Initialising a category in objective-c

Question

I'm writing a category for UITextField to include validation. I wish to change the text field's visual according to validation state (such as having an approved icon as its right view). For this, I keep a validation state property and wish to update the visual on its setter.

Here's what I have (UITextField+Validation.h)

@interface UITextField (Validation)

// Validator registration
- (void)addValidator:(id<HyValidator>)validator;

// Validation
- (void)validate;

@end

UITextField+Validation.m

@interface UITextField (Validation_Private)

@property (nonatomic, strong) NSMutableArray * validators;
@property (nonatomic) HyValidationState validationState;

@end

@implementation UITextField (Validation_Private)

- (NSMutableArray*)validators
{
    if (self.validators == nil) {
        self.validators = [[NSMutableArray alloc] init];
    }
    return self.validators;
}

- (void)setValidators:(NSMutableArray *)validators
{
    self.validators = validators;
}

- (HyValidationState)validationState
{

}

- (void)setValidationState:(HyValidationState)validationState
{

}

- (void)addValidator:(id<HyValidator>)validator
{
    [[self validators] addObject:validator];
}

- (void)validate
{

}

@end

The question is: how do I initialise validators and validationState?

Answer 1

Don't use categories for this. Subclass instead. Or, better yet, use the UITextField's delegate to do the validation, as intended.

Using categories to extend the behavior of existing system classes is generally considered to be bad design.

By using delegation, you can decouple input validation from a specific input class and, thus, your validation can be easily re-used across other input mechanisms.