GAS action-combat demo (ILS_Demo)

Technical breakdown · Unreal Engine 5.4 · C++ · Gameplay Ability System

What

Single-player third-person action-combat demo

Built for

My technical test for Indie Level Studio

Engine

Unreal Engine 5.4

Code

C++ (gameplay framework, GAS, input, AI tasks) + Blueprints / data assets for content

Source

github.com/danielcorredorm/ILS_Demo

A player character fights AI enemies with a melee combo, a dash, jump and double jump. Everything the character can do is granted by the Gameplay Ability System from data, and every input reaches its ability through a Gameplay Tag, so adding a new ability is data work, not new input code.

How input reaches an ability

Enhanced Input action
   │  (UDPInputConfig: InputAction ↔ InputTag)
   ▼
UDPEnhancedInputComponent::BindAbilityByTag   ── Started → pressed, Completed → released
   ▼
ADPCharacter::AbilityInputTag_Trigger(InputTag, bPressed)
   ▼
UDPAbilitySystemComponent::AbilityInputTagTrigger
   │  finds every granted ability whose spec carries that InputTag
   ▼
TryActivateAbility  (or InputPressed / InputReleased on an active ability)
   ▼
UDPGameplayAbility subclass: Attack · FinalAttack · Dash · Jump · DoubleJump

1. The ability set is a data asset

UDPAbilitySet is a UPrimaryDataAsset that lists what a character gets: abilities (each with a level and an InputTag), attribute sets (each initialized from a data table) and gameplay effects. The character only calls GiveToAbilitySystem when it is possessed. The pattern follows Epic’s Lyra sample.

FGameplayAbilitySpec AbilitySpec(AbilityCDO, AbilityToGrant.AbilityLevel);
AbilitySpec.SourceObject = SourceObject;
AbilitySpec.DynamicAbilityTags.AddTag(AbilityToGrant.InputTag);
abilitySystem->GiveAbility(AbilitySpec);

Invalid entries are logged with their index and skipped instead of crashing, so a designer who leaves a slot empty gets a clear error.

2. Abilities bound to input by tag

UDPInputConfig is a data asset that pairs each Enhanced Input action with a tag, split into native actions (move, look, crouch) and ability actions. UDPEnhancedInputComponent::BindAbilityByTag binds every ability action twice, on Started and Completed, and passes the tag plus a pressed/released flag.

UDPAbilitySystemComponent::AbilityInputTagTrigger looks for the granted abilities whose spec has that tag. If the ability is not active it tries to activate it; if it is active, it forwards the press or release (AbilitySpecInputPressed/Released and the replicated input events), so abilities can react to holding or releasing a button.

3. Native Gameplay Tags

Tags used from C++ are declared as native tags with UE_DECLARE_GAMEPLAY_TAG_EXTERN / UE_DEFINE_GAMEPLAY_TAG in DPGameplayTags: native input (Input.Native.Move, Look, Crouch), SetByCaller magnitudes (SetByCaller.Damage, SetByCaller.Respawn) and animation windows (ANS.HandleCollision, ANS.HandleCombo, ANS.FinalAttack). Code never looks tags up by string, and a typo is a compile error. Content tags (abilities, character states, ability input) live in DefaultGameplayTags.ini.

4. Dash: a custom root-motion Ability Task

UDPTApplyRootMotion_Dash extends UAbilityTask_ApplyRootMotion_Base. It builds a FRootMotionSource_ConstantForce (direction × strength, duration, optional strength curve, gravity on/off) and applies it to the character movement component, so the dash goes through the movement component instead of setting the actor’s location.

• On finish it clamps the velocity to the walk speed, so the character does not slide after the dash.
• It can switch the movement mode during the dash and restore the previous one.
• OnDestroy removes the root motion source by ID and marks it for removal, so cancelling the ability never leaves a force running.

UDPGADash creates the task along the actor’s forward vector, plays the dash montage and ends the ability when the task broadcasts OnFinish.

5. Combo and hit windows from an AnimNotifyState

UDPANS_WindowNotify is one reusable AnimNotifyState with a tag field: it adds that tag to the owner’s ability system as a loose tag when the window opens and removes it when it closes. Animators place the windows on the montage; code only reacts to tags.

UDPGAAttack plays its montage and waits on those tags with WaitGameplayTagAdded / WaitGameplayTagRemoved:

• ANS.HandleCollision turns the weapon’s hit box on and off.
• ANS.HandleCombo shows the combo prompt and raises the weapon’s damage multiplier for the next hit.
• UDPGAFinalAttack listens to ANS.FinalAttack instead: a sphere overlap launches nearby characters away and applies damage through a gameplay effect with a SetByCaller.Damage magnitude.

6. Ability cost from a Magnitude Calculation

Each ability has an AbilityCost property. UDPCostMagnitudeCalculation, a UGameplayModMagnitudeCalculation, reads it from the ability in the effect context:

if (const auto GameplayAbility = Cast<UDPGameplayAbility>(Spec.GetContext().GetAbility()))
{
    return GameplayAbility->AbilityCost;
}
return 0.0f;

One shared cost gameplay effect serves every ability; tuning a cost is a number on the ability, not a new effect asset. The attribute set clamps Health and Stamina between 0 and their max in PreAttributeChange.

Also in the project

• Activation policies on the base ability (on input, while input is active, on spawn, passive), with spawn activation that respects the ability’s net execution policy.
• Enemy AI with Behavior Tree tasks: find a random location and attack the target, where the attack task activates a GAS ability on the enemy.
• Death and respawn driven by attribute change delegates and a blocking gameplay effect with a SetByCaller.Respawn magnitude.
• UMG character overlay, enemy health bar and combo prompt.