Category usage in Objective-C

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It isn’t the name “private” that makes it private; the methods are private because they are in a category declared within the implementation file.

There are three uses of a category, each of which add methods to a class (note: methods only, not iVars)

Extending an existing Cocoa class

This lets you add your own methods to an existing class.
For example, if you want to extend NSString to apply special capitalization, you could create a new class called, say NSString+Capitals. in the NSString+Capitals.h you would have:

@interface NSString (Capitals)

-(NSString *)alternateCaps:(NSString *)aString;

@end

and in NSString+Capitals.m you would implement the method

@implementation NSString (Capitals)
-(NSString *)alternateCaps:(NSString *)aString
{
    // Implementation
}

Private methods on a class

This is the same as above, except that the extra methods are declared and defined in the implementation file (.m) Usually a way of having private methods – because they are not in the .h file (which is the one #imported by other classes) they are simply not visible. In this case, the implementation of the methods are done in their own implementation block. e.g

// someClass.m
@interface someClass (extension)
-(void)extend;
@end

@implementation someClass
    // all the methods declared in the .h file and any superclass
    // overrides in this block
@end

@implementation someClass (extension)
-(void)extend {
    // implement private method here;
}

Class Extension (New for 10.5 Leopard)

A simpler way of having private methods. In this special case, the category name is empty and the private methods are implemented in the same block as all the other class methods.

// someClass.m
@interface someClass ()
-(void)extend;
@end

@implementation someClass
    // all the methods declared in the .h file and any superclass
    // overrides in this block
    // Implement private methods in this block as well.
-(void)extend {
    // implement private method here;
}

@end

Here’s a link to the Apple docs on Categories and extensions.

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