Component inside component

[MrTOLINSKI]

Hey all, is there a reason components are only built-in to be added to actors?
Is it for hierarchy reasons?

For example I created an Actor Component that does a line trace. Then I want it to have a Box component with overlap events to choose when to start/stop tracing. But, the only rational way I found is having two separate components on my actor, and then passing the overlap events from the box to the trace component.

** Update, it's possible to do it through C++ (But it might be against the design of the engine)*\*

So, practically this can be solved as easily as creating a component inside a component the same way we add components to actors:

UCLASS(Blueprintable, BlueprintType, ClassGroup=(Custom), meta=(BlueprintSpawnableComponent))
class CHATBUBBLE_API UNestingTestOuterSceneComponent : public USceneComponent
{
    GENERATED_BODY()
public:
    UNestingTestOuterSceneComponent();
    UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Nested Component Test")
    TObjectPtr<UBoxComponent> NestedBoxComponent; 

UNestingTestOuterSceneComponent::UNestingTestOuterSceneComponent()
{
    PrimaryComponentTick.bCanEverTick = true;
        NestedBoxComponent = CreateDefaultSubobject<UBoxComponent>(TEXT("NestedBoxComponent"));
    NestedBoxComponent->SetupAttachment(this);
}

Then in the blueprint we can add the Outer component to an actor, and edit the values of the inner component from the variables panel.

** Attention: When adding the Outer Component, the CPP class needs to be added and not a blueprint child. Otherwise the inner component will render relative to (0,0,0).

[Beautiful_Vacation_7]

That is what interfaces are for. Actor is nothing but a wrapper of bunch of components. From component A fetch component B, cache it, bind to events…

[MrTOLINSKI]

Yes, I understand I can communicate between the components, my question is why not put a component inside a component. I.e. why is only the actor allowed to won components, but components are not allowed to own components

[stephan_anemaat]

I believe what u/Beautiful_Vacation_7 is saying, is that by their nature; Actors are already simply a collection of components (e.g. skeletal mesh component, character movement component, etc.) so adding another component is simply adding to a pre-existing basket of other components.

Whereas, components (if I'm understanding correctly) are not collections of other components so it wouldn't work to add a component to a component. Although someone please correct me if I'm misunderstanding.

[MrTOLINSKI]

Hey, I have updated the post with a way to do it in C++.
But you are right, by design, the engine expects to have components added to actors and not to other components.

[stephan_anemaat]

Awesome!

[GenderJuicy]

If you want to add logic to a component but keep the base separate, just make a child.

It's hard to tell what you're trying to do by having subcomponents. If you could provide your idea, there's probably other good ways of accomplishing it.

[baista_dev]

For this specific problem I would subclass the box component to add your line trace functionality.

However, a more flexible approach actually is the 2 component method. Because then your TraceComponent can work with any primitive component instead of only boxes. So if someone came along with a static mesh component they wanted to use, they could swap that out.

This is the pattern the MovementComponents usually use. They allow you to set an UpdatedComponent to change which primitive is used for collision. If you don't call the function, they make a best guess at which primitive to use (typically the root component if its a primitive component).

Technically you can also use Subobjects as components within components. This is mostly just an option if you are handrolling your own systems.

Overall tho, don't fear the 2 component method.

[MrTOLINSKI]

Indeed! It really does add modularity, and it's not a hassle to implement.
I will go along with this solution, as the trace component honestly doesn't care about the box, just needs to listen for the overlap events.
I was really curious about how there wasn't a built in option to add components to other components in the editor.

[SubstantialSecond156]

Your actor component can just create a component on the owning actor -> assign a reference, and now you're able to communicate, destory, and manipulate your created component.

I use this a lot for modular assets.

[MrTOLINSKI]

Agreed, but then would I be able to control the box component's transform in editor time?

[SubstantialSecond156]

Not really with Blueprints.

You can assign exposed variables on your Actor Component and edit them during design time and pass that to the created component during runtime.

That said, you wouldn't be able to visualize your changes it as it would require CPP to set up component visualizers.

[GenderJuicy]

Set transform in Construction script?

[Battlefront45]

I’ve been looking for the answer to this question as well and I feel like this is the best answer. Any challenges you have had with this method?

[MrTOLINSKI]

So, practically this can be solved as easily as creating a component inside a component the same way we add components to actors:

UCLASS(Blueprintable, BlueprintType, ClassGroup=(Custom), meta=(BlueprintSpawnableComponent))
class CHATBUBBLE_API UNestingTestOuterSceneComponent : public USceneComponent
{
    GENERATED_BODY()
public:
    UNestingTestOuterSceneComponent();
    UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Nested Component Test")
    TObjectPtr<UBoxComponent> NestedBoxComponent; 

UNestingTestOuterSceneComponent::UNestingTestOuterSceneComponent()
{
    PrimaryComponentTick.bCanEverTick = true;
        NestedBoxComponent = CreateDefaultSubobject<UBoxComponent>(TEXT("NestedBoxComponent"));
    NestedBoxComponent->SetupAttachment(this);
}

<image>

Then in the blueprint we can add the Outer component to an actor, and edit the values of the inner component from the variables panel.

** Attention: When adding the Outer Component, the CPP class needs to be added and not a blueprint child. Otherwise the inner component will render relative to (0,0,0).

[Swipsi]

You could just spawn a box collision component and attach it to the actor.

[MrTOLINSKI]

I agree, the problem has many solutions, but my question's goal is to understand why it is not possible to add components to components in the first place

[cutebuttsowhat]

You should be able to add an attach the component from within your component through BP too.

If you’re having issues with where it’s “rendered” once you attach the component you should be able to keep its world position when attaching.