+ (NSString *)randomStringWithLength:(int)length;

+ (NSString *)randomStringWithLength:(int)length{
   
   
    NSString *letters = @"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    NSMutableString *randomString = [NSMutableString stringWithCapacity: length];
    for (int i = 0; i < length; i++) {
        [randomString appendFormat: @"%C", [letters characterAtIndex: arc4random_uniform((int)[letters length])]];
    }
   
    //获取系统时间
    NSDate * senddate=[NSDate date];
    NSDateFormatter  *dateformatter=[[NSDateFormatter alloc] init];
    [dateformatter setDateFormat:@"YYYY-MM-dd"];
    NSString *  locationString=[dateformatter stringFromDate:senddate];
    NSDate * now = [dateformatter dateFromString:locationString];
    //转成时间戳
    NSString *timeSp = [NSString stringWithFormat:@"%ld", (long)[now timeIntervalSince1970]];

    //把时间戳和随机字符进行拼接组合
    return  [NSString stringWithFormat:@"%@_%@_%@.png",timeSp,randomString,@"用户uid"];

}