Creating components in constructor.md

2020-11-30_10:36:27

Creating components in constructor

An Actor's Components are often created in the Actor's constructor. Create such components with CreateDefaultSubobject. Only components created in an Actor's constructor with CreateDefaultSubobject will show up in the Blueprint Editor. (I think.) The function may only be called in the constructor of the owning object. It is a template function, with the type parameter being the the type to instantiate. It takes (at least?) one parameter, which is the label/name of the created object. The member variable that the return value of CreateDefaultSubobject is assigned to should generally be a UPROPERTY.

MyProperty = CreateDefaultSubobject<UMyType>(TEXT("MyLabel"));

If the Component is a USceneComponent, or one of its subclasses, then it should be added to the Component hierarchy. Make it the root with SetRootComponent. Attach to another Component already in the hierarchy with SetupAttachment. SetupAttachment can be passed a socket name, to attach the new component at that socket on the parent Component.

It is common to not configure a lot of details on the created components. In particular, one rarely assign assets to e.g. meshes in C++. It is better to assign assets in a Blueprint subclass of the C++ class, or in instance of the C++ class in the Level Editor. Because asset references in C++ are clunky and cannot be automatically updated by Unreal Editor the way Blueprint references can.

Header:

#pragma once

#include "GameFramework/Actor.h"

#include "MyActor.generated.h"

class UCamera;
class USpringArm;
class UStaticMeshComponent;


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

public:
    UPROPERTY()
    UStaticMeshComponent* Mesh;

    UPROPERTY()
    USpringArmComponent* Arm;

    UPROPERTY()
    UCameraComponent* Camera;
};

Source:

AMyActor::AMyActor()
{
    Mesh = CreateDefaultSubobject<UStaticMeshComponent>(TEXT("Mesh"));
    SetRootComponent(Mesh);

    Arm = CreateDefaultSubobject<UStaticMeshComponent>(TEXT("Arm"));
    Arm->SetupAttachment(Mesh);

    Camera = CreateDefaultSubobject<UCameraComponent>(TEXT("Camera"));
    Camera->SetupAttachment(Arm, USpringArmComponent::SocketName);
}

Always close Unreal Editor when compiling changes to constructors or header files. Really, always close Unreal Editor when compiling.