The Apple documentation actually states that it’s a bad idea to use categories to override instance or class methods: http://developer.apple.com/mac/library/documentation/cocoa/conceptual/objectivec/articles/occategories.html

The relevant bit for you is the first bullet point in this section:

“Although the language currently allows you to use a category to override methods the class inherits, or even methods declared in the class interface, you are strongly discouraged from using this functionality. A category is not a substitute for a subclass. There are several significant shortcomings:

* When a category overrides an inherited method, the method in the category can, as usual, invoke the inherited implementation via a message to super. However, if a category overrides a method that already existed in the category’s class, there is no way to invoke the original implementation.

* A category cannot reliably override methods declared in another category of the same class.

This issue is of particular significance since many of the Cocoa classes are implemented using categories. A framework-defined method you try to override may itself have been implemented in a category, and so which implementation takes precedence is not defined.

* The very presence of some methods may cause behavior changes across all frameworks. For example, if you add an implementation of windowWillClose: to NSObject, this will cause all window delegates to respond to that method and may modify the behavior of all instances of NSWindow instances. This may cause mysterious changes in behavior and can lead to crashes.”

…hope that helps!
Nick

I just a very efficient way to reverse a string and started with your lines of codes. Here is what I came up at the end. Hope this will help for some people. :-)

@implementation NSString (reverse)
- (NSString *) reverseString {
NSMutableString *reversedStr;
int len = [self length];

// Auto released string
reversedStr = [NSMutableString stringWithCapacity:len];

while (len–) {
[reversedStr appendFormat:@"%C", [self characterAtIndex:len]];
}

return reversedStr;
}

I have used categories to breakup a class into multiple files with no problems in the past. I tried to do the same with a class name (“Game_2ViewController+Animation.h”) that had an underscore character and the methods and functions defined within the subcategory class file could not be called by the program. I simply made a new subcategory class file and used no underscore character (“Game2ViewController+Animation.h”) and the problem went away?

To access the original method you must subclass.

I tried to use the same method to have a category overwrite viewDidLoad of a UIViewController class. But the problem is that the category implementation is unable to call the [super viewDidLoad method]. The compiler gives me a warning that the viewDidLoad method is not part of UIResponder. Strange.

Here is the category code

// Header
@interface UIViewController (UIViewController_TestingCategory)
– (void) viewDidLoad;

@end

// Implementation file

#import “UIViewController+TestingCategory.h”

@implementation UIViewController (UIViewController_TestingCategory)

- (void)viewDidLoad {
[super viewDidLoad];

// Do my own special behavior of view did load
….
}

-> The compiler gives me the warning on [super viewDidLoad] and of course things crash.

Do I have to revert to subclassic. I would like to use categories so I don’t have to change my view controllers to inherit from my own UIViewController class.

Thanks,
Leifur