字符串截取

        1. NSString *beforeString = [successString substringToIndex:3];// 截取下标为3之前的字符串

        2. NSString *afterString = [successString substringFromIndex:4];// 截取从下标为4开始之后的字符串

        3. NSArray *array = [successString componentsSeparatedByString:@"-"];//从字符A中分隔成n个元素的数组

        4. NSRange range = [successString rangeOfString:@"--"];//匹配得到的下标

            NSLog(@"rang:%@",NSStringFromRange(range));

            NSString *string = [successString substringWithRange:range];//截取范围类的字符串(--之后的字符串)