The Resource Component: Harvestable Actors and the NavMesh Gotcha

A harvestable actor is just an actor with a Resource component on it. That one component makes a tree or a rock something a worker can detect, harvest over time, and use up. This card covers UResourceComponent and the two gotchas that trip up every first run: navigation and replication. It is part of Resource Gathering AI in Unreal Engine and pairs with the Gatherer component, which searches for it by tag.

A sphere that is also the timer

The component is a USphereComponent subclass, which is the neat trick: the same object is both the proximity trigger and the harvest logic.

UCLASS(Blueprintable, ClassGroup=(Custom), meta=(BlueprintSpawnableComponent))
class RESOURCEGATHERER_API UResourceComponent : public USphereComponent
{
    UPROPERTY(EditAnywhere, Category = "Resource Gathering Settings")
    FGameplayTag ResourceTag;

    UPROPERTY(ReplicatedUsing=OnRep_ExtractionsRemaining, EditAnywhere, Category = "Resource Gathering Settings")
    int32 ExtractionsRemaining = 3;

    UPROPERTY(EditAnywhere, Category = "Resource Gathering Settings")
    float ExtractTimeInSeconds = 4.f;
};

The sphere generates the overlap that the worker’s capsule reacts to, so set its radius a little larger than the mesh, otherwise the worker can run into the trunk without ever “arriving”. The ResourceTag is the label the worker’s search matches (see Assignments and Gameplay Tags). The two numbers are the whole economy of one node: how many times it can be harvested, and how long each harvest takes.

Extraction is a server timer

When the worker arrives and asks to harvest, the component starts a timer and hands back a delegate the Gatherer listens to:

bool UResourceComponent::TryBeginExtraction(FOnExtractionComplete*& OutExtractionDelegate)
{
    if (!GetOwner()->HasAuthority() || ExtractionsRemaining == 0) return false;

    GetWorld()->GetTimerManager().SetTimer(
        ExtractionTimerHandle, this, &UResourceComponent::CompleteExtraction,
        ExtractTimeInSeconds, false);

    OutExtractionDelegate = &OnExtractionComplete;
    return true;
}

This is why the Behavior Tree’s extract task has to wait: nothing has been harvested when TryBeginExtraction returns, only scheduled. The task keeps running until the delegate fires ExtractTimeInSeconds later.

When the timer fires, the component broadcasts completion, decrements the count, and destroys the actor if it is spent:

void UResourceComponent::CompleteExtraction()
{
    if (!OwnerActor || !OwnerActor->GetIsReplicated())
    {
        UE_LOG(LogTemp, Error, TEXT("Owner actor is not replicated; cannot destroy on clients."));
        return;
    }
    OnExtractionComplete.Broadcast();
    ExtractionsRemaining--;
    if (ExtractionsRemaining == 0) GetOwner()->Destroy();
}

The count is ReplicatedUsing=OnRep_ExtractionsRemaining, so clients also destroy the actor the moment it hits zero, keeping every machine’s world in sync.

Gotcha one: replication

Notice the guard at the top of CompleteExtraction. If the resource actor is not replicated, the component cannot tell clients to destroy it, so it refuses and logs. On the first run of a fresh setup this is the classic symptom: the worker reaches the tree, the extract task spins, and nothing finishes. The Output Log says exactly why, tick Replicates on the resource actor, and it works.

Gotcha two: navigation

The second trap is that the worker stops short of the resource and never overlaps it. The cause is navigation: by default a static mesh affects the NavMesh, so the walkable surface is carved out around your resource and the path ends just outside it. The search even filters resources by reachability:

if (!IsNavigablePath(GetOwner()->GetActorLocation(), Actor->GetActorLocation())) continue;

so a carved-out resource may not even be considered. The fix is to open the resource’s mesh and turn off Can Ever Affect Navigation, and to make sure the map has a NavMesh at all (press P in the viewport; the floor turns green).

What to take away

• UResourceComponent is a Sphere component, so one object is both the overlap trigger and the extraction timer. Size the sphere past the mesh.
• ExtractionsRemaining and ExtractTimeInSeconds set yield and pace; the replicated count destroys the actor on every machine when depleted.
• The two first-run failures are a non-replicated resource actor (extraction never finishes) and a mesh that affects navigation (the worker cannot reach it). The Output Log flags the first; the green NavMesh check flags the second.

Next, follow what spawns when extraction finishes in The carried resource, or read the full ownership story in Multiplayer. The packaged system is Resource Gathering Minions on FAB.