StackOverflow2013

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copied!<p>The two syntaxes serve different purposes.</p> <p>A named category -- <strong>@interface Foo(FooCategory)</strong> -- is generally used to:</p> <p>(1) extend an existing class by adding functionality. Example: NSAttributedString in Foundation is extended by a category in AppKit that adds AppKit specific RTF-like text formatting API.</p> <p>(2) declare a set of methods that might or might not be implemented by a delegate. Example: Various classes declare -- but don't implement -- @interface NSObject(SetODelegateMethods).</p> <p>Form (2) has fallen out of favor now that @protocol has been extended to support @optional methods in Objective-C 2.0.</p> <p>A class extension -- <strong>@interface Foo()</strong> -- is designed to allow you to declare additional private API -- SPI or System Programming Interface -- that is used to implement the class innards. This typically appears at the top of the .m file. Any methods / properties declared in the class extension must be implemented in the @implementation, just like the methods/properties found in the public @interface.</p> <p>Class extensions can also be used to redeclare a publicly readonly @property as readwrite prior to @synthesize'ing the accessors.</p> <p>Example:</p> <p>Foo.h</p> <pre><code>@interface Foo:NSObject @property(readonly, copy) NSString *bar; -(void) publicSaucing; @end </code></pre> <p>Foo.m</p> <pre><code>@interface Foo() @property(readwrite, copy) NSString *bar; - (void) superSecretInternalSaucing; @end @implementation Foo @synthesize bar; .... must implement the two methods or compiler will warn .... @end </code></pre>

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