Categories

Categories are an Objective-C syntax for adding features to a class without subclassing it and without even having access to its code (its .m file).

This comes in handy when you want to keep using a specific class (for example NSString) but would like to add an extra feature.

We have seen categories before, when using the AFNetworking framework. UIImageView+AFNetworking is a category on UIImage that allows setting an image using a URL. It does this by adding methods to UIImage such as setImageWithURL: and setImageWithURL:placeholderImage:.

Categories have their own .h and .m files. These are usually named +. These can be easily created through the New File menu in Xcode.

#import <Foundation/Foundation.h>

//--------------------------------------------------------------
//--------------------------------------------------------------
@interface NSString (Validation)

@end

#import "NSString+Validation.h"

//--------------------------------------------------------------
//--------------------------------------------------------------
@implementation NSString (Validation)

@end

Categories can have their own methods and properties, but no instance variables. This means that properties must be implemented and cannot be synthesized.

#import <Foundation/Foundation.h>

//--------------------------------------------------------------
//--------------------------------------------------------------
@interface NSString (Validation)

- (BOOL)validateNotEmpty;
- (BOOL)validateMinimumLength:(NSInteger)length;
- (BOOL)validateMaximumLength:(NSInteger)length;

@end

#import "NSString+Validation.h"

//--------------------------------------------------------------
//--------------------------------------------------------------
@implementation NSString (Validation)

//--------------------------------------------------------------
- (BOOL)validateNotEmpty
{
    return ([self length] != 0);
}

//--------------------------------------------------------------
- (BOOL)validateMinimumLength:(NSInteger)length
{
    return ([self length] >= length);
}

//--------------------------------------------------------------
- (BOOL)validateMaximumLength:(NSInteger)length
{
    return ([self length] <= length);
}

@end

To use a category, we just need to import the header into the class where we need the additional functionality. This will make the extra methods available to use.

#import "ViewController.h"

#import "NSString+Validation.h"

//--------------------------------------------------------------
//--------------------------------------------------------------
@implementation ViewController

//--------------------------------------------------------------
- (void)viewDidLoad
{
    [super viewDidLoad];

    NSString *myString = @"This is a string";

    if ([myString validateNotEmpty]) {
        NSLog(@"1. Not empty!");
    }
    else {
        NSLog(@"1. Empty!");
    }

    if ([myString validateMinimumLength:6]) {
        NSLog(@"2. Min length OK");
    }
    else {
        NSLog(@"2. Too short!");
    }

    if ([myString validateMaximumLength:10]) {
        NSLog(@"3. Max length OK");
    }
    else {
        NSLog(@"3. Too long!");
    }
}

@end