iOS实现银行卡输入格式化

最近项目要做一个绑定银行卡的功能，于是我打开微信和支付宝瞄了一眼，对比后发现微信和支付宝在卡号输入的一些细节上处理是不一样的，举个例子：输入卡号的时候，我发现中间有一位数输错了，然后试着去修改，微信的处理是删除完这个数字，光标直接移动到末尾。。这不科学啊，我还没有修改完呢。。而支付宝的处理就比较合理了，删除完后光标还在那个位置，可以继续输入正确的数字。
本着虚心学习的态度，就想着怎么实现支付宝的这个效果比较好的卡号输入，本以为很简单，可以很快搞定，最后却用了大半天才实现了比较完美的效果。废话不多说，直接上代码：

/**
 * UITextField Category 用于控制光标位置
 */
@interface UITextField (ExtendRange)

- (NSRange)selectedRange;

- (void)setSelectedRange:(NSRange)range;

@end

@implementation UITextField (ExtendRange)
- (NSRange)selectedRange {
    UITextPosition *beginning = self.beginningOfDocument;
    UITextRange *selectedRange = self.selectedTextRange;
    UITextPosition *selectionStart = selectedRange.start;
    UITextPosition *selectionEnd = selectedRange.end;
    const NSInteger location = [self offsetFromPosition:beginning toPosition:selectionStart];
    const NSInteger length = [self offsetFromPosition:selectionStart toPosition:selectionEnd];
    return NSMakeRange(location, length);
}
- (void)setSelectedRange:(NSRange)range {
    UITextPosition *beginning = self.beginningOfDocument;
    UITextPosition *startPosition = [self positionFromPosition:beginning offset:range.location];
    UITextPosition *endPosition = [self positionFromPosition:beginning offset:range.location + range.length];
    UITextRange *selectionRange = [self textRangeFromPosition:startPosition toPosition:endPosition];
    [self setSelectedTextRange:selectionRange];
}
@end

//每组个数
static NSInteger const kGroupSize = 4;

#pragma mark - UITextFieldDelegate
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    if (textField == self.bankCardNoField) {
        NSString *text = textField.text;
        NSString *beingString = [textField.text stringByReplacingCharactersInRange:range withString:string];
        NSString *cardNo = [self removingSapceString:beingString];
        //校验卡号只能是数字，且不能超过20位
        if ( (string.length != 0 && ![self isValidNumbers:cardNo]) || cardNo.length > 20) {
            return NO;
        }
        //获取【光标右侧的数字个数】
        NSInteger rightNumberCount = [self removingSapceString:[text substringFromIndex:textField.selectedRange.location + textField.selectedRange.length]].length;
        //输入长度大于4 需要对数字进行分组，每4个一组，用空格隔开
        if (beingString.length > kGroupSize) {
            textField.text = [self groupedString:beingString];
        } else {
            textField.text = beingString;
        }
        text = textField.text;
        /**
         * 计算光标位置(相对末尾)
         * 光标右侧空格数 = 所有的空格数 - 光标左侧的空格数
         * 光标位置 = 【光标右侧的数字个数】+ 光标右侧空格数
         */
        NSInteger rightOffset = [self rightOffsetWithCardNoLength:cardNo.length rightNumberCount:rightNumberCount];
        NSRange currentSelectedRange = NSMakeRange(text.length - rightOffset, 0);
        
        //如果光标左侧是一个空格，则光标回退一格
        if (currentSelectedRange.location > 0 && [[text substringWithRange:NSMakeRange(currentSelectedRange.location - 1, 1)] isEqualToString:@" "]) {
            currentSelectedRange.location -= 1;
        }
        [textField setSelectedRange:currentSelectedRange];
        return NO;
    }
    return YES;
}

#pragma mark - Helper
/**
 *  计算光标相对末尾的位置偏移
 *
 *  @param length           卡号的长度(不包括空格)
 *  @param rightNumberCount 光标右侧的数字个数
 *
 *  @return 光标相对末尾的位置偏移
 */
- (NSInteger)rightOffsetWithCardNoLength:(NSInteger)length rightNumberCount:(NSInteger)rightNumberCount {
    NSInteger totalGroupCount = [self groupCountWithLength:length];
    NSInteger leftGroupCount = [self groupCountWithLength:length - rightNumberCount];
    NSInteger totalWhiteSpace = totalGroupCount -1 > 0? totalGroupCount - 1 : 0;
    NSInteger leftWhiteSpace = leftGroupCount -1 > 0? leftGroupCount - 1 : 0;
    return rightNumberCount + (totalWhiteSpace - leftWhiteSpace);
}

/**
 *  校验给定字符串是否是纯数字
 *
 *  @param numberStr 字符串
 *
 *  @return 字符串是否是纯数字
 */
- (BOOL)isValidNumbers:(NSString *)numberStr {
    NSString* numberRegex = @"^[0-9]+$";
    NSPredicate *numberPre = [NSPredicate predicateWithFormat:@"SELF MATCHES %@",numberRegex];
    return [numberPre evaluateWithObject:numberStr];
}

/**
 *  去除字符串中包含的空格
 *
 *  @param str 字符串
 *
 *  @return 去除空格后的字符串
 */
- (NSString *)removingSapceString:(NSString *)str {
    return [str stringByReplacingOccurrencesOfString:@" " withString:@"" options:NSCaseInsensitiveSearch range:NSMakeRange(0, str.length)];
}

/**
 *  根据长度计算分组的个数
 *
 *  @param length 长度
 *
 *  @return 分组的个数
 */
- (NSInteger)groupCountWithLength:(NSInteger)length {
    return (NSInteger)ceilf((CGFloat)length /kGroupSize);
}

/**
 *  给定字符串根据指定的个数进行分组，每一组用空格分隔
 *
 *  @param string 字符串
 *
 *  @return 分组后的字符串
 */
- (NSString *)groupedString:(NSString *)string {
    NSString *str = [self removingSapceString:string];
    NSInteger groupCount = [self groupCountWithLength:str.length];
    NSMutableArray *components = [[NSMutableArray alloc] init];
    for (NSInteger i = 0; i < groupCount; i++) {
        if (i*kGroupSize + kGroupSize > str.length) {
            [components addObject:[str substringFromIndex:i*kGroupSize]];
        } else {
            [components addObject:[str substringWithRange:NSMakeRange(i*kGroupSize, kGroupSize)]];
        }
    }
    NSString *groupedString = [components componentsJoinedByString:@" "];
    return groupedString;
}

银行卡输入Demo: https://github.com/Nomeqc/BankCardInputFieldDemo