# 重写响应头
下载对象时，可以重写部分HTTP/HTTPS响应头信息。可重写的响应头信息见下表：
| **参数**                     | **作用**                              | **OBS iOS SDK对应方法**                |
|:---|:---|:---|
| responseContentType        | 重写HTTP/HTTPS响应中的Content-Type        | request.responseContentType        |
| responseContentLanguage    | 重写HTTP/HTTPS响应中的Content-Language    | request.responseContentLanguage    |
| responseExpires            | 重写HTTP/HTTPS响应中的Expires             | request.responseExpires            |
| responseCacheControl       | 重写HTTP/HTTPS响应中的Cache-Control       | request.responseCacheControl       |
| responseContentDisposition | 重写HTTP/HTTPS响应中的Content-Disposition | request.responseContentDisposition |
| responseContentEncoding    | 重写HTTP/HTTPS响应中的Content-Encoding    | request.responseContentEncoding    |
   
以下代码展示了如何重写响应头：
```
static OBSClient *client;
NSString *endPoint = @"https://your-endpoint";
// 认证用的ak和sk硬编码到代码中或者明文存储都有很大的安全风险，建议在配置文件或者环境变量中密文存放，使用时解密，确保安全；本示例以ak和sk保存在环境变量中为例，运行本示例前请先在本地环境中设置环境变量AccessKeyID和SecretAccessKey。
// 您可以登录访问管理控制台获取访问密钥AK/SK，获取方式请参见https://support.huaweicloud.com/usermanual-ca/ca_01_0003.html
char* ak_env = getenv("AccessKeyID");
char* sk_env = getenv("SecretAccessKey");
if (ak_env == NULL || sk_env == NULL) {
    NSLog(@"Error: AccessKeyID or SecretAccessKey environment variable not set");
    return;
}
NSString *AK = [NSString stringWithUTF8String:ak_env];
NSString *SK = [NSString stringWithUTF8String:sk_env];
    
// 初始化身份验证
OBSStaticCredentialProvider *credentialProvider = [[OBSStaticCredentialProvider alloc] initWithAccessKey:AK secretKey:SK];
    
//初始化服务配置
OBSServiceConfiguration *conf = [[OBSServiceConfiguration alloc] initWithURLString:endPoint credentialProvider:credentialProvider];
    
// 初始化client
client = [[OBSClient alloc] initWithConfiguration:conf];
    
// 流式下载
OBSGetObjectToDataRequest *request = [[OBSGetObjectToDataRequest alloc]initWithBucketName:@"bucketname" objectKey:@"objectname"];
    
//重写ContentType
request.responseContentType = @"image/jpeg";
    
    
// 下载进度
request.downloadProgressBlock = ^(int64_t bytesWritten, int64_t totalBytesWritten, int64_t totalBytesExpectedToWrite) {
    NSLog(@"%0.1f%%", (totalBytesExpectedToWrite > 0) ? (float)(totalBytesWritten)*100/(float)totalBytesExpectedToWrite : 0);
};
    
// 下载的数据
__block NSMutableData *objectData = [NSMutableData new];
request.onReceiveDataBlock = ^(NSData *data) {
    [objectData appendData:data];
};
    
// 下载结果
[client getObject:request completionHandler:^(OBSGetObjectResponse *response, NSError *error){
    NSLog(@"%@",response);
}] ;
```