Creating e new Behaviour Tree composite (AtomsUnreal)

In this section, we are going to create a random sequence behaviour tree composite.

Create an `UAtomsBehaviourTreeCompositeNode`

In your project, create a new class UMyRandomSequenceCompositeNode and inherit from the UAtomsBehaviourTreeCompositeNode.

UCLASS()
class UMyRandomSequenceCompositeNode: public UAtomsBehaviourTreeCompositeNode 
{
	GENERATED_UCLASS_BODY()
	
};

You need to add these arguments to the UCLASS.

UCLASS(ClassGroup = (AtomsBehavioursTreeNode))
class UMyRandomSequenceCompositeNode: public UAtomsBehaviourTreeCompositeNode 
{
	GENERATED_UCLASS_BODY()
	
};

Implement the constructor setting the AtomsBehaviourModule member. This variable contains the name of the Atoms behaviour tree node that this unreal node creates.

UMyRandomSequenceCompositeNode::MyRandomSequenceAtomsCompositeNode(const FObjectInitializer& ObjectInitializer) : Super(ObjectInitializer)
{
  NodeName = "MyRandomSequence";
}

Implement the description string method.

Add properties

Now it is time to add some properties to the composite node. If you are going to use blackboard entries on these properties you need to use the FAtomsBehaviourTreeNode*Property struct that you can find inside “BehaviourTree/AtomsBehaviourTreeNodeProperty.h” header.

For this composite we need those 2 properties:

FAtomsBehaviourTreeNodeBoolProperty lockAgent; //used to lock the agent mutex when accessing the blackboard

FAtomsBehaviourTreeNodeIntProperty seed; // random seed

Property conversion

You need to implement now the GetAtomsAttribute method to pass the properties to the atoms node.

Create atoms node

The last step to do inside the unreal composite node is to implement the GenerateAtomsBehaviour method that creates the equivalent atoms node MyRandomSequenceAtomsCompositeNodewe are going to implement after this step.

Implement the Atoms composite class.

Our UMyRandomSequenceCompositeNode is just a description of a composite node, there is no logic inside, it is used to be able to expose properties used by a native Atoms composite node. Create a new MyRandomSequenceAtomsCompositeNode.

Add attributes to the Atoms composite class

like we did with the UMyRandomSequenceCompositeNode, we need to expose the same attributes here using the Atoms::Blackboard*Value classes. You don’t need to expose the lock attribute since it is already inside the base class.

We can now initialize the attributes from the tree and blackboard definition. You need to implement the setAttributes method. It’s here that the blackboard entries are connected to each attribute.

It is time to implement the node logic. For a composite node, you can implement these methods:

initialize: method called to initialize the node instance.
update: method called at every tick.
terminate: method called when the node returned success/failure.
releaseData: called before the destruction of this node instance. Clear here all data allocated on the heap.
onChildUpdated: called when a child changes its status.

For this wait node, we need to implement the initialize to store the init time and then the update to check the timer is elapsed.

For decorators and composites node always set at the initialization a SUSPENDED state. Because it needs to wait for for a child update to know if it need to switch to RUNNING/SUCCESS or FAILURE state.

Local storage

Each node instance has local storage of 8 bytes that you can use to store data. The storage is defined inside a union inside the State structure.

If you need to allocate more then 8 byte do something like this:

To release the allocated data call

Please remember to release the allocated data at least inside the releaseData method.

Access attribute values

To access an attribute value you need to use the context->getBlackboardValue() method. This method automatically returns a reference to the attribute value or to the blackboard entry connected to this attribute.

Those are the final files:

Create an UAtomsBehaviourTreeCompositeNode