How understand AI Behavior tree system?

Development World Creation
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1

Hello!
Recently I got UE 4. I need to understand Behavior tree system on code level but I cant learn all specific of UE 4. Please, help me! What is blackboard in AI? How work UBehaviorTree? What is UBTDecorator?

2

There is an introduction here.
Also, your timing is good. There is a live stream dedicated to behavior trees starting in a few hours. See here.

3

The intro is not code level. I will see the Stream about Behavior Tree.

4

There is a forum thread on Behavior Trees. Don’t read the first post extensively, though - it has little to do with BT.
Behavior Tree Tutorial
Also, there are some helpful pieces of code from Lucasz here.
That being said, the introduction to behavior trees does a good job explaining the general logic and terms (like Decorator) behind them. Once you have your tasks implemented in code, you can easily combine them in behavior tree editor. In a nutshell, a behavior tree (BT) is a logic sequence of tasks. Blackboard is a collection of variables which is shared between BT nodes. Here is a simple task to get you started with blackboard variables.

//.h
UCLASS()
class UBTTask_WanderAround : public UBTTask_BlackboardBase
{	GENERATED_BODY()
public:
	virtual EBTNodeResult::Type ExecuteTask(UBehaviorTreeComponent* OwnerComp, uint8* NodeMemory) override;
};

//.cpp
EBTNodeResult::Type UBTTask_WanderAround::ExecuteTask(class UBehaviorTreeComponent* OwnerComp, uint8* NodeMemory)
{ 
	UNavigationSystem* NavSys = UNavigationSystem::GetCurrent(OwnerComp);
	AAIController* MyAI = Cast<AAIController>(OwnerComp->GetOwner());
	if (NavSys && MyAI && MyAI->GetPawn())
	{
		const float SearchRadius = 1000.0f;
		FNavLocation RandomPt;
		const bool bFound = NavSys->GetRandomPointInRadius(MyAI->GetPawn()->GetActorLocation(), SearchRadius, RandomPt);
		if (bFound)
		{
			//Set a blackboard variable bound to this task by a key
			OwnerComp->GetBlackboardComponent()->SetValueAsVector(GetSelectedBlackboardKey(), RandomPt.Location);
			return EBTNodeResult::Succeeded;
		}
	}

	return EBTNodeResult::Failed; //there are plenty of other useful results; see declaration
}