Use C-style functions instead of categories to avoid name collisions and reduce Objective-C runtime overhead, unless subclasses are expected to override the category method.
Methods in categories are added to the class on which the category is implemented, replacing any existing methods. If the same method is implemented in multiple categories, which one is used is undefined. This is called a name clash, which can result even when following best practices for category naming. The difference between implementing a C-style function and Objective-C category method is trivial, but the static linking characteristics of C-style avoid any potential undefined behavior.
Separately, C-style functions avoid adding to the Objective-C binding time when dyld loads the application executable.
NSString+XYZExcitementAdditions.h
@interface NSString (XYZExcitementAdditions)
- (NSString *)xyz_stringWithExcitement; // bad: potential name collisions
@end
NSString *XYZStringWithExcitement(NSString *string); // good: symbol safety
NSString+XYZExcitementAdditions.m
#import "NSString+XYZExcitementAdditions.h"
@implementation NSString (XYZExcitementAdditions)
- (NSString *)xyz_stringWithExcitement { // bad: potential name collisions
return [self stringByAppendingString:@"!"];
}
@end
NSString *XYZStringWithExcitement(NSString *string) { // good: symbol safety
return [string stringByAppendingString:@"!"];
}