Unreal Actor-Component Architecture

UObject
  └── AActor                            placeable/spawnable; owns components      GameFramework/Actor.h
        └── UActorComponent             logic only, no transform                  Components/ActorComponent.h
              └── USceneComponent       transform + attachment tree               Components/SceneComponent.h
                    └── UPrimitiveComponent  rendering + collision + physics      Components/PrimitiveComponent.h

AActor::AActor()                       CDO + every instance. No world. CreateDefaultSubobject, tick config, defaults
UObject::PostLoad()                    level-placed actors only (not spawned)
UActorComponent::OnComponentCreated()  native components on spawn (not when loaded from a level)
AActor::PreRegisterAllComponents()  →  RegisterComponent() per component  →  AActor::PostRegisterAllComponents()
AActor::PostActorCreated()             spawned actors only, before construction scripts
UserConstructionScript()  →  AActor::OnConstruction(const FTransform&)
AActor::PreInitializeComponents()      gameplay only
UActorComponent::Activate()            if bAutoActivate
UActorComponent::InitializeComponent() if bWantsInitializeComponent, once per session
AActor::PostInitializeComponents()     components initialized; bind to own components here
AActor::BeginPlay()                    level is ticking; may be delayed for networked or child actors
AActor::Tick(float DeltaSeconds)       if PrimaryActorTick.bCanEverTick
AActor::EndPlay(const EEndPlayReason::Type)  base implementation forwards EndPlay to components
AActor::Destroyed()                    explicit Destroy() only; not on level streaming or game end

// MyActor.h
#pragma once

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

class UStaticMeshComponent;
class UMyHealthComponent;

UCLASS()
class MYGAME_API AMyActor : public AActor
{
    GENERATED_BODY()
public:
    AMyActor();
protected:
    virtual void PostInitializeComponents() override;
    virtual void BeginPlay() override;
    virtual void EndPlay(const EEndPlayReason::Type EndPlayReason) override;

    UFUNCTION()
    void HandleDeath();

    UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category="Components")
    TObjectPtr<UStaticMeshComponent> MeshComp;

    UPROPERTY(VisibleAnywhere, Category="Components")
    TObjectPtr<UMyHealthComponent> HealthComp;
};

// MyActor.cpp
#include "MyActor.h"
#include "Components/StaticMeshComponent.h"
#include "TimerManager.h"
#include "MyHealthComponent.h"

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

    MeshComp = CreateDefaultSubobject<UStaticMeshComponent>(TEXT("Mesh"));
    SetRootComponent(MeshComp);                      // SetupAttachment(MeshComp) parents a scene component here; no world needed

    HealthComp = CreateDefaultSubobject<UMyHealthComponent>(TEXT("Health")); // logic-only: no attachment
}

void AMyActor::PostInitializeComponents()
{
    Super::PostInitializeComponents();
    HealthComp->OnDeath.AddDynamic(this, &AMyActor::HandleDeath); // own components are initialized
}

void AMyActor::BeginPlay()
{
    Super::BeginPlay();
    SetActorTickEnabled(true);                        // gameplay starts here
}

void AMyActor::EndPlay(const EEndPlayReason::Type EndPlayReason)
{
    GetWorldTimerManager().ClearAllTimersForObject(this);
    Super::EndPlay(EndPlayReason);                    // base forwards EndPlay to components
}

void AMyActor::HandleDeath() { Destroy(); }

// MyHealthComponent.h
#pragma once

#include "CoreMinimal.h"
#include "Components/ActorComponent.h"
#include "MyHealthComponent.generated.h"

DECLARE_DYNAMIC_MULTICAST_DELEGATE(FMyOnDeathSignature);

UCLASS(ClassGroup=(MyGame), meta=(BlueprintSpawnableComponent))
class MYGAME_API UMyHealthComponent : public UActorComponent
{
    GENERATED_BODY()
public:
    UMyHealthComponent();

    UPROPERTY(BlueprintAssignable, Category="Health")
    FMyOnDeathSignature OnDeath;

    UFUNCTION(BlueprintCallable, Category="Health")
    void ApplyDamage(float Amount);
protected:
    virtual void InitializeComponent() override;

    UPROPERTY(EditAnywhere, Category="Health")
    float MaxHealth = 100.f;

    UPROPERTY(VisibleInstanceOnly, Transient, Category="Health")
    float CurrentHealth = 0.f;
};

// MyHealthComponent.cpp
#include "MyHealthComponent.h"

UMyHealthComponent::UMyHealthComponent()
{
    PrimaryComponentTick.bCanEverTick = false;   // event-driven: no per-frame work
    bWantsInitializeComponent = true;            // opt in to InitializeComponent()
}

void UMyHealthComponent::InitializeComponent()
{
    Super::InitializeComponent();
    CurrentHealth = MaxHealth;                   // once per gameplay session, before BeginPlay
}

void UMyHealthComponent::ApplyDamage(float Amount)
{
    CurrentHealth = FMath::Max(0.f, CurrentHealth - Amount);
    if (CurrentHealth <= 0.f) { OnDeath.Broadcast(); }
}

// In AMyActor (declared: void AddRuntimeLight();)
#include "Components/PointLightComponent.h"

void AMyActor::AddRuntimeLight()
{
    // NewObject creates the UObject only; RegisterComponent creates render/physics state and enables tick.
    UPointLightComponent* Light = NewObject<UPointLightComponent>(this, TEXT("RuntimeLight"));
    Light->SetupAttachment(GetRootComponent());  // allowed before RegisterComponent
    Light->RegisterComponent();
    AddInstanceComponent(Light);                 // adds to InstanceComponents: visible in Details, serialized per instance
    Light->SetIntensity(5000.f);
}

void AMyCharacter::EquipWeapon(UStaticMeshComponent* WeaponMesh)
{
    WeaponMesh->AttachToComponent(GetMesh(), FAttachmentTransformRules::SnapToTargetNotIncludingScale, TEXT("WeaponSocket"));
}

// AMyEnemySpawner member: UPROPERTY(EditDefaultsOnly) TSubclassOf<AMyEnemy> EnemyClass;
AMyEnemy* AMyEnemySpawner::SpawnEnemy(const FVector& Location, const FRotator& Rotation)
{
    FActorSpawnParameters Params;
    Params.Owner = this;
    Params.Instigator = GetInstigator();
    Params.SpawnCollisionHandlingOverride = ESpawnActorCollisionHandlingMethod::AdjustIfPossibleButAlwaysSpawn;
    return GetWorld()->SpawnActor<AMyEnemy>(EnemyClass, Location, Rotation, Params);
}

// AMyEnemy declares: void SetEnemyData(UMyEnemyData* Data);
AMyEnemy* AMyEnemySpawner::SpawnEnemyFromData(const FTransform& SpawnTransform, UMyEnemyData* Data)
{
    AMyEnemy* Enemy = GetWorld()->SpawnActorDeferred<AMyEnemy>(EnemyClass, SpawnTransform, this, GetInstigator(),
        ESpawnActorCollisionHandlingMethod::AlwaysSpawn);
    if (!Enemy) { return nullptr; }
    Enemy->SetEnemyData(Data);                     // runs before OnConstruction and BeginPlay
    Enemy->FinishSpawning(SpawnTransform, false, nullptr, ESpawnActorScaleMethod::MultiplyWithRoot);
    return Enemy;
}

AMyEnemy::AMyEnemy()
{
    PrimaryActorTick.bCanEverTick = true;
    PrimaryActorTick.bStartWithTickEnabled = false;   // later: SetActorTickEnabled(true)
    PrimaryActorTick.TickInterval = 0.1f;             // later: SetActorTickInterval(0.1f)
    PrimaryActorTick.TickGroup = TG_PostPhysics;
}