Controlling an NPC's Behavior with Dynamic Behavior Subtrees

This page will give an overview of how to utilize the Dynamic Subtrees in our standard behavior trees to modify an NPCs behavior. This page will not go into depth on the function and use of Behavior Trees in general, nor will it go over how to create your own behavior trees.

Behavior Subtrees

In a Behavior Tree you are able to run another tree as part of your behavior. This can be done by either using a RunBehavior task or a RunBehaviorDynamic task. Functionally they are both the same under the hood during a trees execution, the difference is that the RunBehaviorDynamic task allows you to modify what tree you target at runtime.

Specifying a Subtree

Each RunBehaviorDynamic task has an Injection Tag field and a Default Behavior Asset field. The Example from the above selected task:

Injection Tags

The InjectionTag field is used to denote tasks that should use the same subtree when modified at runtime. For example if you have multiple locations in your tree where you want to enter a “Passive” subtree, you would tag each of those with the same InjectionTag . These tags are defined as Gameplay Tags and the default ones can be found in the NPCGameplayTagTable .

Default Behavior Assets

The DefaultBehaviorAsset field is used to describe the default tree that should be run, if the tree is not modified at runtime. This default will apply to any Controller which does not modify them.

Defining Defaults per Controller

Our existing AI Controllers have a DefaultDynamicBehaviorTrees which is used when an NPC begins to run its behavior to set their default dynamic subtrees. The DefaultDynamicBehaviorTrees field is a map pairing FGameplayTag keys to UBehaviorTree values. Here you can define any Injection Tags you want to modify upon startup for instances of this controller, with the associated UBehaviorTree value being the new subtree.

The following is an example using our HumanAIController blueprint:

In the case our NPC runs the BT_SimpleAttack Behavior Tree the Highlighted nodes will be modified, as their injection tags match those found in our Controller’s DefaultDynamicBehaviorTrees map.

Setting a Dynamic Subtree at Runtime

We do not only have to operate by modifying the defaults found in the fields of our Controller or Behavior Tree, we are also able to change the subtrees at runtime. All that is needed to do this is a reference to our NPC’s Controller.

Functions for Managing Dynamic Subtrees

ResetAllBehaviorSubtreesToDefault

• Will utilize our Controller’s DefaultDynamicBehaviorTrees to set any subtrees in our table to their defaults

ResetBehaviorSubtreesByTag

• Will consume a list of Injection Tags which it will search for in our Controller’s DefaultDynamicBehaviorTrees and set any matching tags to their defaults

SetBehaviorSubtree

• Will take in one Injection Tag and one Behavior Tree asset, and will set any subtrees matching the Injection Tag to the given Behavior Tree

  • The given Injection Tag does not need to exist in our Controller’s DefaultDynamicBehaviorTrees map

SetMultipleBehaviorSubtrees

• Consumes a map of Injection Tag keys and Behavior Tree values in the same format of the Controller’s DefaultDynamicBehaviorTrees field. Loops through these pairs and will call SetBehaviorSubtree for each one

Order Matters