Behavior Tree (2. Decorator Nodes)

What Are Decorator Nodes in Unreal Engine?

Can be attached to a Composite Node or task node as a gate keeper

Define if a branch/ Node can be executed base on certain conditions

 

• Execution Context: If a decorator's condition evaluates to true, the child node is allowed to execute. If false, the child node is skipped.
• Customization: Decorators can be either prebuilt (provided by Unreal Engine) or custom-built in C++/Blueprint.

 

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Common Use Cases

1. Conditional Execution:
   • Example: Run a task only if the AI has detected the player or is within a certain range.
   • Decorators like Blackboard-Based Condition are frequently used.
2. Behavior Modification:
   • Example: Retry a task multiple times or invert the success/failure status.
   • Decorators like Retry or Invert Result help refine AI logic.
3. Runtime Checks:
   • Example: Ensure a specific resource is available before proceeding with a task.
   • Decorators evaluate conditions dynamically during execution.

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Built-in Decorator Nodes

Unreal Engine provides several out-of-the-box decorator nodes:

1. Blackboard-Based Condition:
   • Allows you to check a specific value in the Blackboard (AI's shared memory).
   • Example: Ensure the TargetActor key is set before running a task.
2. Cooldown:
   • Prevents a node from being executed too frequently by enforcing a cooldown period.
3. Force Success/Failure:
   • Overrides the result of a child node to always succeed or fail.
4. Loop/Retry:
   • Repeats a child node for a specified number of iterations or until a condition is met.

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Custom Decorator Nodes

You can create custom decorator nodes using Blueprints or C++.

Blueprint-Based Custom Decorator

1. Go to the Content Browser.
2. Create a new Blueprint Class and select BTDecorator.
3. Implement the logic for:
   • PerformConditionCheck: Define the condition that determines whether the child node executes.
   • Optional: Add variables and functionality to make the decorator reusable.

C++ Custom Decorator

#include "BehaviorTree/Decorators/BTDecorator_BlueprintBase.h"

UCLASS()
class YOURGAME_API UCustomDecorator : public UBTDecorator_BlueprintBase
{
    GENERATED_BODY()

protected:
    virtual bool CalculateRawConditionValue(UBehaviorTreeComponent& OwnerComp, uint8* NodeMemory) const override
    {
        // Example: Check if AI has more than 50 health
        AAIController* AIController = OwnerComp.GetAIOwner();
        APawn* Pawn = AIController->GetPawn();
        if (Pawn)
        {
            float Health = Pawn->GetHealth();
            return Health > 50.0f;
        }
        return false;
    }
};

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How to Use a Decorator Node

1. Open your Behavior Tree in the Behavior Tree Editor.
2. Add a task or composite node (like a Selector or Sequence).
3. Right-click on the task or composite node and add a Decorator Node.
4. Configure the decorator in the Details Panel (e.g., select Blackboard keys, set conditions).

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Example Scenario: Patrolling AI

1. Selector Node:
   • Chooses between patrolling or chasing the player.
2. Decorator (Blackboard-Based Condition):
   • On "Chase Player" task: Check if the Blackboard key CanSeePlayer is true.
3. Cooldown Decorator:
   • On "Patrol" task: Limit how often the AI can switch waypoints.

By using decorator nodes, you can create AI that reacts dynamically to its environment and player actions.