identity.h

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

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

#include <memory>

PXR_NAMESPACE_OPEN_SCOPE

class Sdf_IdentityRegistry;
class Sdf_IdRegistryImpl;

SDF_DECLARE_HANDLES(SdfLayer);

/// \class Sdf_Identity
///
/// Identifies the logical object behind an SdfSpec.
///
/// This is simply the layer the spec belongs to and the path to the spec.
///
class Sdf_Identity {
    Sdf_Identity(Sdf_Identity const &) = delete;
    Sdf_Identity &operator=(Sdf_Identity const &) = delete;
public:
    /// Returns the layer that this identity refers to.
    SDF_API
    const SdfLayerHandle &GetLayer() const;

    /// Returns the path that this identity refers to.
    const SdfPath &GetPath() const {
        return _path;
    }
    
private:
    // Ref-counting ops manage _refCount.
    friend void TfDelegatedCountIncrement(Sdf_Identity*);
    friend void TfDelegatedCountDecrement(Sdf_Identity*) noexcept;

    friend class Sdf_IdentityRegistry;
    friend class Sdf_IdRegistryImpl;

    Sdf_Identity(Sdf_IdRegistryImpl *regImpl, const SdfPath &path)
        : _refCount(0), _path(path), _regImpl(regImpl) {}
    
    SDF_API
    static void _UnregisterOrDelete(Sdf_IdRegistryImpl *reg, Sdf_Identity *id)
    noexcept;
    void _Forget();

    mutable std::atomic_int _refCount;
    SdfPath _path;
    Sdf_IdRegistryImpl *_regImpl;
};

// Specialize TfDelegatedCountPtr operations.
inline void TfDelegatedCountIncrement(PXR_NS::Sdf_Identity* p) {
    ++p->_refCount;
}
inline void TfDelegatedCountDecrement(PXR_NS::Sdf_Identity* p) noexcept {
    // Once the count hits zero, p is liable to be destroyed at any point,
    // concurrently, by its owning registry if it happens to be doing a cleanup
    // pass.  Cache 'this' and the impl ptr in local variables so we have them
    // before decrementing the count.
    Sdf_Identity *self = p;
    Sdf_IdRegistryImpl *reg = p->_regImpl;
    if (--p->_refCount == 0) {
        // Cannot use 'p' anymore here.
        Sdf_Identity::_UnregisterOrDelete(reg, self);
    }
}

class Sdf_IdentityRegistry {
    Sdf_IdentityRegistry(const Sdf_IdentityRegistry&) = delete;
    Sdf_IdentityRegistry& operator=(const Sdf_IdentityRegistry&) = delete;
public:
    Sdf_IdentityRegistry(const SdfLayerHandle &layer);
    ~Sdf_IdentityRegistry();

    /// Returns the layer that owns this registry.
    const SdfLayerHandle &GetLayer() const {
        return _layer;
    }

    /// Return the identity associated with \a path, issuing a new
    /// one if necessary. The registry will track the identity
    /// and update it if the logical object it represents moves
    /// in namespace.
    Sdf_IdentityRefPtr Identify(const SdfPath &path);

    /// Update identity in response to a namespace edit.
    void MoveIdentity(const SdfPath &oldPath, const SdfPath &newPath);
    
private:
    friend class Sdf_Identity;

    friend class Sdf_IdRegistryImpl;

    // Remove the identity mapping for \a path to \a id from the registry.  This
    // is invoked when an identity's refcount hits zero.
    SDF_API
    void _UnregisterOrDelete();

    /// The layer that owns this registry, and on behalf of which
    /// this registry tracks identities.
    const SdfLayerHandle _layer;

    // Private implementation.
    const std::unique_ptr<Sdf_IdRegistryImpl> _impl;
};

PXR_NAMESPACE_CLOSE_SCOPE

#endif // PXR_USD_SDF_IDENTITY_H