UAbilityTask

UAbilityTask继承自UGameplayTask（UGameplayTask可以用来写一些行为树中的一些节点），可以用来实现一些异步功能。比如播放montage后各种事件的处理。

你可以去GameplayAbilities\Public\Abilities\Tasks\目录下寻找作者编写的task类作为相关参考，也可以直接使用（GameplayTasks目录下的案例比较少）。

当然我更加推荐学习actionRPG项目中的PlayMontageAndWaitForEvent，原因有：1、这个task用得最多2、涉及Task的代码相对较多。

其他推荐学习的：UGameplayTask_WaitDelay、UAbilityTask_WaitGameplayEvent、UAbilityTask_WaitGameplayTagAdded、UAbilityTask_WaitGameplayEffectApplied

大致过程

声明多个动态多播委托用于处理各种事件。
重写所需的虚函数，并且声明相关变量。
编写主体函数。

代码分析

在PlayMontageAndWaitForEvent中重写了4个虚函数：

//用于在各种委托设置完毕后开始执行真正的Tasks。
virtual void Activate() override;

//从外部取消这个Tasks，默认情况下，会结束任务。
virtual void ExternalCancel() override;

//返回debug字符串，内容为当前播放的Montage名称以及Tasks存储的Montage名称
virtual FString GetDebugString() const override;

//结束并清理Tasks，既可以在Tasks内部调用，可以从该Tasks拥有者调用。
//注意：请不要直接调用该函数，你应该调用EndTask()或者TaskOwnerEnded()
//注意：重写该函数时，请确保最后调用Super::OnDestroy(bOwnerFinished)
virtual void OnDestroy(bool AbilityEnded) override;

Activate()

void URPGAbilityTask_PlayMontageAndWaitForEvent::Activate()
{
    if (Ability == nullptr)
    {
        return;
    }

    bool bPlayedMontage = false;
    URPGAbilitySystemComponent* RPGAbilitySystemComponent = GetTargetASC();

    if (RPGAbilitySystemComponent)
    {
        const FGameplayAbilityActorInfo* ActorInfo = Ability->GetCurrentActorInfo();
        UAnimInstance* AnimInstance = ActorInfo->GetAnimInstance();
        if (AnimInstance != nullptr)
        {
            //绑定事件回调函数
            EventHandle = RPGAbilitySystemComponent->AddGameplayEventTagContainerDelegate(EventTags, FGameplayEventTagMulticastDelegate::FDelegate::CreateUObject(this, &URPGAbilityTask_PlayMontageAndWaitForEvent::OnGameplayEvent));

            //播放montage
            if (RPGAbilitySystemComponent->PlayMontage(Ability, Ability->GetCurrentActivationInfo(), MontageToPlay, Rate, StartSection) > 0.f)
            {
                //播放Montage后，其回调函数可能会导致Ability结束，所以我们需要提前结束
                if (ShouldBroadcastAbilityTaskDelegates() == false)
                {
                    return;
                }
                //绑定OnAbilityCancelled
                CancelledHandle = Ability->OnGameplayAbilityCancelled.AddUObject(this, &URPGAbilityTask_PlayMontageAndWaitForEvent::OnAbilityCancelled);
                //绑定OnMontageBlendingOut
                BlendingOutDelegate.BindUObject(this, &URPGAbilityTask_PlayMontageAndWaitForEvent::OnMontageBlendingOut);
                AnimInstance->Montage_SetBlendingOutDelegate(BlendingOutDelegate, MontageToPlay);
                //绑定OnMontageEnded
                MontageEndedDelegate.BindUObject(this, &URPGAbilityTask_PlayMontageAndWaitForEvent::OnMontageEnded);
                AnimInstance->Montage_SetEndDelegate(MontageEndedDelegate, MontageToPlay);

                ACharacter* Character = Cast<ACharacter>(GetAvatarActor());
                if (Character && (Character->Role == ROLE_Authority ||
                                  (Character->Role == ROLE_AutonomousProxy && Ability->GetNetExecutionPolicy() == EGameplayAbilityNetExecutionPolicy::LocalPredicted)))
                {
                    Character->SetAnimRootMotionTranslationScale(AnimRootMotionTranslationScale);
                }

                bPlayedMontage = true;
            }
        }
        else
        {
            ABILITY_LOG(Warning, TEXT("URPGAbilityTask_PlayMontageAndWaitForEvent call to PlayMontage failed!"));
        }
    }
    else
    {
        ABILITY_LOG(Warning, TEXT("URPGAbilityTask_PlayMontageAndWaitForEvent called on invalid AbilitySystemComponent"));
    }
    //播放失败处理
    if (!bPlayedMontage)
    {
        ABILITY_LOG(Warning, TEXT("URPGAbilityTask_PlayMontageAndWaitForEvent called in Ability %s failed to play montage %s; Task Instance Name %s."), *Ability->GetName(), *GetNameSafe(MontageToPlay),*InstanceName.ToString());
        if (ShouldBroadcastAbilityTaskDelegates())
        {
            OnCancelled.Broadcast(FGameplayTag(), FGameplayEventData());
        }
    }

    SetWaitingOnAvatar();
}

ExternalCancel()

check(AbilitySystemComponent);
OnAbilityCancelled();
Super::ExternalCancel();

OnAbilityCancelled的代码

if (StopPlayingMontage())
{
    // Let the BP handle the interrupt as well
    if (ShouldBroadcastAbilityTaskDelegates())
    {
        OnCancelled.Broadcast(FGameplayTag(), FGameplayEventData());
    }
}

OnDestroy()

void URPGAbilityTask_PlayMontageAndWaitForEvent::OnDestroy(bool AbilityEnded)
{
    // Note: Clearing montage end delegate isn't necessary since its not a multicast and will be cleared when the next montage plays.
    // (If we are destroyed, it will detect this and not do anything)

    // This delegate, however, should be cleared as it is a multicast
    if (Ability)
    {
        Ability->OnGameplayAbilityCancelled.Remove(CancelledHandle);
        if (AbilityEnded && bStopWhenAbilityEnds)
        {
            //停止播放Montage
            StopPlayingMontage();
        }
    }

    URPGAbilitySystemComponent* RPGAbilitySystemComponent = GetTargetASC();
    if (RPGAbilitySystemComponent)
    {
        //移除事件绑定
        RPGAbilitySystemComponent->RemoveGameplayEventTagContainerDelegate(EventTags, EventHandle);
    }
    //这句必须放在最后
    Super::OnDestroy(AbilityEnded);
}

PlayMontageAndWaitForEvent

PlayMontageAndWaitForEvent是Tasks的主体函数。

URPGAbilityTask_PlayMontageAndWaitForEvent* URPGAbilityTask_PlayMontageAndWaitForEvent::PlayMontageAndWaitForEvent(UGameplayAbility* OwningAbility,
    FName TaskInstanceName, UAnimMontage* MontageToPlay, FGameplayTagContainer EventTags, float Rate, FName StartSection, bool bStopWhenAbilityEnds, float AnimRootMotionTranslationScale)
{
    //用于缩放GAS tasks变量的工具函数，此为非shipping功能，用于交互调试。
    UAbilitySystemGlobals::NonShipping_ApplyGlobalAbilityScaler_Rate(Rate);

    //使用NewAbilityTask来创建Tasks，并且设置各个变量。
    URPGAbilityTask_PlayMontageAndWaitForEvent* MyObj = NewAbilityTask<URPGAbilityTask_PlayMontageAndWaitForEvent>(OwningAbility, TaskInstanceName);
    MyObj->MontageToPlay = MontageToPlay;
    MyObj->EventTags = EventTags;
    MyObj->Rate = Rate;
    MyObj->StartSection = StartSection;
    MyObj->AnimRootMotionTranslationScale = AnimRootMotionTranslationScale;
    MyObj->bStopWhenAbilityEnds = bStopWhenAbilityEnds;

    return MyObj;
}