To be Transient or Not To be?

/**
    * Should the spawned actor be marked as transient?
    * @note The spawned actor will also be marked transient if this component or its owner actor are transient, regardless of the state of this flag.
    */
UPROPERTY(EditDefaultsOnly, Category=ChildActorComponent)
uint8 bChildActorIsTransient:1;

这个Flag主要作用是给Spawn出来的ChildActor加上Transient标记。

带上Transient标记，所有序列化和SavePackage之类的函数都会跳过这个ChildActor。 适合ChildActor可能会进行一些会导致PackageDirty的情况。

Bug: PIE下ChildActor消失?

如果带上Transient，摆在场景上的ChildActorComponent在PIE下没法看见创建的ChildActor.

Editor	PIE

仔细分析了一波原因，最后定位到在ULevel::Serialize里:

对于带有Transient标记的Actor不会被序列化到Actors数组里。

	else if (Ar.IsSaving() && Ar.IsPersistent())
	{
		UPackage* LevelPackage = GetOutermost();
		TArray<AActor*> EmbeddedActors;
		EmbeddedActors.Reserve(Actors.Num());

		Algo::CopyIf(Actors, EmbeddedActors, [&](AActor* Actor)
		{
			if (!Actor)
			{
				return false;
			}

			check(Actor->GetLevel() == this);

			if (Actor->HasAnyFlags(RF_Transient))
			{
				return false;
			}

当从Editor进入到PIE时候，会进行UWorld的复制，ULevel的重新初始化，最后进入到

bool ULevel::IncrementalRegisterComponents(bool bPreRegisterComponents, int32 NumComponentsToUpdate, FRegisterComponentContext* Context)
{
	// Find next valid actor to process components registration

	if (OwningWorld)
	{
		OwningWorld->SetAllowDeferredPhysicsStateCreation(true);
	}

	while (CurrentActorIndexForIncrementalUpdate < Actors.Num())
    {
        ....
        Actor->IncrementalRegisterComponents(NumComponentsToUpdate, Context);
    }

由于Editor的ChildActor没有被序列化到ULevels Actor里，新建的PIE UWorld的Level将不会有这个ChildActor，所以这里会跳过Child Actor的Register。

所以，ChildActor虽然存在在PIE World(由UWorld的复制的时候带过来的,所以能正常执行BeginPlay / TickFunction)，但是由于没有注册所以没有创建RenderState和PhysicsState，所以在渲染时看不到，物理上也无法交互。

一个简单的修复就是在 ChildActorComponent的OnRegister中，如果发现ChildActor没注册，重新注册。

diff --git a/Engine/Source/Runtime/Engine/Private/Components/ChildActorComponent.cpp b/Engine/Source/Runtime/Engine/Private/Components/ChildActorComponent.cpp
--- a/Engine/Source/Runtime/Engine/Private/Components/ChildActorComponent.cpp
+++ b/Engine/Source/Runtime/Engine/Private/Components/ChildActorComponent.cpp

@@ -102,6 +102,13 @@
void UChildActorComponent::OnRegister()
{
	Super::OnRegister();

    ....
 	{
 		CreateChildActor();
 	}
+   // register all components if necessary
+	if(ChildActor && !ChildActor->HasActorRegisteredAllComponents())
+	{
+		ChildActor->RegisterAllComponents();
+	}
 }
 
 void UChildActorComponent::Serialize(FArchive& Ar)