changeBlock.h

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//
// Copyright 2016 Pixar
//
// Licensed under the terms set forth in the LICENSE.txt file available at
// https://openusd.org/license.
//
#ifndef PXR_USD_SDF_CHANGE_BLOCK_H
#define PXR_USD_SDF_CHANGE_BLOCK_H

#include "pxr/pxr.h"
#include "pxr/usd/sdf/api.h"

PXR_NAMESPACE_OPEN_SCOPE

/// \class SdfChangeBlock
///
/// <b>DANGER DANGER DANGER</b>
///
/// Please make sure you have read and fully understand the
/// issues below before using a changeblock!  They are very
/// easy to use in an unsafe way that could make the system
/// crash or corrupt data.  If you have any questions, please
/// contact the USD team, who would be happy to help!
///
/// SdfChangeBlock provides a way to group a round of related changes to
/// scene description in order to process them more efficiently.
///
/// Normally, Sdf sends notification immediately as changes are made so
/// that downstream representations like UsdStage can update accordingly.
///
/// However, sometimes it can be advantageous to group a series of Sdf
/// changes into a batch so that they can be processed more efficiently,
/// with a single round of change processing.  An example might be when
/// setting many avar values on a model at the same time.
///
/// Opening a changeblock tells Sdf to delay sending notification about
/// changes until the outermost changeblock is exited.  Until then,
/// Sdf internally queues up the notification it needs to send.
///
/// \note  It is *not* safe to use Usd or other downstream API
/// while a changeblock is open!!  This is because those derived
/// representations will not have had a chance to update while the
/// changeblock is open.  Not only will their view of the world be
/// stale, it could be unsafe to even make queries from, since they
/// may be holding onto expired handles to Sdf objects that no longer
/// exist.
///
/// If you need to make a bunch of changes to scene description,
/// the best approach is to build a list of necessary changes that
/// can be performed directly via the Sdf API, then submit those all
/// inside a changeblock without talking to any downstream libraries.
/// For example, this is how many mutators in Usd that operate on more 
/// than one field or Spec work.
///

class SdfChangeBlock {
public:
    SDF_API
    SdfChangeBlock();
    ~SdfChangeBlock() {
        if (_key) {
            _CloseChangeBlock(_key);
        }
    }
private:
    SDF_API
    void _CloseChangeBlock(void const *key) const;
    
    void const *_key;
};

PXR_NAMESPACE_CLOSE_SCOPE

#endif // PXR_USD_SDF_CHANGE_BLOCK_H