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protocolclass.html
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<h1>Protocolclass</h1>
<div>
<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>
</div>
</article>
</div>
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