1、實施GC的關鍵因素是由Object間的引用關係決定的UObject狀態。我們先從上層分析UObject的引用關係和類型之間相互引用的注意事項：
A、UObject間的引用關係需要用指針強引用加UPROPERTY標籤完成例如：
UCLASS(abstract, notplaceable, NotBlueprintable, HideCategories=(Collision,Rendering,"Utilities|Transformation"))
class ENGINE_API AController : public AActor, public INavAgentInterface
{
GENERATED_UCLASS_BODY()
private:
/** Pawn currently being controlled by this controller. Use Pawn.Possess() to take control of a pawn */
UPROPERTY(replicatedUsing=OnRep_Pawn)
APawn* Pawn;
...
}；
UPROPERTY標籤生成UProperty對象，UProperty對象可以控制對屬性的訪問等。也通過UProperty對象保存引用關係
不加UPROPERTY的如果想持久的保存指針的話，需要重寫UObject的AddReferencedObjects接口爲UObject對象添加引用。例如AActor類中爲非UPROPERTY的成員變量OwnedComponents添加reference：
void AActor::AddReferencedObjects(UObject* InThis, FReferenceCollector& Collector)
{
AActor* This = CastChecked<AActor>(InThis);
Collector.AddReferencedObjects(This->OwnedComponents);
#if WITH_EDITOR
if (This->CurrentTransactionAnnotation.IsValid())
{
This->CurrentTransactionAnnotation->AddReferencedObjects(Collector);
}
#endif
Super::AddReferencedObjects(InThis, Collector);
}
B、引擎中的代碼UCLass類引用的非Object對象的變量比如UStruct UEnum 或者FVector等可是數據類型而定，如果是BlurprintType的類型；則需要寫成UPROPERTY才能使用UObject的GC機制進行管理；目前BlurprintType的結構體，枚舉都會被引擎編譯成UStruct UEnum的對象。
C、F Class引用UObject的時候注意；爲避免內存泄漏FClass應繼承與FGCObject，並實現AddReferencedObjects接口，在接口中給UObject的指針添加reference例如FWidget中：
class FWidget
: public FGCObject
{
public:
...
private:
UMaterialInterface* TransparentPlaneMaterialXY;
UMaterialInterface* GridMaterial;
UMaterialInstanceDynamic* AxisMaterialX;
UMaterialInstanceDynamic* AxisMaterialY;
UMaterialInstanceDynamic* AxisMaterialZ;
UMaterialInstanceDynamic* CurrentAxisMaterial;
}；
void FWidget::AddReferencedObjects( FReferenceCollector& Collector )
{
Collector.AddReferencedObject( AxisMaterialX );
Collector.AddReferencedObject( AxisMaterialY );
Collector.AddReferencedObject( AxisMaterialZ );
Collector.AddReferencedObject( OpaquePlaneMaterialXY );
Collector.AddReferencedObject( TransparentPlaneMaterialXY );
Collector.AddReferencedObject( GridMaterial );
Collector.AddReferencedObject( CurrentAxisMaterial );
}
D、智能指針一般不持久保存UObject的指針對象如果有需求我的習慣是使用TWeakObjectPtr的形式保存；可以用Isvalid先判斷指針是否有效，但是因爲是weakreference不對GC產生阻止的作用。
2、下面說下我對於UObject的GarbageCollection的理解，代碼太多我不一定理解的全面。
A、先說最基本的UObject對象的內存空間的分配和釋放；目前FUObjectAllocator處理UObject對象的內存分配和回收 不論上層多複雜但歸根結底都要到關於內存的問題都會歸到這裏。比如我們runtime的時候執行destroy的操作，這時其實對象所佔的空間依然存在，如果有指針引用到該對象的話我們可以看到內容爲none；指針並不是nullptr；只有等執行了GC之後纔會真正執行釋放。同樣UObject實例的時候會分配內存
GUObjectAllocator.FreeUObject(Object);
Obj = (UObject *)GUObjectAllocator.AllocateUObject(TotalSize,Alignment,GIsInitialLoad);
B、綜上GC的最終目標其實就是對那些狀態爲Unreachable或者NoStrongReference等的Object對象執行銷燬ConditionalFinishDestroy；另一個過程對已經執行銷燬過程的Object釋放內存FreeUObject。詳見IncrementalPurgeGarbage在../Private\UObject\GarbageCollection.cpp
C、關於UObject的狀態標記Flag
/** Objects flags for internal use (GC, low level UObject code) */
enum class EInternalObjectFlags : int32
{
None = 0,
// All the other bits are reserved, DO NOT ADD NEW FLAGS HERE!
ReachableInCluster = 1 << 23, /// External reference to object in cluster exists
ClusterRoot = 1 << 24, ///< Root of a cluster
Native = 1 << 25, ///< Native (UClass only).
Async = 1 << 26, ///< Object exists only on a different thread than the game thread.
AsyncLoading = 1 << 27, ///< Object is being asynchronously loaded.
Unreachable = 1 << 28, ///< Object is not reachable on the object graph.
PendingKill = 1 << 29, ///< Objects that are pending destruction (invalid for gameplay but valid objects)
RootSet = 1 << 30, ///< Object will not be garbage collected, even if unreferenced.
NoStrongReference = 1 << 31, ///< The object is not referenced by any strong reference. The flag is used by GC.
GarbageCollectionKeepFlags = Native | Async | AsyncLoading,
// Make sure this is up to date!
AllFlags = ReachableInCluster | ClusterRoot | Native | Async | AsyncLoading | Unreachable | PendingKill | RootSet | NoStrongReference
};
flag作爲GC的狀態保存在FUObjectItem中
D、FUObjectItem和FUObjectArray以及UGCObjectReferencer
FUObjectItem封裝了單個的UObject指針在FUObjectArray中；FUObjectArray包含所有的live的Object對象
FUObjectItem:
/**
* Single item in the UObject array.
*/
struct FUObjectItem
{
// Pointer to the allocated object
class UObjectBase* Object;
// Internal flags
int32 Flags;
...
};
class FFixedUObjectArray
{
/** Static master table to chunks of pointers **/
FUObjectItem* Objects;
...
};
FUObjectArray:
class COREUOBJECT_API FUObjectArray
{
public:
...
//typedef TStaticIndirectArrayThreadSafeRead<UObjectBase, 8 * 1024 * 1024 /* Max 8M UObjects */, 16384 /* allocated in 64K/128K chunks */ > TUObjectArray;
typedef FFixedUObjectArray TUObjectArray;
...
TUObjectArray ObjObjects;
...
};
UGCObjectReferencer爲非UClass的對象引用UObject對象指針提供了宿主；從而構成引用關係；GCObjectReference不依賴與任何對象因爲設置了RootSet
void Init()
{
// Some objects can get created after the engine started shutting down (lazy init of singletons etc).
if (!GIsRequestingExit)
{
StaticInit();
check(GGCObjectReferencer);
// Add this instance to the referencer's list
GGCObjectReferencer->AddObject(this);
}
}
在執行GC CollectReference的過程中會把非UClass類的Uobject指針的引用關係指到GGCObjectReferencer上從而保證Uobject不會被清理。
E、分析GC的過程
入口CollectGarbageInternal GarbageCollection.cpp
/**
* Deletes all unreferenced objects, keeping objects that have any of the passed in KeepFlags set
*
* @param KeepFlags objects with those flags will be kept regardless of being referenced or not
* @param bPerformFullPurge if true, perform a full purge after the mark pass
*/
void CollectGarbageInternal(EObjectFlags KeepFlags, bool bPerformFullPurge)
{
...
};
關鍵的部分在ReachabilityAnalysis和執行IncrementalPurgeGarbage
ReachabilityAnalysis：
執行reachability分析的入口在CollectGarbageInternal 中
const double StartTime = FPlatformTime::Seconds();
FRealtimeGC TagUsedRealtimeGC;
TagUsedRealtimeGC.PerformReachabilityAnalysis(KeepFlags, bForceSingleThreadedGC);
UE_LOG(LogGarbage, Log, TEXT("%f ms for GC"), (FPlatformTime::Seconds() - StartTime) * 1000);
先執行MarkObjectsAsUnreachable把異常的ObjectItem狀態修改：
ObjectItem->SetFlags(EInternalObjectFlags::Unreachable | EInternalObjectFlags::NoStrongReference);
正常的放進ObjectsToSerialize
然後執行CollectReferences ，遍歷ObjectsToSerialize中的Object；從中取出ReferenceTokenStream
// Get pointer to token stream and jump to the start.
FGCReferenceTokenStream* RESTRICT TokenStream = &CurrentObject->GetClass()->ReferenceTokenStream;
然後遍歷referenceTokenStream中的ReferenceInfo
FGCReferenceInfo ReferenceInfo = TokenStream->AccessReferenceInfo(ReferenceTokenStreamIndex);
再根據不同的referencetype執行ReferenceProcessor的HandleTokenStreamObjectReference
或者執行AddreferenceObject 構成ClusterReference
這步處理的結果會把一些ReferencedClusterRootObjectItem的EInternalObjectFlags::NoStrongReference | EInternalObjectFlags::Unreachable flag清理掉
ReferencedClusterRootObjectItem->ClearFlags(EInternalObjectFlags::NoStrongReference | EInternalObjectFlags::Unreachable);
IncrementalPurgeGarbage：
入口在CollectGarbageInternal 中：
if (bPerformFullPurge || GIsEditor)
{
IncrementalPurgeGarbage(false);
}
分爲兩個過程對那些狀態爲Unreachable或者NoStrongReference等的Object對象執行銷燬ConditionalFinishDestroy；和另一個過程對已經執行銷燬過程的Object釋放內存FreeUObject。詳見IncrementalPurgeGarbage
if (ObjectItem->IsUnreachable())
{
UObject* Object = static_cast<UObject*>(ObjectItem->Object);
// Object should always have had BeginDestroy called on it and never already be destroyed
check( Object->HasAnyFlags( RF_BeginDestroyed ) && !Object->HasAnyFlags( RF_FinishDestroyed ) );
// Only proceed with destroying the object if the asynchronous cleanup started by BeginDestroy has finished.
if(Object->IsReadyForFinishDestroy())
{
#if PERF_DETAILED_PER_CLASS_GC_STATS
// Keep track of how many objects of a certain class we're purging.
const FName& ClassName = Object->GetClass()->GetFName();
int32 InstanceCount = GClassToPurgeCountMap.FindRef( ClassName );
GClassToPurgeCountMap.Add( ClassName, ++InstanceCount );
#endif
// Send FinishDestroy message.
Object->ConditionalFinishDestroy();
}
else
{
// The object isn't ready for FinishDestroy to be called yet. This is common in the
// case of a graphics resource that is waiting for the render thread "release fence"
// to complete. Just calling IsReadyForFinishDestroy may begin the process of releasing
// a resource, so we don't want to block iteration while waiting on the render thread.
// Add the object index to our list of objects to revisit after we process everything else
GGCObjectsPendingDestruction.Add(Object);
GGCObjectsPendingDestructionCount++;
}
}
free的過程
FUObjectItem* ObjectItem = *GObjCurrentPurgeObjectIndex;
checkSlow(ObjectItem);
if (ObjectItem->IsUnreachable())
{
UObject* Object = (UObject*)ObjectItem->Object;
check(Object->HasAllFlags(RF_FinishDestroyed|RF_BeginDestroyed));
GIsPurgingObject = true;
Object->~UObject();
GUObjectAllocator.FreeUObject(Object);
GIsPurgingObject = false;
// Keep track of purged stats.
GPurgedObjectCountSinceLastMarkPhase++;
}