Simple implementation of NSFormatter class.

LimitFormatter.h

#import <Foundation/Foundation.h>

@interface LimitFormatter : NSFormatter {

}
@property int maxLength;

@end

LimitFormatter.m

#import "LimitFormatter.h"


@implementation LimitFormatter : NSFormatter

@synthesize maxLength;

- (id)init {
    if([super init]){
        maxLength = INT_MAX;
    }
    return self;
}

- (NSString *)stringForObjectValue:(id)object {
    return (NSString *)object;
}

- (BOOL)getObjectValue:(id *)object 
      forString:(NSString *)string 
      errorDescription:(NSString **)error {
    *object = string;
    return YES;
}

-(BOOL)isPartialStringValid:(NSString *)partialString 
       newEditingString:(NSString **)newString 
           errorDescription:(NSString **)error{
    if([partialString length] > maxLength){
        return NO;
    }
    return YES;
}
@end

Simple implementation of NSFormatter class, just set maximum character's limit that user can enter into NSTextField.