Creating Components at runtime.md

2020-08-14_09:32:03

Creating Components at runtime

Blueprint

Create new Components in an Actor Blueprint with the Add COMPONENT_NAME Component function. For example: Add Static Mesh Component.

This doesn't seem to work when not within the Actor Blueprint itself. One cannot create Components in an Actor that one has a reference to.

C++

Cannot use CreateDefaultSubobject outside of a constructor. Instead use NewObject and add/register manually.

void AMyActor::CreateMyComponent()
{
    UMyComponent* MyComponent = NewObject<UMyComponent>(
        this, TEXT("MyComponent"));
    MyComponent->SetRelativeLocation(
        FVector(0, 0, 0.0));
    MyComponent->SetFlags(RF_NoFlags);
    MyComponent->RegisterComponent();
    MyComponent->AttachToComponent(
        RootComponent,
        FAttachmentTransformRules::KeepRelativeTransform,
        NAME_None)
    AddInstanceComponent(MyComponent);
}

Worrying comment in the Unreal Slackers Discord:

For anyone interested in a solution to what I was working on yesterday: it turns out using NewObject<> blindly is insufficient for creating sub-objects in editor assets. To set the proper flags for UE4 to treat new instances as if they were created by the editor, use SetFlags(GetMaskedFlags(RF_PropagateToSubObjects)). For example:

virtual void PostEditChangeChainProperty(
    FPropertyChangedChainEvent& PropertyChangedEvent) override
{
    UBBSIntAttributeNode* IntNode = NewObject<UBBSIntAttributeNode>(this);
    IntNode->SetFlags(GetMaskedFlags(RF_PropagateToSubObjects));
    IntNode->intValue = value;
    Attributes.Add(AttributeClass, IntNode);
}

I don't understand anything of the above.

If you want to create Components in C++ then the Root Component must also be created in C++, not the Blueprint. Otherwise there's no Root Component to add the newly created Component to. Blueprint initialization happens after the C++ object has been fully constructed. Delayed Component creation should be in BeginPlay. ( I don't think the above is quite true. Seems odd to me. There are many places in which Components can be created. I guess... )

The relationship between AttachTo(Compoent?) and SetupAttachment is unclear to me.

Another suggestion for how to create a new Component instance:

void AMyActor::MakeSomeComponent() {
  if (!my_comp) {
    my_comp = NewObject<USplineComponent>(this, FName{ "some spline" });
    my_comp->RegisterComponent();
    auto rootcomp = GetRootComponent();
    my_comp->SetWorldLocationAndRotation(rootcomp->GetComponentLocation(), FRotator::ZeroRotator);
    my_comp->AttachTo(rootcomp, NAME_None, EAttachLocation::Type::KeepWorldPosition);
  }
}

void ASomeActor::MakeSomeComponent() {
  if (!my_comp) {
    my_comp = NewObject<USplineComponent>(this, FName{ "some spline" });
    my_comp->SetupAttachment(RootComponent);
    my_comp->RegisterComponent();
  }
}

void ASomeActor::MakeSomeComponent() {
  if (!my_comp) {
    my_comp = NewObject<USplineComponent>(this, FName{ "some spline" });
    my_comp->RegisterComponent();
    my_comp->AttachToComponent(RootComponent, NAME_None, EAttachLocation::SnapTotarget);
  }
}

EAttachLocation::SnapToTarget does what the two last lines do, FWIW I think SetupAttachment works okay so long as you call it prior to RegisterComponent() btw

Optional Components are created with something along the lines of

AParent::AParent(const FObjectInitializer& OI)
    Super(OI)
{
    MyCompA = OI.CreateOptionalDefaultSubobject<UMyComponentA>(
        this, UMyComponentA::MyComponentAName);
    MyCompB = OI.CreateOptionalDefaultSubobject<UMyComponentB>(
        this, UMyComponentB::MyComponentAName);
}

AChild::AChild(const FObjectInitializer& OI)
    : Super(
        OI
        .DoNotCreateDefaultSubobject(UMyComponentA::MyComponentAName)
        .DoNotCreateDefaultSubobject(UMyComponentB::MyComponentBName))
    {}