Basic Physics On/Off for Actor

Development World Creation
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1

Hello, I’m struggling to pickup and put down objects in VR because I don’t understand the physics simulation design.

How do I turn physics on/off for an Actor?

Is there a single API call for the Actor?

I can DropActor->GetRootComponent(), but it doesn’t have SetSimulatePhysics(). Do I have to turn off physics for all child components simulating as well?

In this code, I check for a “FoundGripComponent” - a special component for props like tools and whatnot.

When I test this code picking up an actor w/o that component (GripFoundActor && !GripFoundComponent), I can pickup and drop the actor once - but I cannot pick up the actor again (BlueprintActor->DefaultSceneRoot->StaticMesh)

void ARealityMotionController::GripBegin()
{
	// Grab previously identified item.
	WantsToGrip = true;

	if (GripFoundComponent != nullptr)
	{
		UKismetSystemLibrary::PrintString(this, TEXT("MotionController->GripBegin PROP"));
		// This is an interesting PROP, 
		GrippedActor = GripFoundActor;
		GrippedGripComponent = GripFoundComponent;
		// GripComponent will handle physics and attach the GripComponent properly
		GrippedGripComponent->ReceiveGripBegin(this, HandMesh, FName("GRIP_ATTACHMENT"));  // GripComponent will handle physics and attachment.
	} 
	else if (GripFoundActor != nullptr)
	{
		UKismetSystemLibrary::PrintString(this, TEXT("MotionController->GripBegin DECOR"));
		// This is random DECOR, and we should hold onto it till the player gets bored.
		GrippedActor = GripFoundActor;
		
		// Turn off physics
		TArray<UPrimitiveComponent*> AllComponents;
		GrippedActor->GetComponents(AllComponents, true);
		for (UPrimitiveComponent* MyComp : AllComponents)
		{
			MyComp->SetSimulatePhysics(false);
		}

		// Attach to Motion Controller
		GrippedActor->GetRootComponent()->AttachToComponent(HandMesh, FAttachmentTransformRules::SnapToTargetNotIncludingScale, FName("GRIP_ATTACHMENT"));
		//GrippedActor->AttachToActor(this, FAttachmentTransformRules::SnapToTargetNotIncludingScale);
	}
	else 
	{
		UKismetSystemLibrary::PrintString(this, TEXT("MotionController->GripBegin NOTHING"));
	}
}

void ARealityMotionController::GripEnd()
{
	// Drop previously grabbed item
	if (GrippedGripComponent != nullptr)
	{
		UKismetSystemLibrary::PrintString(this, TEXT("MotionController->GripEnd PROP"));
		// GripComponent knows how to detach and simulate physics.
		GrippedGripComponent->ReceiveGripEnd(this);
	}
	else if (GrippedActor != nullptr)
	{
		UKismetSystemLibrary::PrintString(this, TEXT("MotionController->GripEnd DECOR"));
		// Get list of all components
		TArray<UPrimitiveComponent*> AllComponents;
		GrippedActor->GetComponents(AllComponents, true);
		// Enable physics
		for (UPrimitiveComponent* MyComp : AllComponents)
		{
			// "Beginning simulation will detach it" https://docs.unrealengine.com/en-US/API/Runtime/Engine/Components/UPrimitiveComponent/SetSimulatePhysics/index.html
			MyComp->SetSimulatePhysics(true);
		}
	}
	GrippedGripComponent = nullptr;
	GrippedActor = nullptr;
	WantsToGrip = false;
}

Thanks!

2

And what is the DropActor root component? A static mesh component?

UPrimitiveComponent* comp = Cast<UPrimitiveComponent>(GetRootComponent());
if (comp)
{
	comp->SetSimulatePhysics(true);
}
3

Hi EvilCleric, thanks for answering. Good idea for the Cast. Thank you.

My Intention was to by default allow the player to pickup anything (Blueprints have a default SceneComponent). I also made a GripComponent for special props/tools. Even if I don’t allow the player to pickup items w/o a GripComponent, I’d like to understand how to enable/disable physics for the actor in a general case. I updated the code and the question a bit.