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Registering a COM Callback

Instead of polling for changes in the status of a job, you can register to receive notification when the job's status changes. To receive notification, you must implement the IBackgroundCopyCallback2 interface. The interface contains the following methods that BITS calls depending on your registration:

For an example that implements the IBackgroundCopyCallback2 interface, see Example Code in the IBackgroundCopyCallback interface topic.

The IBackgroundCopyCallback2 interface provides notification for when a file is transferred. Typically, you use this method to validate the file so that the file is available for peers to download; otherwise, the file is not available to peers until you call the IBackgroundCopyJob::Complete method. To validate the file, call the IBackgroundCopyFile3::SetValidationState method.

To register your implementation with BITS, call the IBackgroundCopyJob::SetNotifyInterface method. To specify which methods BITS calls, call the IBackgroundCopyJob::SetNotifyFlags method.

The notification interface becomes invalid when your application terminates; BITS does not persist the notify interface. As a result, your application's initialization process should register existing jobs for which you want to receive notification. If you need to capture state and progress information that occurred since the last time your application was run, poll for state and progress information during application initialization.

Before exiting, your application should clear the callback interface pointer (SetNotifyInterface(NULL)). It is more efficient to clear the callback pointer than to let BITS discover that it is no longer valid.

Note that if more than one application calls the SetNotifyInterface method to set the notification interface for the job, the last application to call the SetNotifyInterface method is the one that will receive notifications—the other applications will not receive notifications.

The following example shows how to register for notifications. The example assumes the IBackgroundCopyJob interface pointer is valid. For details on the CNotifyInterface example class used in the following example, see the IBackgroundCopyCallback interface.

IBackgroundCopyJob* pJob;
CNotifyInterface *pNotify = new CNotifyInterface();

if (pNotify)
    hr = pJob->SetNotifyInterface(pNotify);
    if (SUCCEEDED(hr))
        hr = pJob->SetNotifyFlags(BG_NOTIFY_JOB_TRANSFERRED | 
                                  BG_NOTIFY_JOB_ERROR );
    pNotify = NULL;

    if (FAILED(hr))
        //Handle error - unable to register callbacks.



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