector:withObject:withObject:</code></td>
<td>Perform method taking two arguments</td>
</tr>
</tbody>
</table>
<table>
<thead>
<tr>
<th>Function/Method Name</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCObjectPerformSelector0</code></td>
<td>Call method with no arguments</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCObjectPerformSelector</code></td>
<td>Call method with one argument</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCObjectPerformSelector2</code></td>
<td>Call method with two arguments</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCIMPCall0</code></td>
<td>Call implementation with no arguments</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCIMPCall</code></td>
<td>Call implementation with one argument</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCIMPCall2</code></td>
<td>Call implementation with two arguments</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCIMPCallWithFloat</code></td>
<td>Call implementation with float argument</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCIMPCallWithDouble</code></td>
<td>Call implementation with double argument</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCIMPCallWithLong</code></td>
<td>Call implementation with long argument</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCMakeObjectsPerformSelector0</code></td>
<td>Call method on array of objects with no args</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCMakeObjectsPerformSelector</code></td>
<td>Call method on array of objects with one arg</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCMakeObjectsPerformSelector2</code></td>
<td>Call method on array of objects with two args</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCObjectSearchSuperIMP</code></td>
<td>Find superclass implementation of a method</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCObjectSearchSpecificIMP</code></td>
<td>Find method implementation in specific class/category</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCObjectSearchOverriddenIMP</code></td>
<td>Find overridden implementation of a method</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCObjectSearchClobberedIMP</code></td>
<td>Find implementation clobbered by a category</td>
</tr>
</tbody>
</table>]]></content><author><name></name></author><summary type="html"><![CDATA[Syntax topic]]></summary></entry><entry><title type="html">Outro</title><link href="https://www.mulle-kybernetik.com/objc-runtime-in-pictures/outro.html" rel="alternate" type="text/html" title="Outro" /><published>2024-11-30T00:00:00+01:00</published><updated>2024-11-30T00:00:00+01:00</updated><id>https://www.mulle-kybernetik.com/objc-runtime-in-pictures/outro</id><content type="html" xml:base="https://www.mulle-kybernetik.com/objc-runtime-in-pictures/outro.html"><![CDATA[<h2 id="how-to-create-your-own-runtime-pictures">How to create your own runtime pictures</h2>
<p>Create a new <a href="https://mulle-objc.github.io/">mulle-objc</a> with
<a href="https://github/mulle-sde">mulle-sde</a>:</p>
<div class="language-console highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="go">mulle-sde init -d picture -m mulle-objc/objc-developer executable
</span></code></pre></div></div>
<p>Edit <code class="language-plaintext highlighter-rouge">src/main.m</code> with a class hierarchy of your choice, then change <code class="language-plaintext highlighter-rouge">main</code> to:</p>
<div class="language-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kt">int</span> <span class="nf">main</span><span class="p">(</span> <span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">argv</span><span class="p">[])</span>
<span class="p">{</span>
<span class="n">MulleObjCDotdumpClass</span><span class="p">(</span> <span class="s">"MyClass"</span><span class="p">);</span>
<span class="k">return</span><span class="p">(</span> <span class="mi">0</span><span class="p">);</span>
<span class="p">}</span>
</code></pre></div></div>
<p>Now build and run:</p>
<div class="language-console highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="go">mulle-sde craft
mulle-sde run
</span></code></pre></div></div>
<p>Finally run the output <code class="language-plaintext highlighter-rouge">MyClass.dot</code> through <a href="https://graphviz.org/">graphviz</a>
to produce a graphics file:</p>
<div class="language-console highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="go">dot -Tsvg -o MyClass.svg MyClass.dot
</span></code></pre></div></div>]]></content><author><name></name></author><summary type="html"><![CDATA[How to create your own runtime pictures]]></summary></entry><entry><title type="html">Quiz</title><link href="https://www.mulle-kybernetik.com/objc-runtime-in-pictures/quiz.html" rel="alternate" type="text/html" title="Quiz" /><published>2024-11-29T00:00:00+01:00</published><updated>2024-11-29T00:00:00+01:00</updated><id>https://www.mulle-kybernetik.com/objc-runtime-in-pictures/quiz</id><content type="html" xml:base="https://www.mulle-kybernetik.com/objc-runtime-in-pictures/quiz.html"><![CDATA[<p>Armed with this knowledge, here is an example project. Assume that
you are only linking against the <code class="language-plaintext highlighter-rouge">mulle-objc-runtime</code>, which doesn’t provide
any classes.</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@interface</span> <span class="nc">A</span>
<span class="k">@property</span> <span class="kt">char</span> <span class="o">*</span><span class="n">x</span><span class="p">;</span>
<span class="k">+</span> <span class="p">(</span><span class="n">instancetype</span><span class="p">)</span> <span class="n">new</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">A</span>
<span class="k">+</span> <span class="p">(</span><span class="n">instancetype</span><span class="p">)</span> <span class="n">new</span>
<span class="p">{</span>
<span class="k">return</span><span class="p">(</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="p">}</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="p">)</span> <span class="n">x</span> <span class="p">{</span> <span class="k">return</span><span class="p">(</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="p">)</span> <span class="n">__FUNCTION__</span><span class="p">);</span> <span class="p">}</span>
<span class="k">@end</span>
<span class="k">@protocol</span> <span class="nc">B</span>
<span class="k">@optional</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="p">)</span> <span class="n">x</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">B</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="p">)</span> <span class="n">x</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">B</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="p">)</span> <span class="n">x</span> <span class="p">{</span> <span class="k">return</span><span class="p">(</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="p">)</span> <span class="n">__FUNCTION__</span><span class="p">);</span> <span class="p">}</span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">C</span> <span class="p">:</span> <span class="nc">A</span> <span class="o"><</span> <span class="n">B</span> <span class="o">></span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">C</span>
<span class="k">@end</span>
</code></pre></div></div>
<div class="quiz_container">
<!-- Question 1 -->
<div class="quiz_question">
<p>1. How many <i>metaclasses</i> will exist at runtime ?</p>
<label class="quiz_label"><input type="radio" name="q1" value="wrong" /> none</label>
<label class="quiz_label"><input type="radio" name="q1" value="wrong" /> two</label>
<label class="quiz_label"><input type="radio" name="q1" value="right" /> three</label>
<label class="quiz_label"><input type="radio" name="q1" value="wrong" /> four</label>
</div>
<!-- Question 2 -->
<div class="quiz_question">
<p>2. How many <i>methods</i> does <i>infraclass</i> <b>C</b> have in its <i>methodlists</i> ?</p>
<label class="quiz_label"><input type="radio" name="q2" value="right" /> none</label>
<label class="quiz_label"><input type="radio" name="q2" value="wrong" /> one</label>
<label class="quiz_label"><input type="radio" name="q2" value="wrong" /> two</label>
<label class="quiz_label"><input type="radio" name="q2" value="wrong" /> <b>C</b> has no methodlists</label>
</div>
<!-- Question 3 -->
<div class="quiz_question">
<p>3. What is the result of <code>[[C new] x]</code> ?</p>
<label class="quiz_label"><input type="radio" name="q3" value="right" /> -[A x]</label>
<label class="quiz_label"><input type="radio" name="q3" value="wrong" /> -[B x]</label>
<label class="quiz_label"><input type="radio" name="q3" value="wrong" /> NULL</label>
<label class="quiz_label"><input type="radio" name="q3" value="wrong" /> a crash</label>
</div>
<!-- Question 4 -->
<div class="quiz_question">
<p>4. What is the result of <code>[C x]</code> ?</p>
<label class="quiz_label"><input type="radio" name="q4" value="wrong" /> +[A x]</label>
<label class="quiz_label"><input type="radio" name="q4" value="right" /> -[A x]</label>
<label class="quiz_label"><input type="radio" name="q4" value="wrong" /> -[B x]</label>
<label class="quiz_label"><input type="radio" name="q4" value="wrong" /> a crash</label>
</div>
<!-- Submit Button -->
<button class="quiz_button" onclick="calculateScore()">Submit Quiz</button>
<!-- Result -->
<div class="quiz_result" id="quiz_result"></div>
</div>
<script>
function calculateScore() {
let score = 0;
let allAnswered = true;
const questions = ["q1", "q2", "q3", "q4"];
questions.forEach((question) => {
const selectedAnswer = document.querySelector(`input[name="${question}"]:checked`);
if (!selectedAnswer) {
allAnswered = false;
} else if (selectedAnswer.value === "right") {
score++;
}
});
const resultDiv = document.getElementById("quiz_result");
// Array of messages for each possible score
const messages = [
"Why don't you find yourself a nice scripting language ?",
"Don't worry, there are other programming languages in the sea. Maybe rather checkout Go or Rust.",
"Well the test is tricky, it can't hurt to peruse the pamphlet one more time.",
"Almost there, but you had to guess at one time. And you chose poorly.",
"Welcome to the Objective-C Pantheon!"
];
// Check if all questions were answered
if (!allAnswered) {
resultDiv.textContent = "Please answer all questions before submitting.";
} else {
// Display result with corresponding message
resultDiv.textContent = `${messages[score]}`;
}
}
document.querySelector('.quiz_button').addEventListener('click', function(e) {
this.classList.add('clicked');
setTimeout(() => this.classList.remove('clicked'), 300);
});
</script>]]></content><author><name></name></author><summary type="html"><![CDATA[Armed with this knowledge, here is an example project. Assume that you are only linking against the mulle-objc-runtime, which doesn’t provide any classes.]]></summary></entry><entry><title type="html">Protocolclass</title><link href="https://www.mulle-kybernetik.com/objc-runtime-in-pictures/protocolclass.html" rel="alternate" type="text/html" title="Protocolclass" /><published>2024-11-09T00:00:00+01:00</published><updated>2024-11-09T00:00:00+01:00</updated><id>https://www.mulle-kybernetik.com/objc-runtime-in-pictures/protocolclass</id><content type="html" xml:base="https://www.mulle-kybernetik.com/objc-runtime-in-pictures/protocolclass.html"><![CDATA[<h2 id="syntax-topic">Syntax topic</h2>
<div class="language-objective-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@protocol</span> <span class="nc">SomeProtocolClass</span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">SomeProtocolClass</span> <span class="o"><</span><span class="n">SomeProtocolClass</span><span class="o">></span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">SomeClass</span> <span class="o"><</span> <span class="n">SomeProtocolClass</span><span class="o">></span>
<span class="k">@end</span>
</code></pre></div></div>
<h2 id="protocolclass">protocolclass</h2>
<p>A <strong>protocolclass</strong> is a <em>rootclass</em> that implements a <em>protocol</em> of the same
name. The methods declared as <code class="language-plaintext highlighter-rouge">@optional</code> will be defined by the protocolclass,
for reuse by protocol adopting classes:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code>
<span class="k">@interface</span> <span class="nc">RootClass</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">instanceMethod</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">RootClass</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">instanceMethod</span>
<span class="p">{</span>
<span class="n">printf</span><span class="p">(</span> <span class="s">"%s</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">__FUNCTION__</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">@end</span>
<span class="k">@protocol</span> <span class="nc">ProtocolClass</span>
<span class="k">@optional</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">instanceMethod</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">ProtocolClass</span> <span class="o"><</span> <span class="n">ProtocolClass</span><span class="o">></span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">ProtocolClass</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">instanceMethod</span>
<span class="p">{</span>
<span class="n">IMP</span> <span class="n">imp</span><span class="p">;</span>
<span class="n">printf</span><span class="p">(</span> <span class="s">"%s</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">__FUNCTION__</span><span class="p">);</span>
<span class="c1">// can't call super</span>
<span class="n">imp</span> <span class="o">=</span> <span class="n">MulleObjCOverriddenIMP</span><span class="p">;</span>
<span class="k">if</span><span class="p">(</span> <span class="n">imp</span><span class="p">)</span>
<span class="n">MulleObjCIMPCall0</span><span class="p">(</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="p">}</span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">SomeClass</span> <span class="p">:</span> <span class="nc">RootClass</span> <span class="o"><</span> <span class="n">ProtocolClass</span><span class="o">></span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">SomeClass</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">instanceMethod</span>
<span class="p">{</span>
<span class="n">printf</span><span class="p">(</span> <span class="s">"%s</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">__FUNCTION__</span><span class="p">);</span>
<span class="p">[</span><span class="n">super</span> <span class="nf">instanceMethod</span><span class="p">];</span>
<span class="p">}</span>
<span class="k">@end</span>
</code></pre></div></div>
<p>This turns out to be akin to multiple inheritance.</p>
<p><img src="images/protocolclass.svg" alt="protocolclass" /></p>
<p>When you run <code class="language-plaintext highlighter-rouge">[instance instanceMethod]</code> the output will be:</p>
<div class="language-console highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="go">-[SomeClass instanceMethod]
-[ProtocolClass instanceMethod]
-[RootClass instanceMethod]
</span></code></pre></div></div>
<p>The protocolclass “wedged” itself between the superclass and the adopting class.. The super call actually reached the protocolclass method, not the superclass
method.</p>
<p>Interestingly, adding protocolclasses to a rootclass is
possible, but the compiler will not allow you to call super in the adopting
rootclass. A protocolclass will not want to call <em>super</em> as it has no idea what the
superclass of the protocol adopting class is. Nevertheless, mulle-objc does not stand in
the way of a good hack, so you can reach the superclass method with
<code class="language-plaintext highlighter-rouge">MulleObjCOverriddenIMP</code>, if so desired.</p>
<p>What is obviously missing is the possibility to add instance variables.There are other solutions for this, but that is a topic for another pamphlet.</p>
<h3 id="the-order-of-protocolclasses-is-important">The order of protocolclasses is important</h3>
<p>Like the <em>methodlists</em> of a class, the search order of protocolclasses is
back to front:</p>
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>@protocol A
@optional
- (void) instanceMethod;
@end
@interface A < A>
@end
@implementation A
- (void) instanceMethod
{
IMP imp;
printf( "%s\n", __FUNCTION__);
imp = MulleObjCOverriddenIMP;
if( imp)
MulleObjCIMPCall0( imp, self, _cmd);
}
@end
@protocol B
@optional
- (void) instanceMethod;
@end
@interface B < B>
@end
@implementation B
- (void) instanceMethod
{
IMP imp;
printf( "%s\n", __FUNCTION__);
imp = MulleObjCOverriddenIMP;
if( imp)
MulleObjCIMPCall0( imp, self, _cmd);
}
@end
@interface SomeClass < A, B>
@end
@implementation SomeClass
@end
</code></pre></div></div>
<p>This produces a class hierarchy:</p>
<p><img src="images/protocolclass-order.svg" alt="protocolclass" /></p>
<p>When you run <code class="language-plaintext highlighter-rouge">[obj instanceMethod]</code> on a “SomeClass” instance, “B” will be
found first. So the output is:</p>
<div class="language-console highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="go">-[B instanceMethod]
-[A instanceMethod]
</span></code></pre></div></div>
<hr />
<h2 id="api">API</h2>
<table>
<thead>
<tr>
<th>Function/Name</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code class="language-plaintext highlighter-rouge">_PROTOCOLCLASS_INTERFACE0</code></td>
<td>Define protocol class without protocols</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">PROTOCOLCLASS_INTERFACE0</code></td>
<td>Define protocol class with NSObject protocol</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">_PROTOCOLCLASS_INTERFACE</code></td>
<td>Define protocol class with custom protocols</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">PROTOCOLCLASS_INTERFACE</code></td>
<td>Define protocol class with NSObject and custom protocols</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">PROTOCOLCLASS_END</code></td>
<td>End protocol class definition</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">PROTOCOLCLASS_IMPLEMENTATION</code></td>
<td>Begin protocol class implementation</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCOverriddenIMP</code></td>
<td>Get overridden implementation in protocol class chain</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCClobberedIMP</code></td>
<td>Get clobbered implementation (stops at class boundary)</td>
</tr>
</tbody>
</table>]]></content><author><name></name></author><summary type="html"><![CDATA[Syntax topic]]></summary></entry><entry><title type="html">Inheritance</title><link href="https://www.mulle-kybernetik.com/objc-runtime-in-pictures/inheritance.html" rel="alternate" type="text/html" title="Inheritance" /><published>2024-11-08T00:00:00+01:00</published><updated>2024-11-08T00:00:00+01:00</updated><id>https://www.mulle-kybernetik.com/objc-runtime-in-pictures/inheritance</id><content type="html" xml:base="https://www.mulle-kybernetik.com/objc-runtime-in-pictures/inheritance.html"><![CDATA[<h2 id="syntax-topic">Syntax topic</h2>
<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="nc">SomeSuperClass</span>
<span class="k">@end</span>
</code></pre></div></div>
<h2 id="inheritance">Inheritance</h2>
<p>In terms of a C <code class="language-plaintext highlighter-rouge">struct</code>, inheritance is akin to nesting a struct within another
struct:</p>
<div class="language-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">struct</span> <span class="n">a</span>
<span class="p">{</span>
<span class="kt">int</span> <span class="n">v</span><span class="p">;</span>
<span class="p">};</span>
<span class="k">struct</span> <span class="n">b</span>
<span class="p">{</span>
<span class="k">struct</span> <span class="n">a</span> <span class="n">a</span><span class="p">;</span>
<span class="kt">int</span> <span class="n">w</span><span class="p">;</span>
<span class="p">};</span>
</code></pre></div></div>
<p>The Objective-C syntax <code class="language-plaintext highlighter-rouge">@interface <class> : <otherClass></code> is used
to define the inheritance:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@interface</span> <span class="nc">A</span>
<span class="p">{</span>
<span class="kt">int</span> <span class="n">u</span><span class="p">;</span>
<span class="kt">int</span> <span class="n">v</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">getU</span><span class="p">;</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">getV</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">A</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">getU</span>
<span class="p">{</span>
<span class="k">return</span><span class="p">(</span> <span class="n">u</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">getV</span>
<span class="p">{</span>
<span class="k">return</span><span class="p">(</span> <span class="n">v</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">B</span> <span class="p">:</span> <span class="nc">A</span>
<span class="p">{</span>
<span class="kt">int</span> <span class="n">w</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">getW</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">B</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">getV</span>
<span class="p">{</span>
<span class="k">return</span><span class="p">(</span> <span class="p">[</span><span class="n">super</span> <span class="nf">getU</span><span class="p">]</span> <span class="o">+</span> <span class="p">[</span><span class="n">super</span> <span class="nf">getV</span><span class="p">]);</span>
<span class="p">}</span>
<span class="k">-</span> <span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="n">getW</span>
<span class="p">{</span>
<span class="k">return</span><span class="p">(</span> <span class="n">w</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">@end</span>
</code></pre></div></div>
<p>The runtime will now look like this:</p>
<p><img src="images/inheritance.svg" alt="Inheritance" />There is a uni-directional relationship <strong>super</strong> between the class B and its <strong>superclass</strong> A..</p>
<p>The instance variables and methods of class A are <em>inherited</em> by B.As you can see, the contents of the ivarlists and methodlists of A are not copied down to B.Nevertheless, all methods and instance variables of A are now available in B.</p>
<h3 id="super">super</h3>
<p>With the special <code class="language-plaintext highlighter-rouge">[super method]</code> syntax, a method of a superclass can be
invoked, even if there is a method with the same name in the current class.</p>
<h3 id="special-case-the-root-class">Special case: the root class</h3>
<p>A root class is a class that does not have a superclass. There is a special
treatment for ‘+’ class methods on these root classes.</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@interface</span> <span class="nc">RootClass</span>
<span class="k">+</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">classMethod</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">instanceMethod</span><span class="p">;</span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">RootClass</span>
<span class="k">+</span> <span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="n">classMethod</span>
<span class="p">{</span>
<span class="n">printf</span><span class="p">(</span> <span class="s">"%s</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">__FUNCTION__</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="n">instanceMethod</span>
<span class="p">{</span>
<span class="n">printf</span><span class="p">(</span> <span class="s">"%s</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">__FUNCTION__</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">@end</span>
</code></pre></div></div>
<p>When you run</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="p">[</span><span class="n">cls</span> <span class="nf">classMethod</span><span class="p">];</span>
<span class="p">[</span><span class="n">cls</span> <span class="nf">instanceMethod</span><span class="p">];</span>
<span class="p">[</span><span class="n">instance</span> <span class="nf">instanceMethod</span><span class="p">];</span>
<span class="p">[</span><span class="n">instance</span> <span class="nf">classMethod</span><span class="p">];</span>
</code></pre></div></div>
<p>the output will be:</p>
<div class="language-console highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="go">+[RootClass classMethod]
-[RootClass instanceMethod]
-[RootClass instanceMethod]
mulle_objc_universe 0x5B74C1D11080 fatal: unknown method c3074566 "-classMethod" in class 4b9217f9 "RootClass"
</span></code></pre></div></div>
<p>So what is going on here?</p>
<p><img src="images/RootClass.svg" alt="Root class" /></p>
<p>If you follow the <code class="language-plaintext highlighter-rouge">isa/meta</code> path, you will see that the metaclass of the
metaclass as actually the infraclass again. So a rootclass class methods
are augmented with the infraclass methods.</p>
<p>This is something to be aware of. So root classes usually have
a class method implementation for every instance method to avoid surprises.
Another option is to put a empty root class on top.</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">-superclass</code></td>
<td>Get receiver’s superclass</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">+superclass</code></td>
<td>Get class’s superclass</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">+isSubclassOfClass:</code></td>
<td>Check if class inherits from given class</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">-isKindOfClass:</code></td>
<td>Check if receiver inherits from or is instance of class</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">-isMemberOfClass:</code></td>
<td>Check if receiver is direct instance of class</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">[super ...]</code></td>
<td>Language feature to call superclass implementation</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCClassIsSubclassOfClass</code></td>
<td>C function to check class inheritance relationship</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">NSObjectIsKindOfClass</code></td>
<td>C function for instance/class relationship check</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCInstanceIsMemberOfClass</code></td>
<td>C function for direct instance/class check</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">MulleObjCSuperIMP</code></td>
<td>Get superclass implementation of current method</td>
</tr>
</tbody>
</table>]]></content><author><name></name></author><summary type="html"><![CDATA[Syntax topic]]></summary></entry><entry><title type="html">Protocol</title><link href="https://www.mulle-kybernetik.com/objc-runtime-in-pictures/protocol.html" rel="alternate" type="text/html" title="Protocol" /><published>2024-11-07T00:00:00+01:00</published><updated>2024-11-07T00:00:00+01:00</updated><id>https://www.mulle-kybernetik.com/objc-runtime-in-pictures/protocol</id><content type="html" xml:base="https://www.mulle-kybernetik.com/objc-runtime-in-pictures/protocol.html"><![CDATA[<h2 id="syntax-topics">Syntax topics</h2>
<div class="language-objective-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@protocol</span> <span class="nc">SomeProtocol</span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">SomeClass</span> <span class="o"><</span><span class="n">SomeProtocol</span><span class="o">></span>
<span class="k">@end</span>
<span class="p">[</span><span class="n">objc</span> <span class="nf">conformsToProtocol</span><span class="p">:</span><span class="k">@protocol</span><span class="err">(</span> <span class="nc">SomeProtocol</span><span class="p">)];</span>
</code></pre></div></div>
<h2 id="protocol">@protocol</h2>
<p>A <strong>protocol</strong> is a way to declare a set of methods. A class that
<em>adopts</em> a protocol, declares that it implements all of the methods in the
protocol:</p>
<div class="language-objc highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">@protocol</span> <span class="nc">EmptyProtocol</span>
<span class="k">@end</span>
<span class="k">@interface</span> <span class="nc">SomeClass</span> <span class="o"><</span><span class="n">EmptyProtocol</span><span class="o">></span>
<span class="k">@end</span>
<span class="k">@implementation</span> <span class="nc">SomeClass</span>
<span class="c1">// protocol EmptyProtocol is empty, so no need to implement anything</span>
<span class="k">@end</span>
</code></pre></div></div>
<p>The actual output of the compiler is just an identifier that can be queried
with <code class="language-plaintext highlighter-rouge">[Someclass conformsToProtocol:@protocol( EmptyProtocol)]</code>.</p>
<p><a href="images/protocol.svg"><img src="images/protocol.svg" alt="Protocol" /></a></p>
<p>At runtime, the information is lost, which methods are part of a protocol..</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">MulleObjCProtocolGetNameUTF8String</code></td>
<td>Get protocol name as UTF8 string</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">-conformsToProtocol:</code></td>
<td>Check if receiver conforms to protocol (including inheritance)</td>
</tr>
<tr>
<td><code class="language-plaintext highlighter-rouge">-mulleContainsProtocol:</code></td>
<td>Check if receiver directly implements protocol (no inheritance)</td>
</tr>
</tbody>
</table>]]></content><author><name></name></author><summary type="html"><![CDATA[Syntax topics]]></summary></entry><entry><title type="html">Category</title><link href="https://www.mulle-kybernetik.com/objc-runtime-in-pictures/category.html" rel="alternate" type="text/html" title="Category" /><published>2024-11-06T00:00:00+01:00</published><updated>2024-11-06T00:00:00+01:00</updated><id>https://www.mulle-kybernetik.com/objc-runtime-in-pictures/category</id><content type="html" xml:base="https://www.mulle-kybernetik.com/objc-runtime-in-pictures/category.html"><![CDATA[<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>