既然如此，这几个文件都看看
先看看FirstPersonProjectile头文件
定义了几个函数

然后是两个component

这个projectilemovement应该是控制物理运动的
看看CPP文件
sphere那个就创建了一个subobject，初始化了一下，然后这里

CollisionComp->OnComponentHit.AddDynamic(this, &AFirstPersonProjectile::OnHit);		// set up a notification for when this component hits something blocking

看起来是将触发函数绑定到这个hit事件上
然后他就是rootComponent了
然后是另外一个component，就是子弹那个

ProjectileMovement = CreateDefaultSubobject<UProjectileMovementComponent>(TEXT("ProjectileComp"));
ProjectileMovement->UpdatedComponent = CollisionComp;
ProjectileMovement->InitialSpeed = 3000.f;
ProjectileMovement->MaxSpeed = 3000.f;
ProjectileMovement->bRotationFollowsVelocity = true;
ProjectileMovement->bShouldBounce = true;

这几个赋值操作看起来分别

1. 是指定了他更新谁
2. 发射速度
3. 最大速度
4. 是否考虑旋转
5. 反弹
   完整构造函数

AFirstPersonProjectile::AFirstPersonProjectile() 
{
	// Use a sphere as a simple collision representation
	CollisionComp = CreateDefaultSubobject<USphereComponent>(TEXT("SphereComp"));
	CollisionComp->InitSphereRadius(5.0f);
	CollisionComp->BodyInstance.SetCollisionProfileName("Projectile");
	CollisionComp->OnComponentHit.AddDynamic(this, &AFirstPersonProjectile::OnHit);		// set up a notification for when this component hits something blocking

	// Players can't walk on it
	CollisionComp->SetWalkableSlopeOverride(FWalkableSlopeOverride(WalkableSlope_Unwalkable, 0.f));
	CollisionComp->CanCharacterStepUpOn = ECB_No;

	// Set as root component
	RootComponent = CollisionComp;

	// Use a ProjectileMovementComponent to govern this projectile's movement
	ProjectileMovement = CreateDefaultSubobject<UProjectileMovementComponent>(TEXT("ProjectileComp"));
	ProjectileMovement->UpdatedComponent = CollisionComp;
	ProjectileMovement->InitialSpeed = 3000.f;
	ProjectileMovement->MaxSpeed = 3000.f;
	ProjectileMovement->bRotationFollowsVelocity = true;
	ProjectileMovement->bShouldBounce = true;

	// Die after 3 seconds by default
	InitialLifeSpan = 3.0f;
}

另外就是onHit函数实现
如果碰到啥了，且那个对象也进行物理模拟，就给那个对象一个刺激？脉冲？动力？emmm反正就是让他也受力，传过去的是自己的速度*100和自己的位置

void AFirstPersonProjectile::OnHit(UPrimitiveComponent* HitComp, AActor* OtherActor, UPrimitiveComponent* OtherComp, FVector NormalImpulse, const FHitResult& Hit)
{
	// Only add impulse and destroy projectile if we hit a physics
	if ((OtherActor != nullptr) && (OtherActor != this) && (OtherComp != nullptr) && OtherComp->IsSimulatingPhysics())
	{
		OtherComp->AddImpulseAtLocation(GetVelocity() * 100.0f, GetActorLocation());

		Destroy();
	}
}