本文介绍如何使用上传回调。

OSS在上传文件完成的时候可以提供回调（Callback）给应用服务器。您只需要在发送给OSS的请求中携带相应的Callback参数，即能实现回调。因为加入了回调请求和响应的过程，相比简单上传，使用回调通知机制一般会导致客户端花费更多的等待时间。

具体说明参考：Callback

以下代码用于上传回调：

OSSPutObjectRequest * request = [OSSPutObjectRequest new];
request.bucketName = @"<bucketName>";
request.objectKey = @"<objectKey>";
request.uploadingFileURL = [NSURL fileURLWithPath:@<filepath>"];
// 设置回调参数
request.callbackParam = @{
                          @"callbackUrl": @"<your server callback address>",
                          @"callbackBody": @"<your callback body>"
                          };
// 设置自定义变量
request.callbackVar = @{
                        @"<var1>": @"<value1>",
                        @"<var2>": @"<value2>"
                        };
request.uploadProgress = ^(int64_t bytesSent, int64_t totalByteSent, int64_t totalBytesExpectedToSend) {
    NSLog(@"%lld, %lld, %lld", bytesSent, totalByteSent, totalBytesExpectedToSend);
};
OSSTask * task = [client putObject:request];
[task continueWithBlock:^id(OSSTask *task) {
    if (task.error) {
        OSSLogError(@"%@", task.error);
    } else {
        OSSPutObjectResult * result = task.result;
        NSLog(@"Result - requestId: %@, headerFields: %@, servercallback: %@",
              result.requestId,
              result.httpResponseHeaderFields,
              result.serverReturnJsonString);
    }
    return nil;
}];