[C++/UE] UE5中AI開發的一點功能梳理（一） - 行為樹創建

首先，這裡先放幾條在研究過程中得到特別大幫助的一些參考資料鏈接：

Artificial Intelligence in Unreal Engine 5.3 https://dev.epicgames.com/documentation/en-us/unreal-engine/artificial-intelligence-in-unreal-engine
Unreal Engine 5 Tutorial – AI https://www.youtube.com/watch?v=IDZh0epFTRY&list=PL4G2bSPE_8uklDwraUCMKHRk2ZiW29R6e&index=2
UE4源码-AI感知系统AIPerception https://zhuanlan.zhihu.com/p/569297977
ue4 自定义行为树的Service， Task，Decorator节点 https://blog.csdn.net/u010385624/article/details/89339958
Navigation Modifiers and Links in UE4 https://www.vikram.codes/blog/ai/02-nav-modifiers-links
UE的AI基础（2）EQS https://blog.csdn.net/qq_45617648/article/details/130938339
UE4场景询问系统浅析（EQS与行为树） https://www.uejoy.com/?p=500

行為樹 Behavior Tree

事前準備

創建繼承了AI Controller父類的藍圖

創建AI角色（Character）
- 在Character -> Pawn設置中進行AI Controller Class和 Auto Possess AI的配置

參數說明
- AI Controller Class -> 這個角色對應的繼承了AI Controller父類的藍圖
- Auto Possess AI -> 指的是AI Controller Class要怎樣接管Character（AI Controller接管了Character才能使AI生效）
  - Placed in World：一開始就放在場景的Character會被自動接管
    - 如果Character後面被卸載了，再重新生成出來，就不會再被AI Controller Class接管
  - Spawned：場景加載完之後，再被生成出來的Character會被自動接管，也就是說一開始就放置在場景裡的Character不會被AI Controller Class接管
  - Placed in World or Spawned：只要Character被創建出來就自動接管
  - Disable -> 不自動接管，需要在AI Controller裡手動接管
    - AI Controller裡使用Possess節點（Server Only），接受一個接管對象參數

//Pawn.cpp

void APawn::PostInitializeComponents()
{
    QUICK_SCOPE_CYCLE_COUNTER(STAT_Pawn_PostInitComponents);

    Super::PostInitializeComponents();
    
    if (IsValid(this))
    {
        UWorld* World = GetWorld();

        // Automatically add Controller to AI Pawns if we are allowed to.
        if (AutoPossessPlayer == EAutoReceiveInput::Disabled
            && AutoPossessAI != EAutoPossessAI::Disabled && Controller == nullptr && GetNetMode() != NM_Client
#if WITH_EDITOR
            && (GIsEditor == false || World->IsGameWorld())
#endif // WITH_EDITOR
            )
        {
            const bool bPlacedInWorld = (World->bStartup);
            if ((AutoPossessAI == EAutoPossessAI::PlacedInWorldOrSpawned) ||
                (AutoPossessAI == EAutoPossessAI::PlacedInWorld && bPlacedInWorld) ||
                (AutoPossessAI == EAutoPossessAI::Spawned && !bPlacedInWorld))
            {
                SpawnDefaultController();
            }
        }

        // update movement component's nav agent values
        UpdateNavAgent();
    }
}

void APawn::SpawnDefaultController()
{
    if ( Controller != nullptr || GetNetMode() == NM_Client)
    {
        return;
    }

    if (AIControllerClass != nullptr)
    {
        FActorSpawnParameters SpawnInfo;
        SpawnInfo.Instigator = GetInstigator();
        SpawnInfo.SpawnCollisionHandlingOverride = ESpawnActorCollisionHandlingMethod::AlwaysSpawn;
        SpawnInfo.OverrideLevel = GetLevel();
        SpawnInfo.ObjectFlags |= RF_Transient;    // We never want to save AI controllers into a map
        AController* NewController = GetWorld()->SpawnActor<AController>(AIControllerClass, GetActorLocation(), GetActorRotation(), SpawnInfo);
        if (NewController != nullptr)
        {
            // if successful will result in setting this->Controller 
            // as part of possession mechanics
            NewController->Possess(this);
        }
    }
}