在TextField中只能输入数字和字母以及监听textFiel

1.使用分类判断一个字符是不是字母或者数字（regex为正则表达式，可根据需求更改）

#import "NSString+String.h"
@implementation NSString (String)
-(BOOL)deptNumInputShouldNumberAndString{
    if (self.length == 0) {
        return NO;
    }
    NSString *regex = @"^[A-Za-z0-9]+$";
    NSPredicate *pred = [NSPredicate predicateWithFormat:@"SELF MATCHES %@",regex];
    if ([pred evaluateWithObject:self]) {
        return YES;
    }
    return NO;
}
@end

2.在textField代理方法中调用分类方法，如果不是字母和数字，则返回no

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    if (![string deptNumInputShouldNumberAndString]) {
        return NO;
    }
    return YES;
}

3.监听位数。对代理方法中的string做判断，如果是空（@""）则输入的是X按钮即清除按钮，不能用textfield.text值的长度去判断

-(NSInteger)getCountSum:(NSInteger)countSum{
    if ([self isEqual:@""]) {
        if (countSum != 0) {
            countSum--;
        }
    }
    else{
        countSum++;
    }
    return countSum;
}

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string{
    _sum = [string getCountSum:_sum];
    return YES;
}