Unreal Engine

In the Unreal Engine, when building an animal AI system, the core difficulty often lies in the underlying state management and interface abstraction, rather than the behavior tree itself.

The base class plus subclass override pattern is a typical data-driven design in UE; after BP_Wolf and BP_Snake inherit from BP_AnimalBase, they only modify default values without changing logic, thereby achieving behavioral diversity while keeping a single code implementation.

The variables marked as replicated in the article (AnimalState, CurrentHealth, bIsDeath, TargetActor) are synchronized from the server to clients in multiplayer games, while static configurations such as MaxHealth and WalkSpeed do not need replication in order to save bandwidth.

Unreal comes with the ApplyDamage and AnyDamage event systems, and BPI_Damageable is essentially an object-oriented encapsulation of them; the benefit is that interface calls do not depend on UGameplayStatics' global functions, the boundaries of responsibility are clearer, and it is also easier to unit test and swap implementations.

The BPI_ prefix in Unreal indicates that this is a Blueprint Interface; it only declares function signatures and contains no implementation, and any blueprint class implementing this interface must fill in the logic itself, similar to a C++ pure virtual class but more lightweight.

The Fleeing state is often used together with EQS (Environment Query System), calculating a threat value by querying the number and distance of surrounding hostile Actors, and switching state once it exceeds the threshold.

Roaming is usually implemented with the NavMesh and the MoveToRandomLocation node; if the scene has not baked a NavMesh, the animal will get stuck in place.

In Unreal's AI system, enums such as E_AnimalState usually do not directly drive behavior, but instead serve as the core data of blackboard keys, which the behavior tree reads to decide which branch to execute.

The core state enum E_AnimalState defines six behavioral states, but actually lists seven in total: Idle, Roaming, Investigating, Chasing, Attacking, Fleeing, and Death.

The developer created the AI_Animals directory structure under the AI folder, containing subfolders such as Base, Interfaces, Enums, and Structures, dividing the ownership of AI assets by responsibility.