UE常见数据（UStruct、UObject、AActor）同步应用

引子

这个问题的探究来自于日常开发过程中遇到的一次崩溃

示例代码

//1. 结构体1
USTRUCT(BlueprintType)
struct FSubStructParamParent
{
	GENERATED_BODY()
        
	UPROPERTY(BlueprintReadWrite, EditAnywhere)
	int64 SubParentIntParam = 0;
    
	virtual bool NetSerialize(FArchive& Ar, UPackageMap* Map, bool& bOutSuccess)
	{
		Ar<<SubParentIntParam;
		return true;
	}
};

template <>
struct TStructOpsTypeTraits<FSubStructParamParent> : public TStructOpsTypeTraitsBase2<FSubStructParamParent>
{
	enum
	{
		WithNetSerializer = true,
	};
};

//2. 结构体2
USTRUCT(BlueprintType)
struct FSubStructParam : public FSubStructParamParent
{
	GENERATED_BODY()

	UPROPERTY(BlueprintReadWrite, EditAnywhere)
	int32 SubIntParam = 0;

	virtual bool NetSerialize(FArchive& Ar, UPackageMap* Map, bool& bOutSuccess) override
	{
		Super::NetSerialize(Ar, Map, bOutSuccess);
		Ar<<SubIntParam;
		return true;
	}
};

template <>
struct TStructOpsTypeTraits<FSubStructParam> : public TStructOpsTypeTraitsBase2<FSubStructParam>
{
	enum
	{
		WithNetSerializer = true,
	};
};

代码中尝试同步FSubStructParam结构体，但是在调用NetSerialize时产生了崩溃。

崩溃堆栈

Fatal error!

Unhandled Exception: EXCEPTION_ACCESS_VIOLATION reading address 0x0000000000000000

0x00007ff912356fc8 UnrealEditor-PackProjectTest.dll!UScriptStruct::TCppStructOps<FSubStructParam>::NetSerialize() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\CoreUObject\Public\UObject\Class.h:1314]
0x00007ff968a6505c UnrealEditor-CoreUObject.dll!FStructProperty::NetSerializeItem() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\CoreUObject\Private\UObject\PropertyStruct.cpp:192]
0x00007ff923afc52d UnrealEditor-Engine.dll!FRepLayout::SerializeProperties_r() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\RepLayout.cpp:6659]
0x00007ff923af6ff2 UnrealEditor-Engine.dll!FRepLayout::SendPropertiesForRPC() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\RepLayout.cpp:6965]
0x00007ff923459794 UnrealEditor-Engine.dll!UNetDriver::ProcessRemoteFunctionForChannelPrivate() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\NetDriver.cpp:2736]
0x00007ff923446c83 UnrealEditor-Engine.dll!UNetDriver::InternalProcessRemoteFunctionPrivate() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\NetDriver.cpp:2557]
0x00007ff92344655b UnrealEditor-Engine.dll!UNetDriver::InternalProcessRemoteFunction() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\NetDriver.cpp:2459]
0x00007ff9234584b8 UnrealEditor-Engine.dll!UNetDriver::ProcessRemoteFunction() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\NetDriver.cpp:7555]
0x00007ff92264bc1f UnrealEditor-Engine.dll!UActorComponent::CallRemoteFunction() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\Components\ActorComponent.cpp:837]
0x00007ff968ab4cc3 UnrealEditor-CoreUObject.dll!UObject::CallFunction() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\CoreUObject\Private\UObject\ScriptCore.cpp:1130]
0x00007ff968acec2e UnrealEditor-CoreUObject.dll!UObject::ProcessContextOpcode() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\CoreUObject\Private\UObject\ScriptCore.cpp:3086]
0x00007ff968ad3088 UnrealEditor-CoreUObject.dll!ProcessLocalScriptFunction() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\CoreUObject\Private\UObject\ScriptCore.cpp:1206]
0x00007ff968aa4fed UnrealEditor-CoreUObject.dll!ProcessScriptFunction<void (__cdecl*)(UObject *,FFrame &,void *)>() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\CoreUObject\Private\UObject\ScriptCore.cpp:1039]
0x00007ff968ad2ad4 UnrealEditor-CoreUObject.dll!ProcessLocalFunction() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\CoreUObject\Private\UObject\ScriptCore.cpp:1276]
0x00007ff968ad3088 UnrealEditor-CoreUObject.dll!ProcessLocalScriptFunction() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\CoreUObject\Private\UObject\ScriptCore.cpp:1206]
0x00007ff968ad1fef UnrealEditor-CoreUObject.dll!UObject::ProcessInternal() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\CoreUObject\Private\UObject\ScriptCore.cpp:1304]
0x00007ff968740962 UnrealEditor-CoreUObject.dll!UFunction::Invoke() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\CoreUObject\Private\UObject\Class.cpp:6847]
0x00007ff968ad0745 UnrealEditor-CoreUObject.dll!UObject::ProcessEvent() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\CoreUObject\Private\UObject\ScriptCore.cpp:2144]
0x00007ff921ee2597 UnrealEditor-Engine.dll!AActor::ProcessEvent() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\Actor.cpp:1092]
0x00007ff921cb58b3 UnrealEditor-Engine.dll!TScriptDelegate<FNotThreadSafeDelegateMode>::ProcessDelegate<UObject>() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Core\Public\UObject\ScriptDelegates.h:448]
0x00007ff9227c0cc4 UnrealEditor-Engine.dll!FInputActionHandlerDynamicSignature_DelegateWrapper() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Intermediate\Build\Win64\UnrealEditor\Inc\Engine\UHT\InputComponent.gen.cpp:73]
0x00007ff9242211e7 UnrealEditor-Engine.dll!FInputActionUnifiedDelegate::Execute() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Classes\Components\InputComponent.h:302]
0x00007ff92421c657 UnrealEditor-Engine.dll!UPlayerInput::EvaluateInputDelegates() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\UserInterface\PlayerInput.cpp:1516]
0x0000027d00d2de0a UnrealEditor-EnhancedInput.dll!UEnhancedPlayerInput::EvaluateInputDelegates() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Plugins\EnhancedInput\Source\EnhancedInput\Private\EnhancedPlayerInput.cpp:749]
0x00007ff9242687d7 UnrealEditor-Engine.dll!UPlayerInput::ProcessInputStack() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\UserInterface\PlayerInput.cpp:1149]
0x00007ff9238c09c9 UnrealEditor-Engine.dll!APlayerController::ProcessPlayerInput() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\PlayerController.cpp:2738]
0x00007ff9238e0a1e UnrealEditor-Engine.dll!APlayerController::TickPlayerInput() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\PlayerController.cpp:5035]
0x00007ff9238bb967 UnrealEditor-Engine.dll!APlayerController::PlayerTick() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\PlayerController.cpp:2342]
0x00007ff9238de29b UnrealEditor-Engine.dll!APlayerController::TickActor() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\PlayerController.cpp:5194]
0x00007ff921eabf96 UnrealEditor-Engine.dll!FActorTickFunction::ExecuteTick() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\Actor.cpp:238]
0x00007ff92404878b UnrealEditor-Engine.dll!FTickFunctionTask::DoTask() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\TickTaskManager.cpp:278]
0x00007ff92404f005 UnrealEditor-Engine.dll!TGraphTask<FTickFunctionTask>::ExecuteTask() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Core\Public\Async\TaskGraphInterfaces.h:1235]
0x00007ff95cb210aa UnrealEditor-Core.dll!FNamedTaskThread::ProcessTasksNamedThread() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Core\Private\Async\TaskGraph.cpp:760]
0x00007ff95cb2174e UnrealEditor-Core.dll!FNamedTaskThread::ProcessTasksUntilQuit() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Core\Private\Async\TaskGraph.cpp:651]
0x00007ff95cb2e4de UnrealEditor-Core.dll!FTaskGraphCompatibilityImplementation::WaitUntilTasksComplete() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Core\Private\Async\TaskGraph.cpp:2122]
0x00007ff9240786c5 UnrealEditor-Engine.dll!FTickTaskSequencer::ReleaseTickGroup() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\TickTaskManager.cpp:556]
0x00007ff92407fa8b UnrealEditor-Engine.dll!FTickTaskManager::RunTickGroup() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\TickTaskManager.cpp:1583]
0x00007ff922ff6fff UnrealEditor-Engine.dll!UWorld::RunTickGroup() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\LevelTick.cpp:775]
0x00007ff9230059b6 UnrealEditor-Engine.dll!UWorld::Tick() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Engine\Private\LevelTick.cpp:1644]
0x00007ff943afcdde UnrealEditor-UnrealEd.dll!UEditorEngine::Tick() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Editor\UnrealEd\Private\EditorEngine.cpp:2041]
0x00007ff944785ba6 UnrealEditor-UnrealEd.dll!UUnrealEdEngine::Tick() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Editor\UnrealEd\Private\UnrealEdEngine.cpp:550]
0x00007ff7adc38e02 UnrealEditor.exe!FEngineLoop::Tick() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Launch\Private\LaunchEngineLoop.cpp:5924]
0x00007ff7adc5e1bc UnrealEditor.exe!GuardedMain() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Launch\Private\Launch.cpp:180]
0x00007ff7adc5e2aa UnrealEditor.exe!GuardedMainWrapper() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:118]
0x00007ff7adc61724 UnrealEditor.exe!LaunchWindowsStartup() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:258]
0x00007ff7adc76ed4 UnrealEditor.exe!WinMain() [D:\Trunk\trunk\UnrealEngine-5.4\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:298]
0x00007ff7adc7a17a UnrealEditor.exe!__scrt_common_main_seh() [D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288]
0x00007ffa2db17374 KERNEL32.DLL!UnknownFunction []

最后定位到导致该崩溃原因为：在同步结构体时，自定义了结构体的NetSerialize方法，且该方法被定为了虚方法导致的。

解决方案：避免使用virtual；或者在外层包一层结构体的Handle，在Handle层定义NetSerialize方法，类似于GAS中FGameplayEffectContextHandle和FGameplayEffectContext

下面对UE常见数据同步进行一次回顾，主要讨论结构体、UObject、Actor三种数据的同步，基础类型数据不再赘述。

UE的同步方式主要是：RPC和属性同步。

一、结构体的同步

需要同步的结构体

USTRUCT(BlueprintType)
struct FRPCStruct
{
	GENERATED_BODY()
	UPROPERTY(BlueprintReadWrite, EditAnywhere)
	int32 IntParam = 0;

	UPROPERTY(BlueprintReadWrite, EditAnywhere)
	float FloatParam = 0.0f;

	UPROPERTY(BlueprintReadWrite, EditAnywhere)
	FSubStructParam StructParam;

	UPROPERTY(BlueprintReadWrite, EditAnywhere)
	TObjectPtr<UPackProjectParam> ParamObj = nullptr;

	FString ToString() const
	{
		FString Result = FString::Printf(
			TEXT("FRPCStructInfo,Int:%d, Float:%f, SubInt:%d, SubParamInt:%lld"), IntParam, FloatParam,
			StructParam.SubIntParam, StructParam.SubParentIntParam);
		if (IsValid(ParamObj))
		{
			Result += FString::Printf(TEXT("___ExtraParamObj:%d"), ParamObj->ObjIntParam);
		}
		return Result;
	}

	bool NetSerialize(FArchive& Ar, class UPackageMap* Map, bool& bOutSuccess)
	{
		Ar<<IntParam;
		Ar<<FloatParam;
		Ar<<StructParam.SubIntParam;
		Ar<<StructParam.SubParentIntParam;
		uint8 RepBits = 0;
		if (Ar.IsSaving())
		{
			if (IsValid(ParamObj))
			{
				RepBits |= 1 << 0;
			}
		}
		Ar.SerializeBits(&RepBits, 1);

		if (RepBits & (1 << 0))
		{
			Ar << ParamObj;
		}
		bOutSuccess = true;
		return bOutSuccess;
	}
};

template <>
struct TStructOpsTypeTraits<FRPCStruct> : public TStructOpsTypeTraitsBase2<FRPCStruct>
{
	enum
	{
		WithNetSerializer = true,
	};
};

Note：这里NetSerialize方法一定不能为Virtual

RPC 同步Struct

C2S

UFUNCTION(BlueprintCallable, Server, Reliable)
void SendStructToServer(const FRPCStruct& InStruct);

S2C

UFUNCTION(BlueprintCallable, Client, Reliable)
void SendStructToClient(const FRPCStruct& InStruct);

属性同步Struct

Note：

在属性同步中，无论对结构体整体进行赋值，或者修改结构体中某一字段(基础类型)，都会触发OnRep方法。

若结构体中嵌套UObject指针，但UObject为新构造对象，则无法一同同步。

若UObject本身支持同步，仅修改了UObject对象中某字段的值也是无法触发结构体的OnRep方法。

关于自定义结构体NetSerialize方法

结构体本身若是基础数据类型的集合，不用自定义NetSerialize方法也是可以同步的，还可以通过UPROPERTY，NotReplicated标记符控制是否某字段参与同步。

若需要更深度定义同步数据内容，则可通过自定义NetSerialize方法，而要支持自定义NetSerialize，需要定义以下结构体，这样引擎会根据配置的枚举去萃取结构体的属性。

自定义数据较为常见的场景应用：支持TMap的同步、某些字段满足条件后才参与同步等
- 常见方法
  - 正在进行序列化：Ar.IsSaving()
  - 正在进行反序列化：Ar.IsLoading()
  - 序列化、反序列比特：Ar.SerializeBits
- TMap同步方式实现思路：TMap本身是不可同步的，但是TArray可同步，因此常常通过在IsSaving的时候将TMap的Key、Value通过打包成结构体数组同步，在IsLoading时，再将同步来的TArray数据重新解析为TMap。
- 条件同步
```
uint8 RepBits = 0;
if (Ar.IsSaving())
{
    if (IsValid(ParamObj))
    {
        RepBits |= 1 << 0;
    }
}
Ar.SerializeBits(&RepBits, 1);

if (RepBits & (1 << 0))
{
    Ar << ParamObj;
}
```
  上述代码的思路，在序列化结构体（Ar.IsSaving()）时，判断ParamObj的有效性，若有效，则将RepBits的第一位置为1。
  
  无论序列化还是反序列化，都会调用Ar.SerializeBits()，将RepBits读入或写出后，判断ParamObj数据是否进行序列化和反序列化。

扩展思考

Struct 中嵌套 UObject

若UObject本身不支持同步，也无法跟随Struct同步。
Struct中嵌套Actor

FArchive*、UPackageMap* 的作用是什么？

Map->GetObjectFromNetGUID()
Map->GetNetGUIDFromObject()

二、UObject同步

UObject对象默认在UE中是不支持同步的，但通过某些方式或某些特殊的UObject也支持同步。

RPC UObject对象
- C2S
```
UFUNCTION(BlueprintCallable, Server, Reliable)
void SendUObjectToServer(const UObject* Param);
```
  这里分为三种UObject类型
  - 新构造的UObject，无法同步
  - InstancedObject，可以同步，但是修改Object某字段值是无法同步的
  - 支持ReplicatedObject，可以同步，但是修改Object某字段值是无法同步的
- S2C
```
UFUNCTION(BlueprintCallable, Client, Reliable)
void SendUObjectToClient(const UObject* Param);
```
  这里分为三种UObject类型
  - 新构造的UObject，无法同步
  - InstancedObject，可以同步，但是修改Object某字段值是无法同步的
  - 支持ReplicatedObject，可以同步，若修改了某字段值，因为RPC快于属性同步，其在RPC中获取Object该字段值是为旧值。（其本质是因为RPC并未同步数据内容）

属性同步UObject对象

UObject对象定义

这里需要重写IsSupportedForNetworkding方法，返回为true
Actor中标记该UObject为网络复制

在服务将UObject对象创建后，调用AddReplicatedSubObject方法

UStructRPCComponent::UStructRPCComponent()
{
	bReplicateUsingRegisteredSubObjectList = true;
}

void UStructRPCComponent::BeginPlay()
{
	Super::BeginPlay();
	if(GetOwner()->HasAuthority())
	{
		PackProjectParam = NewObject<UPackProjectParam>(this, UPackProjectParam::StaticClass());
		PackProjectParam->ObjIntParam = 10;
		AddReplicatedSubObject(PackProjectParam);
	}
	FTimerHandle TimerHandle;
	GetWorld()->GetTimerManager().SetTimer(TimerHandle, FTimerDelegate::CreateWeakLambda(this,[this]()
	{
		if(IsValid(PackProjectParam))
		{
			PackProjectParam->ObjIntParam = 20;
		}
	}), 2, false);
}

Note：在Actor中定义的OnRep方法只会在创建赋值后执行一次，若后续若只是修改对象里的值，则此OnRep不会再执行。

而定义在UObject对象内部的OnRep函数，不仅会在初次赋值UObject对象指针时调用一次，后续变更值也会调用，且时序早于上述Actor中定义的OnRep函数。

获取可复制UObject NetworkGUID的方法

GetWorld()->GetNetDriver()->GuidCache->GetObjectFromNetGUID();
GetWorld()->GetNetDriver()->GuidCache->GetNetGUID();

扩展思考

为什么InstancedObject的UObject对象可以支持同步呢？但在服务器上修改UObject对象中某字段值确不能同步呢？

这里其实不难理解，InstancedObject实际是通过Package路径序列化在本地的，这里同步的其实是Package路径，同步后，通过Load恢复UObject指针内容。

三、Actor同步（UE同步的基本单位）

Actor需被标记为Replicates，本质上Actor是特殊的UObject对象，为什么他可以被同步呢？

RPC Actor对象
```
UFUNCTION(BlueprintCallable, Client, Reliable)
void SendActorToClient(const AActor* InActor);
UFUNCTION(BlueprintCallable, Server, Reliable)
void SendActorToServer(const AActor* InActor);
```
同上述同步UObject类似，如果Actor本身支持同步，才能通过RPC上行发到服务器，且客户端修改的内容并不会发送到服务器

这里更多讨论下行，即SendActorToClient，这里有两种情景
- case1:Actor是早已同步到客户端的Actor，通过RPC再发送Actor指针，能正确在客户端上获取该Actor指针。
- case2:Actor是在服务器上临时SpawnActor，立刻通过RPC发送到客户端，那能否获得该Actor的指针呢？
  
  答案是获取不到。这种情况的应用其实挺多的，比如在制作坐骑系统时，可能需要临时在服务器上spawn一个坐骑actor，然后possess该actor，但是该actor还未同步到客户端，就会有问题；还比如我们网络刚连接进入游戏，会在服务器创建玩家的Pawn，此时Pawn还未复制到客户端，服务器就进行了Possess。
那如何解决case2的问题呢？
- 其实引擎提供了一个控制台变量，如下
```
[ConsoleVariables]
net.DelayUnmappedRPCs=1
```
  其会保证RPC中Actor*若还未复制到客户端，会延迟到Actor复制到客户端后再调用对应的RPC。
属性同步Actor对象

Actor本身未标记未Replicates，那么则不可复制。

Actor本身标记为Replicates
- case1:Actor早已复制到客户端，那么在服务器赋值该字段，可以正常在OnRep方法中获取到Actor对象。
- case2:Actor为临时Spawn的Actor，能否正确在OnRep中获取到Actor对象。
  
  答案是OnRep可以正确获取到该AActor对象。那说明AActor的复制早于当前属性同步吗？
获取ActorNetworkGUID的方法

GetWorld()->GetNetDriver()->GetGUIDForActor()
GetWorld()->GetNetDriver()->GetActorForGUID()

扩展讨论

UE后续版本支持了InstancedStruct，那么InstancedStruct支持直接同步吗？
UE的同步单位是什么呢？
网络通信相关对象：UNetDriver、UNetConnection、Actor、ActorChannel、FOutBunch
Socket、网络底层模型(TCP/IP、OSI)

UE中DS的常见优化方式

结语

网络同步的基础，是基于OSI七层模型或TCP/IP四层模型，在传输层基于TCP或UDP协议来提供端到端的通信。

而一般网络序列化的思路：将数据结构序列化为二进制数据流进行网络传输，再将二进制数据流反序列化为数据结构。

以UE为例子：

序列化：将UStruct->FString(数据结构格式：Json、Xml等)->TArray
端到端通信：TCP/UDP
反序列化：TArray->FString->UStruct