vault backup: 2024-10-12 17:19:45

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

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+# SaveGame
+
+- **功能描述：** 在SaveGame存档的时候，只序列化有SaveGame标记的属性，而不序列化别的属性。
+- **元数据类型：** bool
+- **引擎模块：** Serialization
+- **常用程度：** ★★★★★
+
+在SaveGame存档的时候，只序列化有SaveGame标记的属性，而不序列化别的属性。
+
+特别的标识哪些属性是用于存档保存的。
+
+对于子结构或子对象属性，也必须要加上SaveGame标记。
+
+NoExportTypes.h里的很多基础结构内部属性都被标上了SaveGame。
+
+## 测试代码：
+
+```cpp
+struct FMySaveGameArchive : public FObjectAndNameAsStringProxyArchive
+{
+    FMySaveGameArchive (FArchive& InInnerArchive)
+        :   FObjectAndNameAsStringProxyArchive(InInnerArchive)
+    { 
+        ArIsSaveGame = true;
+    }
+};
+
+USTRUCT(BlueprintType)
+struct FMySaveGameStruct
+{
+	GENERATED_BODY()
+public:
+	UPROPERTY(EditAnywhere, BlueprintReadWrite)
+		FString MyString_Default;
+	UPROPERTY(EditAnywhere, BlueprintReadWrite,SaveGame)
+		FString MyString_SaveGame;
+};
+
+UCLASS(Blueprintable, BlueprintType)
+class INSIDER_API UMyProperty_SaveGame :public USaveGame
+{
+public:
+	GENERATED_BODY()
+public:
+	UPROPERTY(EditAnywhere, BlueprintReadWrite)
+		int32 MyInt_Default = 123;
+	//PropertyFlags:	CPF_Edit | CPF_BlueprintVisible | CPF_ZeroConstructor | CPF_SaveGame | CPF_IsPlainOldData | CPF_NoDestructor | CPF_HasGetValueTypeHash | CPF_NativeAccessSpecifierPublic 
+	UPROPERTY(EditAnywhere, BlueprintReadWrite, SaveGame)
+		int32 MyInt_SaveGame = 123;
+	UPROPERTY(EditAnywhere, BlueprintReadWrite,SaveGame)
+		FMySaveGameStruct MyStruct;
+};
+
+UMyProperty_SaveGame* UMyProperty_SaveGame_Test::LoadGameFromMemory(const TArray<uint8>& InSaveData)
+{
+	FMemoryReader MemoryReader(InSaveData, true);
+
+	FObjectAndNameAsStringProxyArchive Ar(MemoryReader, true);
+		Ar.ArIsSaveGame = true;//必须手动加上这个标记
+
+	UMyProperty_SaveGame* OutSaveGameObject = NewObject<UMyProperty_SaveGame>(GetTransientPackage(), UMyProperty_SaveGame::StaticClass());
+	OutSaveGameObject->Serialize(Ar);
+
+	return OutSaveGameObject;
+}
+
+bool UMyProperty_SaveGame_Test::SaveGameToMemory(UMyProperty_SaveGame* SaveGameObject, TArray<uint8>& OutSaveData)
+{
+	FMemoryWriter MemoryWriter(OutSaveData, true);
+
+	// Then save the object state, replacing object refs and names with strings
+	FObjectAndNameAsStringProxyArchive Ar(MemoryWriter, false);
+	Ar.ArIsSaveGame = true;//必须手动加上这个标记
+	SaveGameObject->Serialize(Ar);
+
+	return true; // Not sure if there's a failure case here.
+}
+
+void UMyProperty_SaveGame_Test::RunTest()
+{
+	UMyProperty_SaveGame* saveGame = Cast<UMyProperty_SaveGame>(UGameplayStatics::CreateSaveGameObject(UMyProperty_SaveGame::StaticClass()));
+	saveGame->MyInt_Default = 456;
+	saveGame->MyInt_SaveGame = 456;
+	saveGame->MyStruct.MyString_Default = TEXT("Hello");
+	saveGame->MyStruct.MyString_SaveGame = TEXT("Hello");
+
+	TArray<uint8> outBytes;
+	UMyProperty_SaveGame_Test::SaveGameToMemory(saveGame, outBytes);
+
+	UMyProperty_SaveGame* saveGame2 = UMyProperty_SaveGame_Test::LoadGameFromMemory(outBytes);
+}
+```
+
+测试结果，只有SaveGame标记的属性这个值才序列化进去。
+
+![Untitled](Untitled.png)
+
+等价于在蓝图的细节面板里表示：
+
+![Untitled](Untitled%201.png)
+
+## 原理：
+
+只在ArIsSaveGame的时候检测这个标记，意味着这个标记只在检测USaveGame的对象的子对象结构属性的时候才用。但是ArIsSaveGame需要自己手动设置为true，否则默认这个机制是不工作的。实现的一种方式是自己手动加上一句Ar.ArIsSaveGame = true;，或者自己自定义一个FMySaveGameArchive来进行序列化。
+
+在源码里发现UEnhancedInputUserSettings也是继承于USaveGame，采用存档的方式保存的。
+
+```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;
+}
+```