unreal-gameplay-ability-system

Unreal Gameplay Ability System

Overview

GAS is Unreal's framework for abilities, attributes, buffs, cooldowns and costs, with network prediction built in. It pays off in games with many abilities and stacking effects, especially multiplayer ones; it is heavy machinery for a platformer with one jump. Most GAS bugs are setup bugs: the component initialised on one side only, clamping in the wrong callback, abilities that never end.

Core principle: Abilities are code, numbers are GameplayEffects (data), state is gameplay tags, and every attribute change goes through an effect so it replicates and predicts correctly.

When to use it

• Use when: many abilities with cooldowns/costs, buffs and debuffs that stack and expire, attributes modified by many sources, multiplayer with predicted abilities. If the project already uses GAS, new abilities go through it.
• Skip when: a handful of actions with simple cooldowns in a single-player game — a component with a few functions is clearer.

Setup

Owner of the ASC	Use for
APlayerState	Player characters — attributes and effects survive pawn death and respawn
The ACharacter itself	AI and simple pawns

Replication mode: Full (single-player), Mixed (player-controlled, multiplayer), Minimal (AI in multiplayer).

// Server: when the controller possesses the pawn
void AHeroCharacter::PossessedBy(AController* NewController) {
    Super::PossessedBy(NewController);
    InitAbilitySystem();
}
// Client: when the PlayerState replicates
void AHeroCharacter::OnRep_PlayerState() {
    Super::OnRep_PlayerState();
    InitAbilitySystem();
}
void AHeroCharacter::InitAbilitySystem() {
    auto* PS = GetPlayerState<AHeroPlayerState>();
    if (!PS) return;
    AbilitySystem = PS->GetAbilitySystemComponent();
    AbilitySystem->InitAbilityActorInfo(PS, this);       // owner = PlayerState, avatar = this pawn
    if (HasAuthority()) GrantStartupAbilitiesAndEffects();
}

Missing the client-side InitAbilityActorInfo is the classic "works on the server, abilities do nothing on the client" bug. Implement IAbilitySystemInterface on the actors that own or carry the ASC.