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

@@ -0,0 +1,212 @@
+# Transient
+
+- **功能描述：** 不序列化该属性，该属性初始化时候会被0填充。
+- **元数据类型：** bool
+- **引擎模块：** Serialization
+- **作用机制：** 在PropertyFlags中加入[CPF_Transient](../../../../Flags/EPropertyFlags/CPF_Transient.md)
+- **常用程度：★★★★★**
+
+序列化的时候略过该属性，用0来填充默认值。
+
+二进制和文本都不序列化该属性。
+
+一般用于一些临时中间变量或计算后的结果变量。
+
+## 示例代码：
+
+```cpp
+UCLASS(Blueprintable, BlueprintType)
+class INSIDER_API UMyProperty_Serialization :public UDataAsset
+{
+public:
+	GENERATED_BODY()
+public:
+	UPROPERTY(EditAnywhere, BlueprintReadWrite)
+		int32 MyInt_Default = 123;
+		//PropertyFlags:	CPF_Edit | CPF_BlueprintVisible | CPF_ZeroConstructor | CPF_Transient | CPF_IsPlainOldData | CPF_NoDestructor | CPF_HasGetValueTypeHash | CPF_NativeAccessSpecifierPublic 
+	UPROPERTY(EditAnywhere, BlueprintReadWrite, Transient)
+		int32 MyInt_Transient = 123;
+		//PropertyFlags:	CPF_Edit | CPF_BlueprintVisible | CPF_ZeroConstructor | CPF_DuplicateTransient | CPF_IsPlainOldData | CPF_NoDestructor | CPF_HasGetValueTypeHash | CPF_NativeAccessSpecifierPublic 
+	UPROPERTY(EditAnywhere, BlueprintReadWrite, DuplicateTransient)
+		int32 MyInt_DuplicateTransient = 123;
+		//PropertyFlags:	CPF_Edit | CPF_BlueprintVisible | CPF_ZeroConstructor | CPF_IsPlainOldData | CPF_NoDestructor | CPF_NonPIEDuplicateTransient | CPF_HasGetValueTypeHash | CPF_NativeAccessSpecifierPublic 
+	UPROPERTY(EditAnywhere, BlueprintReadWrite, NonPIEDuplicateTransient)
+		int32 MyInt_NonPIEDuplicateTransient = 123;
+};
+void UMyProperty_Serialization_Test::RunTest()
+{
+	UMyProperty_Serialization* obj = NewObject<UMyProperty_Serialization>(GetTransientPackage());
+
+	obj->MyInt_Default = 456;
+	obj->MyInt_Transient = 456;
+	obj->MyInt_DuplicateTransient = 456;
+	obj->MyInt_NonPIEDuplicateTransient = 456;
+
+	//save obj
+	TArray<uint8> outBytes;
+	FMemoryWriter MemoryWriter(outBytes, true);
+	FObjectAndNameAsStringProxyArchive Ar(MemoryWriter, false);
+	obj->Serialize(Ar);
+
+	//load
+	FMemoryReader MemoryReader(outBytes, true);
+
+	FObjectAndNameAsStringProxyArchive Ar2(MemoryReader, true);
+
+	UMyProperty_Serialization* obj2 = NewObject<UMyProperty_Serialization>(GetTransientPackage());
+	obj2->Serialize(Ar2);
+}
+```
+
+对这么一个BP DataAsset进行AssetActions→Export，
+
+T3D格式：
+
+```cpp
+Begin Object Class=/Script/Insider.MyProperty_Serialization Name="BP_Serialization" ExportPath="/Script/Insider.MyProperty_Serialization'/Game/Property/BP_Serialization.BP_Serialization'"
+   MyInt_Default=456
+   MyInt_DuplicateTransient=456
+End Object
+```
+
+COPY格式：
+
+```cpp
+Begin Object Class=/Script/Insider.MyProperty_Serialization Name="BP_Serialization" ExportPath="/Script/Insider.MyProperty_Serialization'/Game/Property/BP_Serialization.BP_Serialization'"
+   MyInt_Default=456
+End Object
+```
+
+如果是普通的序列化：
+
+可见obj2的MyInt_Transient 属性并没有从序列化中获得新值456.
+
+![Untitled](Untitled.png)
+
+## 原理代码：
+
+判断CPF_Transient的生效，只有在IsPersistent()的时候，并且不是在保存CDO。SetIsPersistent()的调用在很多地方都出现，比如在MemoryReader/MemoryWriter都是IsPersistent。
+
+因此Transient是在序列化的时候会被忽略。
+
+在ExportText的时候发现会进行CPF_Transient的判断，除非强制进行包括PPF_IncludeTransient
+
+```cpp
+bool FProperty::ShouldSerializeValue(FArchive& Ar) const
+{
+	// Skip the property if the archive says we should
+	if (Ar.ShouldSkipProperty(this))
+	{
+		return false;
+	}
+
+	// Skip non-SaveGame properties if we're saving game state
+	if (!(PropertyFlags & CPF_SaveGame) && Ar.IsSaveGame())
+	{
+		return false;
+	}
+
+	const uint64 SkipFlags = CPF_Transient | CPF_DuplicateTransient | CPF_NonPIEDuplicateTransient | CPF_NonTransactional | CPF_Deprecated | CPF_DevelopmentAssets | CPF_SkipSerialization;
+	if (!(PropertyFlags & SkipFlags))
+	{
+		return true;
+	}
+
+	// Skip properties marked Transient when persisting an object, unless we're saving an archetype
+	if ((PropertyFlags & CPF_Transient) && Ar.IsPersistent() && !Ar.IsSerializingDefaults())
+	{
+
+		return false;
+	}
+
+	// Skip properties marked DuplicateTransient when duplicating
+	if ((PropertyFlags & CPF_DuplicateTransient) && (Ar.GetPortFlags() & PPF_Duplicate))
+	{
+		return false;
+	}
+
+	// Skip properties marked NonPIEDuplicateTransient when duplicating, but not when we're duplicating for PIE
+	if ((PropertyFlags & CPF_NonPIEDuplicateTransient) && !(Ar.GetPortFlags() & PPF_DuplicateForPIE) && (Ar.GetPortFlags() & PPF_Duplicate))
+	{
+		return false;
+	}
+
+	// Skip properties marked NonTransactional when transacting
+	if ((PropertyFlags & CPF_NonTransactional) && Ar.IsTransacting())
+	{
+		return false;
+	}
+
+	// Skip deprecated properties when saving or transacting, unless the archive has explicitly requested them
+	if ((PropertyFlags & CPF_Deprecated) && !Ar.HasAllPortFlags(PPF_UseDeprecatedProperties) && (Ar.IsSaving() || Ar.IsTransacting() || Ar.WantBinaryPropertySerialization()))
+	{
+		return false;
+	}
+
+	// Skip properties marked SkipSerialization, unless the archive is forcing them
+	if ((PropertyFlags & CPF_SkipSerialization) && (Ar.WantBinaryPropertySerialization() || !Ar.HasAllPortFlags(PPF_ForceTaggedSerialization)))
+	{
+		return false;
+	}
+
+	// Skip editor-only properties when the archive is rejecting them
+	if (IsEditorOnlyProperty() && Ar.IsFilterEditorOnly())
+	{
+		return false;
+	}
+
+	// Otherwise serialize!
+	return true;
+}
+
+///////////////////////////////////////////
+bool FProperty::ShouldPort( uint32 PortFlags/*=0*/ ) const
+{
+	// if no size, don't export
+	if (GetSize() <= 0)
+	{
+		return false;
+	}
+
+	if (HasAnyPropertyFlags(CPF_Deprecated) && !(PortFlags & (PPF_ParsingDefaultProperties | PPF_UseDeprecatedProperties)))
+	{
+		return false;
+	}
+
+	// if we're parsing default properties or the user indicated that transient properties should be included
+	if (HasAnyPropertyFlags(CPF_Transient) && !(PortFlags & (PPF_ParsingDefaultProperties | PPF_IncludeTransient)))
+	{
+		return false;
+	}
+
+	// if we're copying, treat DuplicateTransient as transient
+	if ((PortFlags & PPF_Copy) && HasAnyPropertyFlags(CPF_DuplicateTransient | CPF_TextExportTransient) && !(PortFlags & (PPF_ParsingDefaultProperties | PPF_IncludeTransient)))
+	{
+		return false;
+	}
+
+	// if we're not copying for PIE and NonPIETransient is set, don't export
+	if (!(PortFlags & PPF_DuplicateForPIE) && HasAnyPropertyFlags(CPF_NonPIEDuplicateTransient))
+	{
+		return false;
+	}
+
+	// if we're only supposed to export components and this isn't a component property, don't export
+	if ((PortFlags & PPF_SubobjectsOnly) && !ContainsInstancedObjectProperty())
+	{
+		return false;
+	}
+
+	// hide non-Edit properties when we're exporting for the property window
+	if ((PortFlags & PPF_PropertyWindow) && !(PropertyFlags & CPF_Edit))
+	{
+		return false;
+	}
+
+	return true;
+}
+```
+
+因为不序列Transient属性，因此该属性修改值也并不会被保存起来。打开Asset的时候依然会是默认值，也并不会被复制。
+
+![Untitled](Untitled%201.png)