对象存储 OSS数据完整性校验

详细解答可以参考官方帮助文档

因为移动端网络环境的复杂性，数据在客户端和服务器之间传输时有可能会出错。因此OSS SDK在下载过程中提供了基于CRC的端到端的数据完整性效验功能。

CRC校验在读取下载过程中时候，如果开启了crc效验，并有在传输中有数据错误的情况。会提示OSSClientErrorCodeInvalidCRC的错误。注：如果用户

OSSGetObjectRequest * request = [OSSGetObjectRequest new];
request.bucketName = ...;
//开启crc效验
request.crcFlag = OSSRequestCRCOpen;
OSSTask * task = [testProxyClient getObject:request];
[[task continueWithBlock:^id(OSSTask *task) {
    //如果开启了crc效验，并有在传输中有数据错误的情况。会提示OSSClientErrorCodeInvalidCRC的错误
    XCTAssertNil(task.error);
    return nil;
}] waitUntilFinished];
注：如果设置onReceiveData block。开启CRC效验后，需要自行比较CRC数值是否一致。范例如下
OSSGetObjectRequest * request = [OSSGetObjectRequest new];
request.bucketName = ....
request.crcFlag = OSSRequestCRCOpen;
....
__block uint64_t localCrc64 = 0;
    //如果设置onReceiveData block
NSMutableData *receivedData = [NSMutableData data];
request.onRecieveData = ^(NSData *data) {
    if (data)
    {
        NSMutableData *mutableData = [data mutableCopy];
        void *bytes = mutableData.mutableBytes;
        localCrc64 = [OSSUtil crc64ecma:localCrc64 buffer:bytes length:data.length];
        [receivedData appendData:data];
    }
};
__block uint64_t remoteCrc64 = 0;
OSSTask * task = [_client getObject:request];
[[task continueWithBlock:^id(OSSTask *task) {
    XCTAssertNil(task.error);
       OSSGetObjectResult *result = task.result;
    if (result.remoteCRC64ecma) 
    {
        NSScanner *scanner = [NSScanner scannerWithString:result.remoteCRC64ecma];
        [scanner scanUnsignedLongLong:&remoteCrc64];
        if (remoteCrc64 == localCrc64)
        {
            NSLog(@"crc64校验成功!");
        }
        else
        {
            NSLog(@"crc64校验失败!");
        }
   }
   return nil;
}] waitUntilFinished];