GameplayAbilitySystem

从这篇文章开始，我们将目光转向Gameplay Ability System。
推荐最新的关于GAS的视频，你也能下载到ppt：
unreal-fest-europe-2019/using-the-gameplay-ability-system
借用其中Concept部分，可以从整体上认知GAS中各个组件的作用。

AbilitySystemComponent

我们回到AbilitySystemComponent上。从上面的说明我们已经可以猜测到AbilitySystemComponent是一个庞大的类，"in charge of managing everything GAS related inside the actor"。事实上，的确如此。AbilitySystemComponent.h有1586行代码。看看官方给出对于AbilitySystemComponent的说明：

/** 
 *  UAbilitySystemComponent 
 *
 *  A component to easily interface with the 3 aspects of the AbilitySystem:
 *  
 *  GameplayAbilities:
 *      -Provides a way to give/assign abilities that can be used (by a player or AI for example)
 *      -Provides management of instanced abilities (something must hold onto them)
 *      -Provides replication functionality
 *          -Ability state must always be replicated on the UGameplayAbility itself, but UAbilitySystemComponent provides RPC replication
 *          for the actual activation of abilities
 *          
 *  GameplayEffects:
 *      -Provides an FActiveGameplayEffectsContainer for holding active GameplayEffects
 *      -Provides methods for applying GameplayEffects to a target or to self
 *      -Provides wrappers for querying information in FActiveGameplayEffectsContainers (duration, magnitude, etc)
 *      -Provides methods for clearing/remove GameplayEffects
 *      
 *  GameplayAttributes
 *      -Provides methods for allocating and initializing attribute sets
 *      -Provides methods for getting AttributeSets
 *  
 */

AbilitySystemComponent提供了丰富的“method”来与Ability、Effect、Attribute交互，随着我们深入GAS，会接触到。

URPGGameAbilitySystemComponent.h

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

#pragma once

#include "ActionRPG.h"
#include "AbilitySystemComponent.h"
#include "Abilities/RPGAbilityTypes.h"
#include "RPGAbilitySystemComponent.generated.h"

class URPGGameplayAbility;

/**
 * Subclass of ability system component with game-specific data
 * Most games will need to make a game-specific subclass to provide utility functions
 */
UCLASS()
class ACTIONRPG_API URPGAbilitySystemComponent : public UAbilitySystemComponent
{
    GENERATED_BODY()

public:
    // Constructors and overrides
    URPGAbilitySystemComponent();

    /** Returns a list of currently active ability instances that match the tags */
       /**   返回匹配tag的所有激活的ability实例**/
    void GetActiveAbilitiesWithTags(const FGameplayTagContainer& GameplayTagContainer, TArray<URPGGameplayAbility*>& ActiveAbilities);

    /** Returns the default level used for ability activations, derived from the character */
      /** 获得当前释放技能的默认等级，从人物角色上获得(这个游戏设定，应该是人物等级==技能等级) */
    int32 GetDefaultAbilityLevel() const;

    /** Version of function in AbilitySystemGlobals that returns correct type */
       /** 使用 AbilitySystemGlobals 类的方法，返回特定actor实例上的abilitysystemcomponent实例 */
    static URPGAbilitySystemComponent* GetAbilitySystemComponentFromActor(const AActor* Actor, bool LookForComponent = false);

};

拓展了三个方法，具体作用看注释。

URPGGameAbilitySystemComponent.cpp

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

#include "Abilities/RPGAbilitySystemComponent.h"
#include "RPGCharacterBase.h"
#include "Abilities/RPGGameplayAbility.h"
#include "AbilitySystemGlobals.h"

URPGAbilitySystemComponent::URPGAbilitySystemComponent() {}

void URPGAbilitySystemComponent::GetActiveAbilitiesWithTags(const FGameplayTagContainer& GameplayTagContainer, TArray<URPGGameplayAbility*>& ActiveAbilities)
{
      /** FGameplayAbilitySpec是对gameplay ability的包装，还包含了ability运行时必要的信息 */
    TArray<FGameplayAbilitySpec*> AbilitiesToActivate;

          /** UAbilitySystemComponent的方法，获得所有可激活的ability specification*/
    GetActivatableGameplayAbilitySpecsByAllMatchingTags(GameplayTagContainer, AbilitiesToActivate, false);

    // Iterate the list of all ability specs
    for (FGameplayAbilitySpec* Spec : AbilitiesToActivate)
    {
        // Iterate all instances on this ability spec
        TArray<UGameplayAbility*> AbilityInstances = Spec->GetAbilityInstances();

        for (UGameplayAbility* ActiveAbility : AbilityInstances)
        {
            ActiveAbilities.Add(Cast<URPGGameplayAbility>(ActiveAbility));
        }
    }
}

int32 URPGAbilitySystemComponent::GetDefaultAbilityLevel() const
{
    ARPGCharacterBase* OwningCharacter = Cast<ARPGCharacterBase>(OwnerActor);

    if (OwningCharacter)
    {
          /** 获得人物等级 */
        return OwningCharacter->GetCharacterLevel();
    }
    return 1;
}

URPGAbilitySystemComponent* URPGAbilitySystemComponent::GetAbilitySystemComponentFromActor(const AActor* Actor, bool LookForComponent)
{
      /** 使用UAbilitySystemGlobals的方法，对结果进行cast */
    return Cast<URPGAbilitySystemComponent>(UAbilitySystemGlobals::GetAbilitySystemComponentFromActor(Actor, LookForComponent));
}

现在，我们回顾一下。发现三个新添加的get方法都是跟actor有关：

• GetActiveAbilitiesWithTags： 获得actor（owner）所有与tag匹配的ability实例
• GetDefaultAbilityLevel：获得character（owner）的等级
• GetAbilitySystemComponentFromActor： 获得任意Actor实例挂载的AbilitySystemComponent实例