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Recipe: Add an Attribute

Goal: Add a new replicated attribute (e.g., Stamina) to your Attribute Set.

Prerequisites: An existing UAttributeSet subclass. See Attributes and Attribute Sets for the concepts.

Steps

1. Declare the Attribute in the Header

// MyAttributeSet.h

UPROPERTY(BlueprintReadOnly, ReplicatedUsing = OnRep_Stamina, Category = "Attributes")
FGameplayAttributeData Stamina;
ATTRIBUTE_ACCESSORS(UMyAttributeSet, Stamina)

ATTRIBUTE_ACCESSORS is a macro that generates GetStamina(), SetStamina(), GetStaminaAttribute(), and InitStamina() helper functions.

2. Initialize in the Constructor

// MyAttributeSet.cpp

UMyAttributeSet::UMyAttributeSet()
{
    InitStamina(100.f);
}

3. Register for Replication

void UMyAttributeSet::GetLifetimeReplicatedProps(
    TArray<FLifetimeProperty>& OutLifetimeProps) const
{
    Super::GetLifetimeReplicatedProps(OutLifetimeProps);

    DOREPLIFETIME_CONDITION_NOTIFY(UMyAttributeSet, Stamina,
        COND_None, REPNOTIFY_Always);
}

REPNOTIFY_Always ensures the OnRep fires even if the value didn't change (needed for prediction reconciliation).

4. Add the OnRep Function

Header: 

UFUNCTION()
void OnRep_Stamina(const FGameplayAttributeData& OldStamina);

Implementation: 

void UMyAttributeSet::OnRep_Stamina(const FGameplayAttributeData& OldStamina)
{
    GAMEPLAYATTRIBUTE_REPNOTIFY(UMyAttributeSet, Stamina, OldStamina);
}

GAMEPLAYATTRIBUTE_REPNOTIFY tells the ASC about the attribute change for internal bookkeeping.

5. (Optional) Add Clamping

Override PreAttributeChange for clamping the current value:

void UMyAttributeSet::PreAttributeChange(
    const FGameplayAttribute& Attribute, float& NewValue)
{
    Super::PreAttributeChange(Attribute, NewValue);

    if (Attribute == GetStaminaAttribute())
    {
        NewValue = FMath::Clamp(NewValue, 0.f, GetMaxStamina());
    }
}

And/or clamp in PostGameplayEffectExecute for base value clamping after instant effects:

void UMyAttributeSet::PostGameplayEffectExecute(
    const FGameplayEffectModCallbackData& Data)
{
    Super::PostGameplayEffectExecute(Data);

    if (Data.EvaluatedData.Attribute == GetStaminaAttribute())
    {
        SetStamina(FMath::Clamp(GetStamina(), 0.f, GetMaxStamina()));
    }
}

6. Compile and Test

  1. Compile your project
  2. In PIE, use showdebug abilitysystem to verify the attribute appears
  3. Apply an effect that modifies Stamina and confirm the value changes

Checklist

  • UPROPERTY with ReplicatedUsing
  • ATTRIBUTE_ACCESSORS macro
  • Init{AttributeName}() in constructor (e.g., InitStamina(100.f))
  • DOREPLIFETIME_CONDITION_NOTIFY in GetLifetimeReplicatedProps
  • OnRep_ function with GAMEPLAYATTRIBUTE_REPNOTIFY
  • Clamping in PreAttributeChange and/or PostGameplayEffectExecute