The right solution to limit length of a text field.

UITextFieldLimitLengthDelegate.m

/*

Why not simply use textField:shouldChangeCharactersInRange:replacementString:?
===

If you google how to limit a length of text field, you'll see many implementations simply using textField:shouldChangeCharactersInRange:replacementString: like

  - (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
  {
      return [[textField.text stringByReplacingCharactersInRange:range withString:string] length] <= MAX_LENGTH;
  }

It looks work, however, it actually doesn't works with CJK and some other languages keyboards.
Actually textField:shouldChangeCharactersInRange:replacementString: should always returns YES, otherwise you'll see some troubles with these keyboards.
I think it's Apple's API design issue.

Because of this behavior, limiting a length of text field is not easy.
This is an example of better implementation which can handle many edge cases and works CJK keyboards.

---

Copyright (c) 2014 Yoshimasa Niwa

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*/

#import <UIKit/UIKit.h>

@interface UITextFieldLimitLengthDelegate : NSObject <UITextFieldDelegate>

@property (nonatomic) NSUInteger maxLength;

/**
 * Use this class as a delegate for the text field.
 */
- (instancetype)initWithTextField:(UITextField *)textField maxLength:(NSUInteger)maxLength;

/**
 * If you're not using this limiter as a delegate for the text field, you should call this method
 * from your textField:shouldChangeCharactersInRange:replacementString: delegate method.
 */
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string;

@end

@implementation UITextFieldLimitLengthDelegate
{
    NSString *_previousText;
    NSRange _lastReplaceRange;
    NSString *_lastReplacementString;
}

- (instancetype)init
{
    return [self initWithTextField:nil maxLength:0];
}

- (instancetype)initWithTextField:(UITextField *)textField maxLength:(NSUInteger)maxLength
{
    if (self = [super init]) {
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(_textFieldDidChange:) name:UITextFieldTextDidChangeNotification object:textField];
        _maxLength = maxLength;
    }
    return self;
}

- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    _previousText = textField.text;
    _lastReplaceRange = range;
    _lastReplacementString = string;

    return YES;
}

- (void)_textFieldDidChange:(NSNotification *)notification
{
    UITextField *textField = (UITextField *)notification.object;

    if (textField.markedTextRange) {
        return;
    }

    if ([textField.text length] > _maxLength) {
        NSInteger offset = _maxLength - [textField.text length];

        NSString *replacementString = [_lastReplacementString substringToIndex:([_lastReplacementString length] + offset)];
        NSString *text = [_previousText stringByReplacingCharactersInRange:_lastReplaceRange withString:replacementString];

        UITextPosition *position = [textField positionFromPosition:textField.selectedTextRange.start offset:offset];
        UITextRange *selectedTextRange = [textField textRangeFromPosition:position toPosition:position];

        textField.text = text;
        textField.selectedTextRange = selectedTextRange;
    }
}

@end