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03-UnrealEngine/Gameplay/UObject/UnrealSpecifiers/Specifier/UCLASS/Serialization/Transient/Transient.md

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+# Transient
+
+- **功能描述：** 指定该类的所有对象都略过序列化。
+
+- **引擎模块：** Serialization
+- **元数据类型：** bool
+- **作用机制：** 在ClassFlags中添加[CLASS_Transient](../../../../Flags/EClassFlags/CLASS_Transient.md)
+- **关联项：** [NonTransient](../NonTransient.md)
+- **常用程度：** ★★★
+
+指定该类的所有对象都略过序列化。
+
+- 从不将属于此类的对象保存到磁盘。此说明符会传播到子类，但是可由NonTransient说明符覆盖。可以在子类被改写.
+- 会造成相应Object的RF_Transient标记。
+- 注意：UPROPERTY(Transient)只是指定这一个特定的属性不序列化。而UCLASS(Transient)是作用于该类的所有对象。
+
+## 示例代码：
+
+```cpp
+UCLASS(Blueprintable, Transient)
+class INSIDER_API UMyClass_Transient :public UObject
+{
+	GENERATED_BODY()
+public:
+	UPROPERTY(EditAnywhere, BlueprintReadWrite)
+		int32 MyProperty = 123;
+};
+
+UCLASS(Blueprintable, NonTransient)
+class INSIDER_API UMyClass_NonTransient :public UObject
+{
+	GENERATED_BODY()
+public:
+	UPROPERTY(EditAnywhere, BlueprintReadWrite)
+		int32 MyProperty = 123;
+};
+
+UCLASS(Blueprintable, BlueprintType)
+class INSIDER_API UMyClass_Transient_Test :public UObject
+{
+	GENERATED_BODY()
+public:
+	UPROPERTY(EditAnywhere, BlueprintReadWrite)
+		UMyClass_Transient* MyTransientObject;
+	UPROPERTY(EditAnywhere, BlueprintReadWrite)
+		UMyClass_NonTransient* MyNonTransientObject;
+
+	UPROPERTY(EditAnywhere, BlueprintReadWrite)
+		int32 MyInt_Normal=123;
+		UPROPERTY(EditAnywhere, BlueprintReadWrite,Transient)
+		int32 MyInt_Transient =123;
+};
+```
+
+序列化对象指针关键是采用FObjectProperty里的SerializeItem方法Slot << ObjectValue;
+
+不能采用Actor的Class Default来测试，因为：
+
+Transient properties are serialized for (Blueprint) Class Default Objects but should not be serialized for any 'normal' instances of classes.
+
+```cpp
+UCLASS(Blueprintable, BlueprintType)
+class INSIDER_API AMyActor_Transient_Test :public AActor
+{
+	GENERATED_BODY()
+public:
+	UPROPERTY(EditAnywhere, BlueprintReadWrite)
+		UMyClass_Transient* MyTransientObject;
+	UPROPERTY(EditAnywhere, BlueprintReadWrite)
+		UMyClass_NonTransient* MyNonTransientObject;
+
+	UPROPERTY(EditAnywhere, BlueprintReadWrite)
+		int32 MyInt_Normal=123;
+		UPROPERTY(EditAnywhere, BlueprintReadWrite,Transient)
+		int32 MyInt_Transient =123;
+};
+```
+
+也不能用FObjectAndNameAsStringProxyArchive测试，因为FObjectAndNameAsStringProxyArchive内部在序列化对象的时候，会先查找名字。
+
+```cpp
+FArchive& FObjectAndNameAsStringProxyArchive::operator<<(UObject*& Obj)
+{
+	if (IsLoading())
+	{
+		// load the path name to the object
+		FString LoadedString;
+		InnerArchive << LoadedString;
+		// look up the object by fully qualified pathname
+		Obj = FindObject<UObject>(nullptr, *LoadedString, false);
+		// If we couldn't find it, and we want to load it, do that
+		if(!Obj && bLoadIfFindFails)
+		{
+			Obj = LoadObject<UObject>(nullptr, *LoadedString);
+		}
+	}
+	else
+	{
+		// save out the fully qualified object name
+		FString SavedString(Obj->GetPathName());
+		InnerArchive << SavedString;
+	}
+	return *this;
+}
+```
+
+因此采用Package的测试：
+
+```cpp
+
+FString packageName = TEXT("/Game/MyTestPackage");
+FString assetPath = FPackageName::LongPackageNameToFilename(packageName, FPackageName::GetAssetPackageExtension());
+UPackage* package = CreatePackage(*packageName);
+FSavePackageArgs saveArgs{};
+saveArgs.Error = GError;
+
+//ObjectFlags:	RF_NoFlags
+UMyClass_Transient_Test* testObject = NewObject<UMyClass_Transient_Test>(package, TEXT("testObject"));	
+//ObjectFlags:	RF_Transient 
+testObject->MyTransientObject = NewObject<UMyClass_Transient>(testObject, TEXT("MyTransientObject"));	
+//ObjectFlags:	RF_NoFlags
+testObject->MyNonTransientObject = NewObject<UMyClass_NonTransient>(testObject, TEXT("MyNonTransientObject"));	
+
+testObject->MyTransientObject->MyProperty = 456;
+testObject->MyNonTransientObject->MyProperty = 456;
+testObject->MyInt_Normal = 456;
+testObject->MyInt_Transient = 456;
+
+bool result = UPackage::SavePackage(package, testObject, *assetPath, saveArgs);
+```
+
+在保存完成之后，重新加载Package:
+
+```cpp
+FString packageName = TEXT("/Game/MyTestPackage");
+FString assetPath = FPackageName::LongPackageNameToFilename(packageName, FPackageName::GetAssetPackageExtension());
+
+UPackage* package = LoadPackage(nullptr, *assetPath, LOAD_None);
+package->FullyLoad();
+
+UMyClass_Transient_Test* newTestObject=LoadObject<UMyClass_Transient_Test>(package, TEXT("testObject"),*assetPath);
+```
+
+## 示例效果：
+
+可以看到MyTransientObject 并没有被序列化到磁盘上，因此不会加载出来。
+
+![Untitled](Untitled.png)
+
+## 原理：
+
+在SavePackage的时候：RF_Transient会导致这个对象不会被HarvestExport，不会被放进SaveContext里
+
+```cpp
+void FPackageHarvester::TryHarvestExport(UObject* InObject)
+{
+	// Those should have been already validated
+	check(InObject && InObject->IsInPackage(SaveContext.GetPackage()));
+
+	// Get the realm in which we should harvest this export
+	EIllegalRefReason Reason = EIllegalRefReason::None;
+	ESaveRealm HarvestContext = GetObjectHarvestingRealm(InObject, Reason);
+	if (!SaveContext.GetHarvestedRealm(HarvestContext).IsExport(InObject))
+	{
+		SaveContext.MarkUnsaveable(InObject);
+		bool bExcluded = false;
+		if (!InObject->HasAnyFlags(RF_Transient))
+		{
+			bExcluded = ConditionallyExcludeObjectForTarget(SaveContext, InObject, HarvestContext);
+		}
+		if (!InObject->HasAnyFlags(RF_Transient) && !bExcluded)
+		{
+			// It passed filtering so mark as export
+			HarvestExport(InObject, HarvestContext);
+		}
+
+		// If we have a illegal ref reason, record it
+		if (Reason != EIllegalRefReason::None)
+		{
+			SaveContext.RecordIllegalReference(CurrentExportDependencies.CurrentExport, InObject, Reason);
+		}
+	}
+}
+```