Understanding Unreal Engine’s Gameplay Ability System

The Gameplay Ability System (GAS) is one of Unreal Engine’s most powerful yet complex frameworks for implementing character abilities, status effects, and interconnected game mechanics. This tutorial will guide you through the fundamentals of GAS and how to leverage it effectively in your projects.

What is the Gameplay Ability System?

At its core, GAS is a flexible framework for implementing:

Abilities: Actions that characters can perform (attacks, spells, skills)
Attributes: Stats that can be modified (health, mana, stamina)
Effects: Temporary or permanent modifications to attributes or gameplay (buffs, debuffs)
Cooldowns, Costs, and Tags: Systems for managing ability timing and requirements

Originally developed for Paragon and Fortnite, GAS has evolved into a robust solution for implementing complex game mechanics with built-in networking.

Core Components of GAS

Understanding GAS begins with its primary components:

1. AbilitySystemComponent (ASC)

// Add to your character class header
#include "AbilitySystemComponent.h"
#include "AbilitySystemInterface.h"

UCLASS()
class YOURGAME_API AYourCharacter : public ACharacter, public IAbilitySystemInterface
{
    GENERATED_BODY()
    
public:
    UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "Abilities")
    UAbilitySystemComponent* AbilitySystemComponent;
    
    // Implementation of IAbilitySystemInterface
    virtual UAbilitySystemComponent* GetAbilitySystemComponent() const override
    {
        return AbilitySystemComponent;
    }
};

The ASC is the central component that manages all ability-related functionality. It should be attached to any actor that will use abilities.

2. GameplayAbility

// Create a custom ability class
#include "Abilities/GameplayAbility.h"

UCLASS()
class YOURGAME_API UYourGameplayAbility : public UGameplayAbility
{
    GENERATED_BODY()
    
public:
    UYourGameplayAbility();
    
    // Override key functions
    virtual void ActivateAbility(const FGameplayAbilitySpecHandle Handle, 
                                 const FGameplayAbilityActorInfo* ActorInfo, 
                                 const FGameplayAbilityActivationInfo ActivationInfo,
                                 const FGameplayEventData* TriggerEventData) override;
};

Each distinct ability in your game (fireball, dash, heal) should be a subclass of UGameplayAbility.

3. AttributeSet

// Create a custom attribute set
#include "AttributeSet.h"
#include "AbilitySystemComponent.h"
#include "Net/UnrealNetwork.h"

UCLASS()
class YOURGAME_API UYourAttributeSet : public UAttributeSet
{
    GENERATED_BODY()
    
public:
    UYourAttributeSet();
    
    // Define attributes
    UPROPERTY(BlueprintReadOnly, Category = "Attributes")
    FGameplayAttributeData Health;
    
    UPROPERTY(BlueprintReadOnly, Category = "Attributes")
    FGameplayAttributeData MaxHealth;
    
    // Getter functions
    GAMEPLAYATTRIBUTE_PROPERTY_GETTER(UYourAttributeSet, Health);
    GAMEPLAYATTRIBUTE_PROPERTY_GETTER(UYourAttributeSet, MaxHealth);
    
    // Property replication
    virtual void GetLifetimeReplicatedProps(TArray<FLifetimeProperty>& OutLifetimeProps) const override;
};

AttributeSets define the stats and resources your characters possess and that abilities can modify.

4. GameplayEffects

GameplayEffects are not typically subclassed in C++ but rather configured in the editor. They define how attributes are modified by abilities.

Setting Up GAS in Your Project

To implement GAS in your Unreal project, follow these steps:

1. Enable the Required Plugins

Ensure you have the following in your project’s .uproject file:

"Plugins": [
    {
        "Name": "GameplayAbilities",
        "Enabled": true
    }
]

2. Configure Your Character Class

Initialize the AbilitySystemComponent and AttributeSet in your character’s constructor:

AYourCharacter::AYourCharacter()
{
    // Create ability system component
    AbilitySystemComponent = CreateDefaultSubobject<UAbilitySystemComponent>("AbilitySystemComponent");
    AbilitySystemComponent->SetIsReplicated(true);
    AbilitySystemComponent->SetReplicationMode(EGameplayEffectReplicationMode::Mixed);
    
    // Create attribute set
    AttributeSet = CreateDefaultSubobject<UYourAttributeSet>("AttributeSet");
}

3. Grant Abilities to Characters

In your character’s BeginPlay or PossessedBy function:

void AYourCharacter::BeginPlay()
{
    Super::BeginPlay();
    
    if (AbilitySystemComponent)
    {
        // Grant abilities if we have authority
        if (HasAuthority())
        {
            for (TSubclassOf<UGameplayAbility>& Ability : DefaultAbilities)
            {
                AbilitySystemComponent->GiveAbility(
                    FGameplayAbilitySpec(Ability, 1, INDEX_NONE, this));
            }
        }
        
        // Initialize attributes
        InitializeAttributes();
        
        // Bind input to abilities if this is locally controlled
        if (IsLocallyControlled())
        {
            BindAbilityInput();
        }
    }
}

Practical Example: Implementing a Dash Ability

Let’s implement a simple dash ability to demonstrate how GAS components work together:

1. Create the Ability Class

UCLASS()
class YOURGAME_API UDashAbility : public UGameplayAbility
{
    GENERATED_BODY()
    
public:
    UDashAbility()
    {
        // Configure ability settings
        InstancingPolicy = EGameplayAbilityInstancingPolicy::InstancedPerActor;
        AbilityTags.AddTag(FGameplayTag::RequestGameplayTag(FName("Ability.Movement.Dash")));
        ActivationBlockedTags.AddTag(FGameplayTag::RequestGameplayTag(FName("State.Debuff.Stun")));
        
        // Configure costs and cooldown
        CooldownDuration = 3.0f;
        DashDistance = 500.0f;
    }
    
    virtual void ActivateAbility(const FGameplayAbilitySpecHandle Handle, 
                                const FGameplayAbilityActorInfo* ActorInfo, 
                                const FGameplayAbilityActivationInfo ActivationInfo,
                                const FGameplayEventData* TriggerEventData) override
    {
        if (CommitAbility(Handle, ActorInfo, ActivationInfo))
        {
            // Apply cooldown
            ApplyCooldown(Handle, ActorInfo, ActivationInfo);
            
            // Get character and perform dash
            ACharacter* Character = Cast<ACharacter>(ActorInfo->AvatarActor.Get());
            if (Character)
            {
                FVector DashDirection = Character->GetActorForwardVector();
                Character->LaunchCharacter(DashDirection * DashDistance, true, true);
            }
            
            // End the ability
            EndAbility(Handle, ActorInfo, ActivationInfo, true, false);
        }
    }
    
private:
    UPROPERTY(EditDefaultsOnly, Category = "Dash")
    float CooldownDuration;
    
    UPROPERTY(EditDefaultsOnly, Category = "Dash")
    float DashDistance;
};

Advanced GAS Topics

Once you’ve mastered the basics, explore these advanced GAS features:

1. Gameplay Cues

Gameplay Cues allow you to trigger visual and audio effects in response to gameplay events:

// Inside your ability activation
FGameplayCueParameters CueParameters;
CueParameters.Location = ActorInfo->AvatarActor->GetActorLocation();
ActorInfo->AbilitySystemComponent->ExecuteGameplayCue(FGameplayTag::RequestGameplayTag("GameplayCue.Dash"), CueParameters);

2. Ability Tasks

For abilities that need to perform actions over time:

UAbilityTask_PlayMontageAndWait* Task = UAbilityTask_PlayMontageAndWait::CreatePlayMontageAndWaitProxy(
    this, NAME_None, AttackMontage, 1.0f, NAME_None);
Task->OnCompleted.AddDynamic(this, &UMeleeAttackAbility::OnMontageCompleted);
Task->OnInterrupted.AddDynamic(this, &UMeleeAttackAbility::OnMontageInterrupted);
Task->ReadyForActivation();

3. Prediction

GAS has built-in support for client-side prediction, which is essential for responsive multiplayer games:

// Set in your ability constructor
NetExecutionPolicy = EGameplayAbilityNetExecutionPolicy::LocalPredicted;

Common Pitfalls and Solutions

Be aware of these common issues when implementing GAS:

Replication Setup: Ensure your ASC replication mode matches your game needs.
Tag Management: Create a structured tag hierarchy to avoid conflicts.
Effect Application: Remember that instant effects apply immediately, while duration effects persist.
Performance: For abilities used frequently, consider instancing policies carefully.

Conclusion

The Gameplay Ability System provides a robust framework for implementing complex game mechanics, from basic attacks to intricate ability combinations. While the learning curve is steep, the flexibility and power it offers make it worth the investment for many game projects.

At AyaDog Games, we’ve built our own extensions on top of GAS to streamline common patterns and improve developer experience. Our open-source Gameplay Framework provides additional utilities that make working with GAS more intuitive.

Remember that effective GAS implementation requires careful planning of your ability, attribute, and tag architecture. Spend time designing these systems before diving into implementation to save yourself refactoring headaches later.

For more GAS resources, check Epic’s documentation and community guides like the GAS Documentation Project.

Understanding Unreal Engine's Gameplay Ability System