Metadata change listeners

#Metadata change listeners

Implement and register IMetadataChangeListener if you want to provide custom logic when file metadata was changed in a session and the metadata update operation is executed. The interface looks as follow:

public interface IMetadataChangeListener
{
/// <summary>
/// Invoked before the written data is sent to the server.
/// </summary>
/// <param name="instance">The file to affect</param>
/// <param name="metadata">The new metadata</param>
/// <param name="original">The original remote object metadata</param>
/// <returns>
/// Whatever the metadata was modified and requires us to re-send it.
/// Returning false would mean that any request of write to the file would be 
/// ignored in the current SaveChanges call.
/// </returns>
bool BeforeChange(FileHeader instance, RavenJObject metadata, RavenJObject original);

/// <summary>
/// Invoked after the metadata is sent to the server.
/// </summary>
/// <param name="instance">The updated file information.</param>
/// <param name="metadata">The current metadata as stored in the server.</param>
void AfterChange(FileHeader instance, RavenJObject metadata);
}

The same like for delete operations you can prevent from updating metadata by returning false from BeforeChange method. The operation will be skipped then and AfterChange won't be executed.

#Example

You can modify the metadata before it will be sent to the server. For example you can remove tags that are banned:

public class RemoveBannedTagsListener : IMetadataChangeListener
{
private readonly string[] bannedTags =
{
	/* ... */
};

public bool BeforeChange(FileHeader instance, RavenJObject metadata, RavenJObject original)
{
	var tags = metadata.Value<RavenJArray>("Tags");

	if (tags == null)
		return true;

	foreach (var banned in bannedTags)
	{
		tags.Remove(banned);
	}

	metadata["Tags"] = tags; 

	return true;
}

public void AfterChange(FileHeader instance, RavenJObject metadata)
{
	
}
}