Detecting overlaps for a static mesh attached to a skeletal mesh

SlimeQ · December 13, 2017, 12:43pm

I made a bug report over on answerhub for this, but honestly I’m not expecting much of an answer over there. Here is my problem:

I have a throwable VR sticky grenade. It uses a ProjectileMovementComponent for physics. I have it set up so that on bounce, if the object it hit is of collision type Pawn, it attaches to the object at the hit bone. The actual function looks like this:
void AProxyWarExplosive::OnBounce(const FHitResult& HitResult, const FVector& NormalImpulse) {
    if (WeaponConfig.bSticksToPawns && HitResult.GetComponent()-&gt;GetCollisionObjectType() == ECollisionChannel::ECC_Pawn) {
        MovementComp-&gt;StopMovementImmediately();
        RotationComp-&gt;StopMovementImmediately();
        AttachToComponent(HitResult.GetComponent(), FAttachmentTransformRules::KeepWorldTransform, HitResult.BoneName);
    }
}
This works 100% fine. However, I’m now finding that it’s very natural when one of these things hits you to want to pull it off of your body. This also works just fine… on clients. Only on clients. On the server, picking up a grenade does not work when the grenade is attached to a player’s body.

Thinking this was surely my fault, I logged out all the picking up logic to debug. The pick up process is very simple, I just have a SphereComponent attached to the player’s hand and I use GetOverlappingActors like so:
TArray&lt;AActor*&gt; Overlapping;
GrabSphere-&gt;GetOverlappingActors(Overlapping, ClassFilter);
Where ClassFilter is the grenade base class. I then Iterate through the Overlapping array and return the nearest one to be picked up. What I’m finding in this particular situation is that these attached grenades do not show up in Overlapping list at all.

Here is a video of me doing some further testing with just a very basic skeletal mesh and a static mesh. No characters or anything involved to get in the way. As you can see, I can pull the grenade off the cube just fine, but when it’s attached to the skeletal mesh I cannot. Again, this behavior is ONLY on the server.

Why in the world would this be happening?

Based on what I’ve read in similar posts and in the code/documentation, it is not really recommended to calculate overlaps on objects attached to a kinematic skeletal mesh. I am assuming that for this reason the default behavior is to disable collision on the grenade when it’s attached. That theory sort of falls apart though when I look at the networked behavior. The fact that collision works properly on clients leads me to believe that the culprit is some non-replicated flag. There is no perceptible difference though in the properties of grenades stuck to static meshes and skeletal meshes, or between client and server. Is this a bug, or am I just missing something? This is one of the weirdest things I’ve encountered in a while.