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<!DOCTYPE html>
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<h1 id="categories">Categories</h1>
<p>Categories are a way to split a single class definition into multiple files.
Their goal is to ease the burden of maintaining large code bases by
modularizing a class. This prevents your source code from becoming monolithic
10000+ line files that are impossible to navigate and makes it easy to assign
specific, well-defined portions of a class to individual developers.</p>
<figure>
<img style="max-width: 250px" src="media/categories/category-overview.png">
<figcaption>Using multiple files to implement a class</figcaption>
</figure>
<p>In this module, we’ll use a category to extend an existing class
without touching its original source file. Then, we’ll learn how this
functionality can be used to emulate protected methods. Extensions are a close
relative to categories, so we’ll be taking a brief look at those,
too.</p>
<h2 id="setting-up">Setting Up</h2>
<p>Before we can start experimenting with categories, we need a base class to
work off of. Create or change your existing <code>Car</code> interface to the
following:</p>
<pre><code class="c1">// Car.h</code>
<code class="cp">#import</code> <code class="l"><Foundation/Foundation.h></code><code class="cp"></code>
<code class="k">@interface</code> <code class="nc">Car</code> : <code class="nc">NSObject</code>
<code class="k">@property</code> <code class="p">(</code><code class="k">copy</code><code class="p">)</code> <code class="nb">NSString</code> <code class="o">*</code><code class="n">model</code><code class="p">;</code>
<code class="k">@property</code> <code class="p">(</code><code class="k">readonly</code><code class="p">)</code> <code class="kt">double</code> <code class="n">odometer</code><code class="p">;</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">startEngine</code><code class="p">;</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">drive</code><code class="p">;</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">turnLeft</code><code class="p">;</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">turnRight</code><code class="p">;</code>
<code class="k">@end</code>
</pre>
<p>The corresponding implementation just outputs some descriptive messages so
we can see when different methods get called:</p>
<pre><code class="c1">// Car.m</code>
<code class="cp">#import</code> <code class="l">"Car.h"</code><code class="cp"></code>
<code class="k">@implementation</code> <code class="nc">Car</code>
<code class="k">@synthesize</code> <code class="n">model</code> <code class="o">=</code> <code class="n">_model</code><code class="p">;</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">startEngine</code> <code class="p">{</code>
<code class="nb">NSLog</code><code class="p">(</code><code class="s">@"Starting the %@'s engine"</code><code class="p">,</code> <code class="n">_model</code><code class="p">);</code>
<code class="p">}</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">drive</code> <code class="p">{</code>
<code class="nb">NSLog</code><code class="p">(</code><code class="s">@"The %@ is now driving"</code><code class="p">,</code> <code class="n">_model</code><code class="p">);</code>
<code class="p">}</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">turnLeft</code> <code class="p">{</code>
<code class="nb">NSLog</code><code class="p">(</code><code class="s">@"The %@ is turning left"</code><code class="p">,</code> <code class="n">_model</code><code class="p">);</code>
<code class="p">}</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">turnRight</code> <code class="p">{</code>
<code class="nb">NSLog</code><code class="p">(</code><code class="s">@"The %@ is turning right"</code><code class="p">,</code> <code class="n">_model</code><code class="p">);</code>
<code class="p">}</code>
<code class="k">@end</code>
</pre>
<p>Now, let’s say you want to add another set of methods related to car
maintenance. Instead of cluttering up these <code>Car.h</code> and
<code>Car.m</code> files, you can place the new methods in a dedicated
category.</p>
<h2 id="creating-categories">Creating Categories</h2>
<p>Categories work just like normal class definitions in that they are composed
of an interface and an implementation. To add a new category to your Xcode
project, create a new file and choose the <em>Objective-C category</em>
template under <em>iOS > Cocoa Touch</em>. Use
<code>Maintenance</code> for the <em>Category</em> field and <code>Car</code>
for <em>Category on</em>.</p>
<figure>
<img style="max-width: 357px" src="media/categories/creating-a-category.png">
<figcaption>Creating the <code>Maintenance</code> category</figcaption>
</figure>
<p>The only restriction on category names is that they don’t conflict
with other categories on the same class. The canonical file-naming convention
is to use the class name and the category name separated by a plus sign, so you
should find a <code>Car+Maintenance.h</code> and a
<code>Car+Maintenance.m</code> in Xcode’s <em>Project Navigator</em>
after saving the above category.</p>
<p>As you can see in <code>Car+Maintenance.h</code>, a category interface looks
exactly like a normal interface, except the class name is followed by the
category name in parentheses. Let’s go ahead and add a few methods to the
category:</p>
<pre><code class="c1">// Car+Maintenance.h</code>
<code class="cp">#import</code> <code class="l">"Car.h"</code><code class="cp"></code>
<code class="k">@interface</code> <code class="nc">Car</code> <code class="nl">(Maintenance)</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">BOOL</code><code class="p">)</code><code class="nf">needsOilChange</code><code class="p">;</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">changeOil</code><code class="p">;</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">rotateTires</code><code class="p">;</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">jumpBatteryUsingCar:</code><code class="p">(</code><code class="n">Car</code> <code class="o">*</code><code class="p">)</code><code class="nv">anotherCar</code><code class="p">;</code>
<code class="k">@end</code>
</pre>
<p>At runtime, these methods become part of the <code>Car</code> class. Even
though they’re declared in a different file, you will be able to access
them as if they were defined in the original <code>Car.h</code>.</p>
<p>Of course, you have to implement the category interface for the above
methods to actually <em>do</em> anything. Again, a category implementation
looks almost exactly like a standard implementation, except the category name
appears in parentheses after the class name:</p>
<pre><code class="c1">// Car+Maintenance.m</code>
<code class="cp">#import</code> <code class="l">"Car+Maintenance.h"</code><code class="cp"></code>
<code class="k">@implementation</code> <code class="nc">Car</code> <code class="nl">(Maintenance)</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">BOOL</code><code class="p">)</code><code class="nf">needsOilChange</code> <code class="p">{</code>
<code class="k">return</code> <code class="kc">YES</code><code class="p">;</code>
<code class="p">}</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">changeOil</code> <code class="p">{</code>
<code class="nb">NSLog</code><code class="p">(</code><code class="s">@"Changing oil for the %@"</code><code class="p">,</code> <code class="p">[</code><code class="n">self</code> <code class="nf">model</code><code class="p">]);</code>
<code class="p">}</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">rotateTires</code> <code class="p">{</code>
<code class="nb">NSLog</code><code class="p">(</code><code class="s">@"Rotating tires for the %@"</code><code class="p">,</code> <code class="p">[</code><code class="n">self</code> <code class="nf">model</code><code class="p">]);</code>
<code class="p">}</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">jumpBatteryUsingCar:</code><code class="p">(</code><code class="n">Car</code> <code class="o">*</code><code class="p">)</code><code class="nv">anotherCar</code> <code class="p">{</code>
<code class="nb">NSLog</code><code class="p">(</code><code class="s">@"Jumped the %@ with a %@"</code><code class="p">,</code> <code class="p">[</code><code class="n">self</code> <code class="nf">model</code><code class="p">],</code> <code class="p">[</code><code class="n">anotherCar</code> <code class="nf">model</code><code class="p">]);</code>
<code class="p">}</code>
<code class="k">@end</code>
</pre>
<p>It’s important to note that a category can also be used to override
existing methods in the base class (e.g., the <code>Car</code> class’s
<code>drive</code> method), but <strong>you should never do this</strong>. The
problem is that categories are a flat organizational structure. If you override
an existing method in <code>Car+Maintenance.m</code>, and then decide you want
to change its behavior again with another category, there is no way for
Objective-C to know which implementation to use. Subclassing is almost always a
better option in such a situation.</p>
<h2 id="using-categories">Using Categories</h2>
<p>Any files that use an API defined in a category need to import that category
header just like a normal class interface. After importing
<code>Car+Maintenance.h</code>, all of its methods will be available directly
through the <code>Car</code> class:</p>
<pre><code class="c1">// main.m</code>
<code class="cp">#import</code> <code class="l"><Foundation/Foundation.h></code><code class="cp"></code>
<code class="cp">#import</code> <code class="l">"Car.h"</code><code class="cp"></code>
<code class="cp">#import</code> <code class="l">"Car+Maintenance.h"</code><code class="cp"></code>
<code class="kt">int</code> <code class="nf">main</code><code class="p">(</code><code class="kt">int</code> <code class="n">argc</code><code class="p">,</code> <code class="kt">const</code> <code class="kt">char</code> <code class="o">*</code> <code class="n">argv</code><code class="p">[])</code> <code class="p">{</code>
<code class="k">@autoreleasepool</code> <code class="p">{</code>
<code class="n">Car</code> <code class="o">*</code><code class="n">porsche</code> <code class="o">=</code> <code class="p">[[</code><code class="n">Car</code> <code class="nf">alloc</code><code class="p">]</code> <code class="nf">init</code><code class="p">];</code>
<code class="n">porsche</code><code class="p">.</code><code class="n">model</code> <code class="o">=</code> <code class="s">@"Porsche 911 Turbo"</code><code class="p">;</code>
<code class="n">Car</code> <code class="o">*</code><code class="n">ford</code> <code class="o">=</code> <code class="p">[[</code><code class="n">Car</code> <code class="nf">alloc</code><code class="p">]</code> <code class="nf">init</code><code class="p">];</code>
<code class="n">ford</code><code class="p">.</code><code class="n">model</code> <code class="o">=</code> <code class="s">@"Ford F-150"</code><code class="p">;</code>
<code class="c1">// "Standard" functionality from Car.h</code>
<code class="p">[</code><code class="n">porsche</code> <code class="nf">startEngine</code><code class="p">];</code>
<code class="p">[</code><code class="n">porsche</code> <code class="nf">drive</code><code class="p">];</code>
<code class="p">[</code><code class="n">porsche</code> <code class="nf">turnLeft</code><code class="p">];</code>
<code class="p">[</code><code class="n">porsche</code> <code class="nf">turnRight</code><code class="p">];</code>
<code class="c1">// Additional methods from Car+Maintenance.h</code>
<code class="k">if</code> <code class="p">([</code><code class="n">porsche</code> <code class="nf">needsOilChange</code><code class="p">])</code> <code class="p">{</code>
<code class="p">[</code><code class="n">porsche</code> <code class="nf">changeOil</code><code class="p">];</code>
<code class="p">}</code>
<code class="p">[</code><code class="n">porsche</code> <code class="nf">rotateTires</code><code class="p">];</code>
<code class="p">[</code><code class="n">porsche</code> <code class="nf">jumpBatteryUsingCar:</code><code class="n">ford</code><code class="p">];</code>
<code class="p">}</code>
<code class="k">return</code> <code class="mi">0</code><code class="p">;</code>
<code class="p">}</code>
</pre>
<p>If you remove the import statement for <code>Car+Maintenance.h</code>, the
<code>Car</code> class will revert to its original state, and the compiler will
complain that <code>needsOilChange</code>, <code>changeOil</code>, and the rest
of the methods from the <code>Maintenance</code> category don’t exist.</p>
<h2 id="protected-methods">“Protected” Methods</h2>
<p>But, categories aren’t just for spreading a class definition over
several files. They are a powerful organizational tool that allow arbitrary
files to “opt-in” to a portion of an API by simply importing the
category. To everybody else, that API remains hidden.</p>
<p>Recall from the <a href="methods.html#protected-and-private-methods">Methods</a> module that
protected methods don’t actually exist in Objective-C; however, the
opt-in behavior of categories can be used to <em>emulate</em> protected access
modifiers. The idea is to define a “protected” API in a dedicated
category, and only import it into subclass implementations. This makes the
protected methods available to subclasses, but keeps them hidden from other
aspects of the application. For example:</p>
<pre><code class="c1">// Car+Protected.h</code>
<code class="cp">#import</code> <code class="l">"Car.h"</code><code class="cp"></code>
<code class="k">@interface</code> <code class="nc">Car</code> <code class="nl">(Protected)</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">prepareToDrive</code><code class="p">;</code>
<code class="k">@end</code>
</pre>
<pre><code class="c1">// Car+Protected.m</code>
<code class="cp">#import</code> <code class="l">"Car+Protected.h"</code><code class="cp"></code>
<code class="k">@implementation</code> <code class="nc">Car</code> <code class="nl">(Protected)</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">prepareToDrive</code> <code class="p">{</code>
<code class="nb">NSLog</code><code class="p">(</code><code class="s">@"Doing some internal work to get the %@ ready to drive"</code><code class="p">,</code>
<code class="p">[</code><code class="n">self</code> <code class="nf">model</code><code class="p">]);</code>
<code class="p">}</code>
<code class="k">@end</code>
</pre>
<p>The <code>Protected</code> category shown above defines a single method for
internal use by <code>Car</code> and its subclasses. To see this in action,
let’s modify <code>Car.m</code>’s <code>drive</code> method to use
the protected <code>prepareToDrive</code> method:</p>
<pre><code class="c1">// Car.m</code>
<code class="cp">#import</code> <code class="l">"Car.h"</code><code class="cp"></code>
<code class="cp">#import</code> <code class="l">"Car+Protected.h"</code><code class="cp"></code>
<code class="k">@implementation</code> <code class="nc">Car</code>
<code class="p">...</code>
<code class="o">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="n">drive</code> <code class="p">{</code>
<code class="p">[</code><code class="n">self</code> <code class="nf">prepareToDrive</code><code class="p">];</code>
<code class="nb">NSLog</code><code class="p">(</code><code class="s">@"The %@ is now driving"</code><code class="p">,</code> <code class="n">_model</code><code class="p">)</code><code class="o">;</code>
<code class="p">}</code>
<code class="p">...</code>
</pre>
<p>Next, let’s take a look at how this protected method works by creating
a subclass called <code>Coupe</code>. There’s nothing special about the
interface, but notice how the implementation opts-in to the protected API by
importing <code>Car+Protected.h</code>. This makes it possible to use the
protected <code>prepareToDrive</code> method in the subclass. If desired, you
can also override the protected method by simply re-defining it in
<code>Coupe.m</code>.</p>
<pre><code class="c1">// Coupe.h</code>
<code class="cp">#import</code> <code class="l">"Car.h"</code><code class="cp"></code>
<code class="k">@interface</code> <code class="nc">Coupe</code> : <code class="nc">Car</code>
<code class="c1">// Extra methods defined by the Coupe subclass</code>
<code class="k">@end</code>
</pre>
<pre><code class="c1">// Coupe.m</code>
<code class="cp">#import</code> <code class="l">"Coupe.h"</code><code class="cp"></code>
<code class="cp">#import</code> <code class="l">"Car+Protected.h"</code><code class="cp"></code>
<code class="k">@implementation</code> <code class="nc">Coupe</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">startEngine</code> <code class="p">{</code>
<code class="p">[</code><code class="n">super</code> <code class="nf">startEngine</code><code class="p">];</code>
<code class="c1">// Call the protected method here instead of in `drive`</code>
<code class="p">[</code><code class="n">self</code> <code class="nf">prepareToDrive</code><code class="p">];</code>
<code class="p">}</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">drive</code> <code class="p">{</code>
<code class="nb">NSLog</code><code class="p">(</code><code class="s">@"VROOOOOOM!"</code><code class="p">);</code>
<code class="p">}</code>
<code class="k">@end</code>
</pre>
<p>To enforce the protected status of the methods in
<code>Car+Protected.h</code>, it should only be made available to subclass
implementations—do <em>not</em> import it into other files. In the
following <code>main.m</code>, you can see the protected
<code>prepareToDrive</code> method called by <code>[ford drive]</code> and
<code>[porsche startEngine]</code>, but the compiler will complain if you try
to call it directly.</p>
<pre><code class="c1">// main.m</code>
<code class="cp">#import</code> <code class="l"><Foundation/Foundation.h></code><code class="cp"></code>
<code class="cp">#import</code> <code class="l">"Car.h"</code><code class="cp"></code>
<code class="cp">#import</code> <code class="l">"Coupe.h"</code><code class="cp"></code>
<code class="kt">int</code> <code class="nf">main</code><code class="p">(</code><code class="kt">int</code> <code class="n">argc</code><code class="p">,</code> <code class="kt">const</code> <code class="kt">char</code> <code class="o">*</code> <code class="n">argv</code><code class="p">[])</code> <code class="p">{</code>
<code class="k">@autoreleasepool</code> <code class="p">{</code>
<code class="n">Car</code> <code class="o">*</code><code class="n">ford</code> <code class="o">=</code> <code class="p">[[</code><code class="n">Car</code> <code class="nf">alloc</code><code class="p">]</code> <code class="nf">init</code><code class="p">];</code>
<code class="n">ford</code><code class="p">.</code><code class="n">model</code> <code class="o">=</code> <code class="s">@"Ford F-150"</code><code class="p">;</code>
<code class="p">[</code><code class="n">ford</code> <code class="nf">startEngine</code><code class="p">];</code>
<code class="p">[</code><code class="n">ford</code> <code class="nf">drive</code><code class="p">];</code> <code class="c1">// Calls the protected method</code>
<code class="n">Car</code> <code class="o">*</code><code class="n">porsche</code> <code class="o">=</code> <code class="p">[[</code><code class="n">Coupe</code> <code class="nf">alloc</code><code class="p">]</code> <code class="nf">init</code><code class="p">];</code>
<code class="n">porsche</code><code class="p">.</code><code class="n">model</code> <code class="o">=</code> <code class="s">@"Porsche 911 Turbo"</code><code class="p">;</code>
<code class="p">[</code><code class="n">porsche</code> <code class="nf">startEngine</code><code class="p">];</code> <code class="c1">// Calls the protected method</code>
<code class="p">[</code><code class="n">porsche</code> <code class="nf">drive</code><code class="p">];</code>
<code class="c1">// "Protected" methods have not been imported,</code>
<code class="c1">// so this will *not* work</code>
<code class="c1">// [porsche prepareToDrive];</code>
<code class="kt">SEL</code> <code class="n">protectedMethod</code> <code class="o">=</code> <code class="k">@selector</code><code class="p">(</code><code class="n">prepareToDrive</code><code class="p">);</code>
<code class="k">if</code> <code class="p">([</code><code class="n">porsche</code> <code class="nf">respondsToSelector:</code><code class="n">protectedMethod</code><code class="p">])</code> <code class="p">{</code>
<code class="c1">// This *will* work</code>
<code class="p">[</code><code class="n">porsche</code> <code class="nf">performSelector:</code><code class="n">protectedMethod</code><code class="p">];</code>
<code class="p">}</code>
<code class="p">}</code>
<code class="k">return</code> <code class="mi">0</code><code class="p">;</code>
<code class="p">}</code>
</pre>
<p>Notice that you <em>can</em> access <code>prepareToDrive</code> dynamically
through <code>performSelector:</code>. Once again, <em>all</em> methods in
Objective-C are public, and there is no way to truly hide them from client
code. Categories are merely a convention-based way to control which parts of an
API are visible to which files.</p>
<h2 id="extensions">Extensions</h2>
<p>Extensions are similar to categories in that they let you add methods to a
class outside of the main interface file. But, in contrast to categories, an
extension’s API must be implemented in the <em>main</em> implementation
file—it <em>cannot</em> be implemented in a category.</p>
<p>Remember that private methods can be emulated by adding them to the
implementation but not the interface. This works when you have only a few
private methods, but can become unwieldy for larger classes. Extensions solve
this problem by letting you declare a <em>formal</em> private API.</p>
<p>For example, if you wanted to formally add a private
<code>engineIsWorking</code> method to the <code>Car</code> class defined
above, you could include an extension in <code>Car.m</code>. The compiler
complains if the method isn’t defined in the main
<code>@implementation</code> block, but since it’s declared in
<code>Car.m</code> instead of <code>Car.h</code>, it remains a <em>private</em>
method. The extension syntax looks like an empty category:</p>
<pre><code class="c1">// Car.m</code>
<code class="cp">#import</code> <code class="l">"Car.h"</code><code class="cp"></code>
<code class="c1">// The class extension</code>
<code class="k">@interface</code> <code class="nc">Car</code> <code class="nl">()</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">BOOL</code><code class="p">)</code><code class="nf">engineIsWorking</code><code class="p">;</code>
<code class="k">@end</code>
<code class="c1">// The main implementation</code>
<code class="k">@implementation</code> <code class="nc">Car</code>
<code class="k">@synthesize</code> <code class="n">model</code> <code class="o">=</code> <code class="n">_model</code><code class="p">;</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">BOOL</code><code class="p">)</code><code class="nf">engineIsWorking</code> <code class="p">{</code>
<code class="c1">// In the real world, this would probably return a useful value</code>
<code class="k">return</code> <code class="kc">YES</code><code class="p">;</code>
<code class="p">}</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">void</code><code class="p">)</code><code class="nf">startEngine</code> <code class="p">{</code>
<code class="k">if</code> <code class="p">([</code><code class="n">self</code> <code class="nf">engineIsWorking</code><code class="p">])</code> <code class="p">{</code>
<code class="nb">NSLog</code><code class="p">(</code><code class="s">@"Starting the %@'s engine"</code><code class="p">,</code> <code class="n">_model</code><code class="p">);</code>
<code class="p">}</code>
<code class="p">}</code>
<code class="p">...</code>
<code class="k">@end</code>
</pre>
<p>In addition to declaring formal private API’s, extensions can be used
to re-declare properties from the public interface. This is often used to make
properties internally behave as read-write properties while remaining read-only
to other objects. For instance, if we change the above class extension to:</p>
<pre><code class="c1">// Car.m</code>
<code class="cp">#import</code> <code class="l">"Car.h"</code><code class="cp"></code>
<code class="k">@interface</code> <code class="nc">Car</code> <code class="nl">()</code>
<code class="k">@property</code> <code class="p">(</code><code class="k">readwrite</code><code class="p">)</code> <code class="kt">double</code> <code class="n">odometer</code><code class="p">;</code>
<code class="k">-</code> <code class="p">(</code><code class="kt">BOOL</code><code class="p">)</code><code class="nf">engineIsWorking</code><code class="p">;</code>
<code class="k">@end</code>
<code class="p">...</code>
</pre>
<p>We can then assign values to <code>self.odometer</code> inside of the
implementation, but trying to do so outside of <code>Car.m</code> will result
in a compiler error.</p>
<p>Again, re-declaring properties as read-write and creating formal private
API’s isn’t all that useful for small classes. Their real utility
comes into play when you need to organize larger frameworks.</p>
<p>Extensions used to see much more action before Xcode 4.3, back when private
methods had to be declared <em>before</em> they were used. This was
inconvenient for many developers, and extensions provided a workaround by
acting as forward-declarations of private methods. So, even if you don’t
use the above pattern in your own projects, you’re likely to encounter it
at some point in your Objective-C career.</p>
<h2 id="summary">Summary</h2>
<p>This module covered Objective-C categories and extensions. Categories are a
way to modularize a class by spreading its implementation over many files.
Extensions provide similar functionality, except its API must be declared in
the <em>main</em> implementation file.</p>
<p>Outside of organizing large code libraries, one of the most common uses of
categories is to add methods to built-in data types like <code>NSString</code>
or <code>NSArray</code>. The advantage of this is that you don’t have to
update existing code to use a new subclass, but you need to be very careful not
to override existing functionality. For small personal projects, categories
really aren’t worth the trouble, and sticking with standard tools like
subclassing and protocols will save you some debugging headaches down the
road.</p>
<p>In the next module, we’ll explore another organizational tool called
blocks. Blocks are a way to represent and pass around arbitrary statements.
This opens the door to a whole new world of programming paradigms.</p>
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