VVMStrongCellRegistry.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#if WITH_VERSE_VM || defined(__INTELLISENSE__)
 
#include "Async/TransactionallySafeMutex.h"
#include "Containers/Map.h"
#include "CoreTypes.h"
#include "VVMCell.h"
#include "VVMGlobalHeapRoot.h"
 
namespace Verse
{
 
// A global registry that prevents cells from getting GC'ed
struct FStrongCellRegistry : FGlobalHeapRoot
{
    COREUOBJECT_API ~FStrongCellRegistry();
 
    // Adds a VCell reference to the set and returns reference count for the VCell after adding
    uint32 Add(FAccessContext Context, VCell* Cell)
    {
        V_DIE_UNLESS(Cell);
 
        // We are the mutator thread so we can read the map without locking
        uint32* ExistingRefCounter = Registry.Find(Cell);
        if (ExistingRefCounter)
        {
            // We can also mutate the counter without lock because GC does not look at that
            return ++*ExistingRefCounter;
        }
 
        AddInternal(Context, Cell);
        return 1;
    }
 
    // Removes a VCell reference from the set and returns reference count for the VCell after removing
    uint32 Remove(VCell* Cell)
    {
        // We are the mutator thread so we can read the map without locking
        FSetElementId ExistingId = Registry.FindId(Cell);
        V_DIE_UNLESS(ExistingId.IsValidId());
        TPair<VCell*, uint32>& Entry = Registry.Get(ExistingId);
        // We can also mutate the counter without lock because GC does not look at that
        uint32 NewRefCount = --Entry.Value;
        if (NewRefCount == 0)
        {
            RemoveInternal(ExistingId);
        }
        return NewRefCount;
    }
 
private:
    void Visit(FMarkStackVisitor& Visitor) override;
    void Visit(FAbstractVisitor& Visitor) override;
 
    template <class VisitorType>
    void VisitImpl(VisitorType& Visitor);
 
    COREUOBJECT_API void AddInternal(FAccessContext Context, VCell* Cell);
    COREUOBJECT_API void RemoveInternal(FSetElementId Id);
 
    UE::FTransactionallySafeMutex Mutex;
    TMap<VCell*, uint32> Registry;
};
 
} // namespace Verse
#endif // WITH_VERSE_VM