// Copyright Epic Games, Inc. All Rights Reserved.

/*=============================================================================
        GCObject.h: Abstract base class to allow non-UObject objects reference
                                UObject instances with proper handling of them by the
                                Garbage Collector.
=============================================================================*/

#pragma once

#include "CoreMinimal.h"
#include "UObject/ObjectMacros.h"
#include "UObject/Object.h"

class FGCObject;

class COREUOBJECT_API FGCObject;

/**
 * This nested class is used to provide a UObject interface between non
 * UObject classes and the UObject system. It handles forwarding all
 * calls of AddReferencedObjects() to objects/ classes that register with it.
 */
class COREUOBJECT_API UGCObjectReferencer: public UObject
{
    /**
     * This is the list of objects that are referenced
     */
    TArray<FGCObject*> ReferencedObjects;
    /** Critical section used when adding and removing objects */
    FCriticalSection ReferencedObjectsCritical;
    /** True if we are currently inside AddReferencedObjects */
    bool bIsAddingReferencedObjects = false;
    /** Currently serializing FGCObject*, only valid if bIsAddingReferencedObjects */
    FGCObject* CurrentlySerializingObject = nullptr;

public:
    DECLARE_CASTED_CLASS_INTRINSIC_WITH_API(UGCObjectReferencer, UObject, CLASS_Transient, TEXT("/Script/CoreUObject"), CASTCLASS_None, NO_API);

    /**
     * Adds an object to the referencer list
     *
     * @param Object The object to add to the list
     */
    void AddObject(FGCObject* Object);

    /**
     * Removes an object from the referencer list
     *
     * @param Object The object to remove from the list
     */
    void RemoveObject(FGCObject* Object);

    /**
     * Get the name of the first FGCObject that owns this object.
     *
     * @param Object The object that we're looking for.
     * @param OutName the name of the FGCObject that reports this object.
     * @param bOnlyIfAddingReferenced Only try to find the name if we are currently inside AddReferencedObjects
     * @return true if the object was found.
     */
    bool GetReferencerName(UObject* Object, FString& OutName, bool bOnlyIfAddingReferenced = false) const;

    /**
     * Forwards this call to all registered objects so they can reference
     * any UObjects they depend upon
     *
     * @param InThis This UGCObjectReferencer object.
     * @param Collector The collector of referenced objects.
     */
    static void AddReferencedObjects(UObject* InThis, FReferenceCollector& Collector);

    /**
     * Destroy function that gets called before the object is freed. This might
     * be as late as from the destructor.
     */
    virtual void FinishDestroy() override;
};

/**
 * This class provides common registration for garbage collection for
 * non-UObject classes. It is an abstract base class requiring you to implement
 * the AddReferencedObjects() method.
 */
class COREUOBJECT_API FGCObject
{
    bool bReferenceAdded = false;

    void Init()
    {
        // Some objects can get created after the engine started shutting down (lazy init of singletons etc).
        if (!IsEngineExitRequested())
        {
            StaticInit();
            check(GGCObjectReferencer);
            // Add this instance to the referencer's list
            GGCObjectReferencer->AddObject(this);
            bReferenceAdded = true;
        }
    }

public:
    /**
     * The static object referencer object that is shared across all
     * garbage collectible non-UObject objects.
     */
    static UGCObjectReferencer* GGCObjectReferencer;

    /**
     * Initializes the global object referencer and adds it to the root set.
     */
    static void StaticInit(void)
    {
        if (GGCObjectReferencer == NULL)
        {
            GGCObjectReferencer = NewObject<UGCObjectReferencer>();
            GGCObjectReferencer->AddToRoot();
        }
    }

    /**
     * Tells the global object that forwards AddReferencedObjects calls on to objects
     * that a new object is requiring AddReferencedObjects call.
     */
    FGCObject(void)
    {
        Init();
    }

    /** Copy constructor */
    FGCObject(FGCObject const&)
    {
        Init();
    }

    /** Move constructor */
    FGCObject(FGCObject&&)
    {
        Init();
    }

    /**
     * Removes this instance from the global referencer's list
     */
    virtual ~FGCObject(void)
    {
        // GObjectSerializer will be NULL if this object gets destroyed after the exit purge.
        // We want to make sure we remove any objects that were added to the GGCObjectReferencer during Init when exiting
        if (GGCObjectReferencer && bReferenceAdded)
        {
            // Remove this instance from the referencer's list
            GGCObjectReferencer->RemoveObject(this);
        }
    }

    /**
     * Pure virtual that must be overloaded by the inheriting class. Use this
     * method to serialize any UObjects contained that you wish to keep around.
     *
     * @param Collector The collector of referenced objects.
     */
    virtual void AddReferencedObjects(FReferenceCollector& Collector) = 0;

    /**
     * Use this method to report a name for your referencer.
     */
    virtual FString GetReferencerName() const
    {
        return "Unknown FGCObject";
    }

    /**
     * Use this method to report how the specified object is referenced, if necessary
     */
    virtual bool GetReferencerPropertyName(UObject* Object, FString& OutPropertyName) const
    {
        return false;
    }
};