No hit event when RootComponent is USceneComponent and no physics simulation

[Masochist]

Create a project Blank from blank template, place PlayerStart at (0,0,91.5) and override the game mode below.

Game mode:

#pragma once

#include "CoreMinimal.h"
#include "GameFramework/GameModeBase.h"
#include "MyGameModeBase.generated.h"


UCLASS()
class BLANK_API AMyGameModeBase : public AGameModeBase
{
	GENERATED_BODY()
	void BeginPlay() override;
};

//=======================================
#include "MyGameModeBase.h"
#include "MyActor.h"
#include "Kismet/GameplayStatics.h"


void AMyGameModeBase::BeginPlay()
{
	Super::BeginPlay();

	GetWorld()->SpawnActor<AMyActor>(FVector(500, -500, 100), FRotator(0));
	GetWorld()->SpawnActor<AMyActor>(FVector(500, 500, 100), FRotator(0))->bToPositiveInfinity = false;
}

Actor:

#pragma once

#include "CoreMinimal.h"
#include "GameFramework/Actor.h"
#include "MyActor.generated.h"

class UBoxComponent;

UCLASS()
class BLANK_API AMyActor : public AActor
{
	GENERATED_BODY()

public:
	AMyActor();
	bool bToPositiveInfinity = true;

protected:
	void BeginPlay() override;
	void Tick(float DeltaTime) override;

	UPROPERTY(VisibleAnywhere)
	USceneComponent* Scene;

	UPROPERTY(VisibleAnywhere)
	UBoxComponent* Box;

	UFUNCTION()
	void OnBoxComponentHit(
		UPrimitiveComponent* HitComp,
		AActor* OtherActor,
		UPrimitiveComponent* OtherComp,
		FVector Norm,
		const FHitResult& Hit);
};

// =============================================
#include "MyActor.h"
#include "Components/BoxComponent.h"



AMyActor::AMyActor()
{
	PrimaryActorTick.bCanEverTick = true;


	bool bUseSceneAsRoot = false; // Change this manually! Using live coding is fine too! 

	Box = CreateDefaultSubobject<UBoxComponent>("BoxComponent");
	Box->SetCollisionProfileName(UCollisionProfile::BlockAll_ProfileName);
	Box->SetHiddenInGame(false);
	Box->SetLineThickness(10.f);
	Box->SetRelativeScale3D(FVector(2.f));

	//Box->SetSimulatePhysics(true);
	//Box->SetNotifyRigidBodyCollision(true);


	if (bUseSceneAsRoot)
	{
		Scene = CreateDefaultSubobject<USceneComponent>("SceneComponent");
		SetRootComponent(Scene);
		Box->SetupAttachment(Scene);
	}
	else
	{
		SetRootComponent(Box);
	}
}


void AMyActor::BeginPlay()
{
	Super::BeginPlay();
	Box->OnComponentHit.AddDynamic(this, &ThisClass::OnBoxComponentHit);
}

void AMyActor::OnBoxComponentHit(UPrimitiveComponent* HitComp, AActor* OtherActor, UPrimitiveComponent* OtherComp, FVector Norm, const FHitResult& Hit)
{
	UE_LOG(LogTemp, Warning, TEXT(__FUNCTION__));
}


void AMyActor::Tick(float DeltaTime)
{
	Super::Tick(DeltaTime);
	if (bToPositiveInfinity)
	{
		AddActorLocalOffset(FVector(0, 5, 0), true);
	}
	else
	{
		AddActorLocalOffset(FVector(0, -5, 0), true);
	}
}

Why can’t we use USceneComponent as RootComponent here?

[DanM]

Due to how AddActorLocalOffset is implemented. It uses the root component. The key part is the call to

MoveComponentImpl, with the SceneComponent as the root it will call USceneComponent::MoveComponentImpl which doesn’t check for collision, with the Box as the root it calls UPrimitiveComponent:MoveComponentImpl which does.