Targeting

技能系統(tǒng)中很重要的一環(huán)就是目標(biāo)的選取，比如LOL中的各色技能。

在GameplayAbilities and You 中，將Targeting放在了Advanced話題中。我主要參考這篇文章來講GAS中的Targeting。
文中以Wait Target Data這個(gè)AbilityTask為例。這個(gè)task在GAS中有舉足輕重的地位，原因如：

• 提供系統(tǒng)用以可視化技能目標(biāo)選擇
• 提供玩家發(fā)送信息到服務(wù)器的框架(Client to Server)

需要注意，task右側(cè)的引腳在客戶端和服務(wù)端都有效，Valid Data Delegate都會(huì)調(diào)用。后端關(guān)于它的實(shí)現(xiàn)采取一種比較有趣的方式： task會(huì)生成actor（GameplayAbilityTargetActor）實(shí)例，而這些實(shí)例不是replicated。（應(yīng)該是客戶端和服務(wù)端都生成actor）實(shí)際上，服務(wù)端通過AbilitySystemComponent將數(shù)據(jù)發(fā)送給客戶端。這種方案導(dǎo)致targeting actor的設(shè)置有些奇特。（暫時(shí)我還沒get到）

Target Actor

/**
 * TargetActors are spawned to assist with ability targeting. They are spawned by ability tasks and create/determine the outgoing targeting data passed from one task to another
 *
 * WARNING: These actors are spawned once per ability activation and in their default form are not very efficient
 * For most games you will need to subclass and heavily modify this actor, or you will want to implement similar functions in a game-specific actor or blueprint to avoid actor spawn costs
 * This class is not well tested by internal games, but it is a useful class to look at to learn how target replication occurs
 */

我們通過繼承AGameplayAbilityTargetActor創(chuàng)建自定義的TargetActor。其中，有兩個(gè)主要的函數(shù)需要我們重寫：

• virtual void StartTargeting(UGameplayAbility* Ability) override
• virtual void ConfirmTargetingAndContinue() override

StartTarget

在這里，你可以訪問Ability實(shí)例，因此你可以使用Ability類里的數(shù)據(jù)。比如：假如你有一個(gè)造墻的技能，那么在目標(biāo)選擇時(shí)，需要從Ability實(shí)例中得到墻的mesh數(shù)據(jù)，這樣TargetActor才能正確顯示墻體指示器。Ability實(shí)例中還包含釋放該技能的Character引用，因此TargetActor還能訪問到Character的信息（人物的tag和attribute可能影響targeting）。

ConfirmTargetingAndContinue

這里有些較難理解的地方，但是我們化繁為簡(jiǎn)，這個(gè)函數(shù)最核心的功能是調(diào)用TargetDataReadyDelegate，同時(shí)傳遞攜帶包含我們target data的負(fù)載。因此，如果我們想傳遞兩個(gè)transforms，分別包含技能施放者位置和目標(biāo)物體位置，那么代碼如下：

// 繼承自FGameplayAbilityTargetData
FGameplayAbilityTargetData_LocationInfo *ReturnData = new FGameplayAbilityTargetData_LocationInfo();

// Source Transform
ReturnData->SourceLocation.LocationType = EGameplayAbilityTargetingLocationType::LiteralTransform;
ReturnData->SourceLocation.LiteralTransform = FTransform(SourceLocation);
// Destination Transform
ReturnData->TargetLocation.LocationType = EGameplayAbilityTargetingLocationType::LiteralTransform;
ReturnData->TargetLocation.LiteralTransform = FTransform((TargetLocation - SourceLocation).ToOrientationQuat(), TargetLocation);

// Handle !! 這就是我們?yōu)槭裁磏ew了卻不要delete的原因，里面用了智能指針
FGameplayAbilityTargetDataHandle Handle(ReturnData);
// Fire delegate with data handle !!!
TargetDataReadyDelegate.Broadcast(Handle);

關(guān)鍵的數(shù)據(jù)結(jié)構(gòu)是FGameplayAbilityTargetData（你的TargetingSystem運(yùn)行后最終的結(jié)果數(shù)據(jù)），引擎里已經(jīng)提供了一系列的子類用于不同的需求：

• FGameplayAbilityTargetData_LocationInfo ：包含最多兩個(gè)位置信息數(shù)據(jù)
• FGameplayAbilityTargetData_ActorArray ： 包含一個(gè)Actor數(shù)組
• FGameplayAbilityTargetData_SingleTargetHit ： 包含一個(gè)碰撞結(jié)果（HitResult）

特別注意： 這個(gè)方法是將數(shù)據(jù)從Client端發(fā)往Server端。雖然Server端也有一個(gè)版本的數(shù)值，但它并不準(zhǔn)確。因?yàn)槭菑腃lient端發(fā)往Server端，所以數(shù)據(jù)有被用戶篡改的風(fēng)險(xiǎn)，記得要添加驗(yàn)證。

接下來我們自定義一個(gè)FGameplayAbilityTargetData類，它包含兩個(gè)location，一個(gè)float，一個(gè)int。這個(gè)數(shù)據(jù)類依舊是用于造墻技能，只是這次，我們用鼠標(biāo)按下的時(shí)間（float）來決定墻的高度。代碼如下：

USTRUCT(BlueprintType)
struct FGameplayAbilityCastingTargetingLocationInfo: public FGameplayAbilityTargetData
{
    GENERATED_USTRUCT_BODY()

        /**
          如果沒記錯(cuò)，屬性需要標(biāo)記UPROPERTY()，才能被序列化
        */

    /** Amount of time the ability has been charged */
    UPROPERTY(BlueprintReadWrite, EditAnywhere, Category = Targeting)
    float ChargeTime;

    /** The ID of the Ability that is performing targeting */
    UPROPERTY()
    uint32 UniqueID;

    /** Generic location data for source */
    UPROPERTY(BlueprintReadWrite, EditAnywhere, Category = Targeting)
    FGameplayAbilityTargetingLocationInfo SourceLocation;

    /** Generic location data for target */
    UPROPERTY(BlueprintReadWrite, EditAnywhere, Category = Targeting)
    FGameplayAbilityTargetingLocationInfo TargetLocation;

    // -------------------------------------

    virtual bool HasOrigin() const override
    {
        return true;
    }

    virtual FTransform GetOrigin() const override
    {
        return SourceLocation.GetTargetingTransform();
    }

    // -------------------------------------

    virtual bool HasEndPoint() const override
    {
        return true;
    }

    virtual FVector GetEndPoint() const override
    {
        return TargetLocation.GetTargetingTransform().GetLocation();
    }

    virtual FTransform GetEndPointTransform() const override
    {
        return TargetLocation.GetTargetingTransform();
    }

    // -------------------------------------

    virtual UScriptStruct* GetScriptStruct() const override
    {
        return FGameplayAbilityCastingTargetingLocationInfo::StaticStruct();
    }

    virtual FString ToString() const override
    {
        return TEXT("FGameplayAbilityCastingTargetingLocationInfo");
    }

    bool NetSerialize(FArchive& Ar, class UPackageMap* Map, bool& bOutSuccess);
};

/**  這一部分具體的運(yùn)作機(jī)制不明，但必須要有 */
template<>
struct TStructOpsTypeTraits<FGameplayAbilityCastingTargetingLocationInfo> : public TStructOpsTypeTraitsBase2<FGameplayAbilityCastingTargetingLocationInfo>
{
    enum
    {
        WithNetSerializer = true    // For now this is REQUIRED for FGameplayAbilityTargetDataHandle net serialization to work
    };
};

NetSerialize的實(shí)現(xiàn)：

bool FGameplayAbilityCastingTargetingLocationInfo::NetSerialize(FArchive& Ar, class UPackageMap* Map, bool& bOutSuccess)
{
    SourceLocation.NetSerialize(Ar, Map, bOutSuccess);
    TargetLocation.NetSerialize(Ar, Map, bOutSuccess);

    Ar << ChargeTime;
    Ar << UniqueID;

    bOutSuccess = true;
    return true;
}

最后，正如官方頭文件所述，TargetActor默認(rèn)是每執(zhí)行一次Ability都會(huì)創(chuàng)建，然后銷毀。為了提高性能，要考慮重用TargetActor。當(dāng)然，你甚至可以不使用TargetActor，ActionRPG中就是這樣。對(duì)于造墻這種技能，墻體指示器可能要跟隨玩家的移動(dòng)，所以你需要保存在StartTarget中獲得Character的指針或引用，然后在TargetActor的Tick函數(shù)中獲取Character的位置信息來更新墻體指示器的位置。

RPGTargetType.h/cpp

我們終于可以回到ActionRPG項(xiàng)目上來了。很不幸的是，前面說了一大段的知識(shí)點(diǎn)。這個(gè)項(xiàng)目并沒有用到，而是另辟蹊徑。

頭文件：

// Copyright 1998-2019 Epic Games, Inc. All Rights Reserved.

#pragma once

#include "ActionRPG.h"
#include "Abilities/GameplayAbilityTypes.h"
#include "Abilities/RPGAbilityTypes.h"
#include "RPGTargetType.generated.h"

class ARPGCharacterBase;
class AActor;
struct FGameplayEventData;

/**
 * Class that is used to determine targeting for abilities
 * It is meant to be blueprinted to run target logic
 * This does not subclass GameplayAbilityTargetActor because this class is never instanced into the world
 * This can be used as a basis for a game-specific targeting blueprint
 * If your targeting is more complicated you may need to instance into the world once or as a pooled actor
 */
UCLASS(Blueprintable, meta = (ShowWorldContextPin))
class ACTIONRPG_API URPGTargetType : public UObject
{
    GENERATED_BODY()

public:
    // Constructor and overrides
    URPGTargetType() {}

    /** Called to determine targets to apply gameplay effects to */
    UFUNCTION(BlueprintNativeEvent)
    void GetTargets(ARPGCharacterBase* TargetingCharacter, AActor* TargetingActor, FGameplayEventData EventData, TArray<FHitResult>& OutHitResults, TArray<AActor*>& OutActors) const;
};

/** Trivial target type that uses the owner */
UCLASS(NotBlueprintable)
class ACTIONRPG_API URPGTargetType_UseOwner : public URPGTargetType
{
    GENERATED_BODY()

public:
    // Constructor and overrides
    URPGTargetType_UseOwner() {}

    /** Uses the passed in event data */
    virtual void GetTargets_Implementation(ARPGCharacterBase* TargetingCharacter, AActor* TargetingActor, FGameplayEventData EventData, TArray<FHitResult>& OutHitResults, TArray<AActor*>& OutActors) const override;
};

/** Trivial target type that pulls the target out of the event data */
UCLASS(NotBlueprintable)
class ACTIONRPG_API URPGTargetType_UseEventData : public URPGTargetType
{
    GENERATED_BODY()

public:
    // Constructor and overrides
    URPGTargetType_UseEventData() {}

    /** Uses the passed in event data */
    virtual void GetTargets_Implementation(ARPGCharacterBase* TargetingCharacter, AActor* TargetingActor, FGameplayEventData EventData, TArray<FHitResult>& OutHitResults, TArray<AActor*>& OutActors) const override;
};

源文件：

// Copyright 1998-2019 Epic Games, Inc. All Rights Reserved.

#include "Abilities/RPGTargetType.h"
#include "Abilities/RPGGameplayAbility.h"
#include "RPGCharacterBase.h"

void URPGTargetType::GetTargets_Implementation(ARPGCharacterBase* TargetingCharacter, AActor* TargetingActor, FGameplayEventData EventData, TArray<FHitResult>& OutHitResults, TArray<AActor*>& OutActors) const
{
    return;
}

void URPGTargetType_UseOwner::GetTargets_Implementation(ARPGCharacterBase* TargetingCharacter, AActor* TargetingActor, FGameplayEventData EventData, TArray<FHitResult>& OutHitResults, TArray<AActor*>& OutActors) const
{
    OutActors.Add(TargetingCharacter);
}

void URPGTargetType_UseEventData::GetTargets_Implementation(ARPGCharacterBase* TargetingCharacter, AActor* TargetingActor, FGameplayEventData EventData, TArray<FHitResult>& OutHitResults, TArray<AActor*>& OutActors) const
{
    const FHitResult* FoundHitResult = EventData.ContextHandle.GetHitResult();
    if (FoundHitResult)
    {
        OutHitResults.Add(*FoundHitResult);
    }
    else if (EventData.Target)
    {
        OutActors.Add(const_cast<AActor*>(EventData.Target));
    }
}

不同于TargetActor，這里定義的TargetType（繼承自UObject）不會(huì)被也不能實(shí)例化到World中（Actor才能被放置到World中）。