中文的 UTF8 是 3 字节，Emoj 是 4 个字节、占 2 个字符，所有不能简单的用length来判断

本文对于字符和字节的输入限制各提供了两种方法

字符限制

• 方法一

- (void)textFieldDidChange:(UITextField *)textField
{
        NSString *text = textField.text;
        UITextRange *selectedRange = [textField markedTextRange];
        UITextPosition *position = [textField positionFromPosition:selectedRange.start offset:0];
        // 中文输入下，选择没有高亮的字
        if (!position)
        {
            if (textField.text.length > MAX_STRING_LENGTH)
            {
                NSRange rangeIndex = [text rangeOfComposedCharacterSequenceAtIndex:MAX_STRING_LENGTH];
                if (rangeIndex.length == 1)
                {
                    textField.text = [text substringToIndex:MAX_STRING_LENGTH];
                }
                else
                {
                    // emoji占两个字符 防止整词被切断
                    NSRange rangeRange = [text rangeOfComposedCharacterSequencesForRange:NSMakeRange(0, MAX_STRING_LENGTH)];
                    if (rangeRange.length > MAX_STRING_LENGTH)
                    {
                        rangeRange.length = rangeRange.length - rangeIndex.length;
                    }
                    textField.text = [text substringWithRange:rangeRange];
                }  
            }
        }
}

• 方法二

- (void)textFieldDidChange:(UITextField *)textField
{
        NSString *text = textField.text;   
        UITextRange *selectedRange = [textField markedTextRange];
        UITextPosition *position = [textField positionFromPosition:selectedRange.start offset:0];
        if (!position)
        {
            if (text.length > MAX_LENGTH)
            {
                NSRange range;
                NSUInteger inputLength = 0;
                for(int i=0; i < text.length && inputLength <= MAX_LENGTH; i += range.length) 
                {
                    range = [textField.text rangeOfComposedCharacterSequenceAtIndex:i];
                    inputLength += [text substringWithRange:range].length;
                    if (inputLength > MAX_LENGTH) 
                    {
                        textField.text = [text substringWithRange:NSMakeRange(0, range.location)];
                    }
                }
            }
        }
}

字节限制

• 方法一

- (void)textFieldDidChange:(UITextField *)textField
{
        UITextRange *selectedRange = [textField markedTextRange];
        UITextPosition *position = [textField positionFromPosition:selectedRange.start offset:0];
        if (!position)
        {
            while ([textField.text lengthOfBytesUsingEncoding:NSUTF8StringEncoding] > MAX_STRING_LENGTH)
            {
                NSRange rangeIndex = [textField.text rangeOfComposedCharacterSequenceAtIndex:textField.text.length - 1];
                if (rangeIndex.length == 1)
                {
                    textField.text = [textField.text substringToIndex:textField.text.length - 1];
                }
                else
                {
                    NSRange rangeRange = [textField.text rangeOfComposedCharacterSequencesForRange:NSMakeRange(0, textField.text.length - 1)];
                    if (rangeRange.length > textField.text.length - 1) 
                    {
                        rangeRange.length = rangeRange.length - rangeIndex.length;
                    }
                    textField.text = [textField.text substringWithRange:rangeRange];
                }
            }
        }
}

• 方法二

- (void)textFieldDidChange:(UITextField *)textField
{
    NSString *text = textField.text;
    UITextRange *selectedRange = [textField markedTextRange];
    UITextPosition *position = [textField positionFromPosition:selectedRange.start offset:0];
    if (!position){
        NSUInteger textBytesLength = [textField.text lengthOfBytesUsingEncoding:NSUTF8StringEncoding];
        if (textBytesLength > MAX_BYTES_LENGTH) {
            NSRange range;
            NSUInteger byteLength = 0;
            for(int i=0; i < text.length && byteLength <= MAX_BYTES_LENGTH; i += range.length) {
                range = [textField.text rangeOfComposedCharacterSequenceAtIndex:i];
                byteLength += strlen([[text substringWithRange:range] UTF8String]);
                if (byteLength > MAX_BYTES_LENGTH) {
                    textField.text = [text substringWithRange:NSMakeRange(0, range.location)];
                }
            }
        }
    }
}