1.只能输入数字和小数点
2.小数点前只能有一个0
3.小数点后两位

// MARK: -- 键盘代理
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string{
    if ([string isEqualToString:@""]) { // 删除操作
        if ([textField.text isEqualToString:@"0"]) {
            textField.text = @"";
        }
    }
    // 输入小数点补0
    if ([textField.text isEqualToString:@""] && [@"." isEqualToString:string]) {
        textField.text = @"0.";
    }
    if ([textField.text isEqualToString:string] && [@"0" isEqualToString:string]) { // 前两位不能为00
        return NO;
    }else{ // 可以是小数点且只能是小数点
        if (![textField.text isEqualToString:@"0"] || [string isEqualToString:@"."]) {
            NSInteger dotLocation = [textField.text rangeOfString:@"."].location;
            if (dotLocation != NSNotFound){
                if ([string isEqualToString:@"."]){
                    return NO;
                }
            }
            //判断小数点后最多两位
            if (dotLocation != NSNotFound && range.location > dotLocation + 2) {
                return NO;
            }
        }else{
            return NO;
        }
        
    }
    return YES;
    
}