The Gameplay Abilities Module

Michael Chapman

(@woppin on Discord)

Gameplay Abilities

• A Common Plugin shared by Paragon and Fortnite
• Handle gameplay mechanics + their replication and prediction

Why?

​

• Allow engineers to handle the network prediction and replication in C++
• Allow designers to work in blueprint to tweak mechanics easily
• Generic enough to support anything the design team comes up with
• Optimised to reduce network load

Gameplay Abilities

Resources available:

• Sample from Epic dev Dave Ratti
• https://github.com/daveratti/GameplayAbilitiesSample
• Forum post and wiki article from Kaz
• https://wiki.unrealengine.com/GameplayAbilities_and_You
• #gameplay-abilities-plugin channel on Unreal Slackers discord community
• Code from this talk:
• https://github.com/michaeltchapman/MCGameplayAbilities
• Sabre Dart Studios training:
• https://www.youtube.com/watch?v=Ev2P6BTUxN0

Concepts

AbilitySystemComponent

AttributeSets

GameplayTags

GameplayEffects

​

​

​

​

​

GameplayCues

GameplayAbilities

GameplayTasks

GameplayEvents

​

​

​

​

​

Gameplay Abilities : AbilitySystemComponent

Central point for coordinating everything

Attach to any actor implementing IAbilitySystemInterface

AbilitySystem = CreateDefaultSubobject<UAbilitySystemComponent>(TEXT("AbilitySystem"));

On pawns after possession: AbilitySystem->InitAbilityActorInfo(this, this);

Relatively lightweight, attach to anything that should receive effects or activate abilities

Gameplay Abilities : AttributeSets

Store replicated floats wrapped in a struct

UPROPERTY(Category = "Attribute", EditAnywhere, ReplicatedUsing = OnRep_Health, BlueprintReadWrite) FGameplayAttributeData Health;

UFUNCTION() void OnRep_Health() { GAMEPLAYATTRIBUTE_REPNOTIFY(UBaseAttributeSet, Health); } static FGameplayAttribute AttributeHealth();

Account for temporary changes without losing base

AbilitySystem->GetNumericAttribute(UBaseAttributeSet::AttributeHealth());

AbilitySystem->GetNumericAttributeBase(UBaseAttributeSet::AttributeHealth());

​

Gameplay Abilities : AttributeSets

Attribute Change Events

void PreAttributeBaseChange(const FGameplayAttribute& Attribute, float& NewValue) const;

void PreAttributeChange(const FGameplayAttribute& Attribute, float& NewValue);

Used to clamp values Eg. health <= max_health

void PostGameplayEffectExecute(const struct FGameplayEffectModCallbackData &Data);

Used to trigger actions as a result of attribute changes, eg health < 0 -> do_ragdoll

bool PreGameplayEffectExecute(struct FGameplayEffectModCallbackData &Data);

Used to modify or discard the values passed into an effect execution.

​

Gameplay Abilities : GameplayTags

If attributes are the ‘float’ part of describing the game state, tags are the ‘bool’ part. Can be applied locally and have state managed manually:

AbilitySystem->AddMinimalReplicationGameplayTags(InitialTags);

More commonly applied via effects and replicated. Can be applied multiple times and stack.

Triggers events on both client and server

FOnGameplayEffectTagCountChanged& LitheCallback = AbilitySystem->RegisterGameplayTagEvent(FGameplayTag::RequestGameplayTag(UGlobalData::LitheTag), EGameplayTagEventType::AnyCountChange);

LitheCallback.AddUObject(this, &ABaseCharacter::SetLithe);

​

Gameplay Abilities : GameplayEffects

Predictively applied when done via an ability

• Can modify attributes
• Can add/remove tags
• Can be instant, have set duration, or be infinite
• Can trigger a Cue
• Can have tags itself
• Can be blocked by tags on the source or target
• Can stack, and overflow the stack
• Can trigger other effects
• Can remove other effects
• Can trigger events
• Can have a context attached to it, including a FHitResult.

​

Gameplay Abilities : GameplayEffects

Can be applied via an ability, or using the ASC of the target. The latter won’t be predicted

• Create GameplayEffectSpec from class
• Add or Remove tags
• SetByCallerMagnitude
• Add Effect Context
• Apply Effect Spec to Self/Target

The blueprint nodes for use within abilities omit the middle 3 steps.

Gameplay Abilities : GameplayCues

Cues encapsulate cosmetic effects

• Driven by a tag namespace ‘GameplayCue’
• Can be triggered by GameplayEffects
• Can be executed by a predicting client in ability and replicated out
• Can be executed locally and not replicated

​

Cues hook into several events from the cue manager

• OnActive
• Called when GameplayCue is activated.
• WhileActive
• Called when GameplayCue is active, even if it wasn't actually just applied (Join in progress, etc)
• Executed
• Called when a GameplayCue is executed: instant effects or periodic tick
• Removed
• Called when GameplayCue is removed

Gameplay Abilities : GameplayAbilities

• Provide a (optionally) network-predicted way of interacting with all of these elements
• Lots of tag options:
• Cancel other abilities
• Block other abilities from activating
• Require tags to activate
• Block from activating
• Apply to owner when activated
• Instancing:
• Per Actor
• Per Execution
• Non Instanced

​

​

• Net Execution Policy
• Local predicted
• Local
• Server Initiated
• Server Only
• Cooldown effect class
• Cost Effect class

​

Gameplay Abilities : GameplayAbilities

• Most player abilities will be local predicted. This will have the ability graph run on both the client and the server.
• Often there will be a WaitTargetData node at the beginning. This allows the player to visualise aiming.
• The cooldown and cost will be applied when ‘CommitAbility’ is used.
• All paths must lead to EndAbility, or the UGameplayAbility instance will not be cleaned up.

​

​

• ​

​

Gameplay Abilities : GameplayAbilities

Simple ability that applies an effect to the caster

​

​

Gameplay Abilities : GameplayAbilities

Simple ability that spawns a projectile.

​

Gameplay Abilities : GameplayTasks

​

• Flexible task running framework, can be used separately from GameplayAbilities
• The GAS component inherits from the task management component
• bSimulatedTask
• bTickingTask
• AbilityTask adds some convenience pieces on top of GameplayTask
• Designer focused

​

​

​

​

Gameplay Abilities : GameplayEvents

​

• Trigger things from other things.
• Associated with a tag and a UObject payload
• Trigger Abilities
• Wait for Event

​

​

​

​

Acknowledgements

​

• Thanks to the many friendly folks on the discord channel:
• @Kaz for taking the time to write everything up initially
• @jackblue for helping me through the early stages
• @Roy Awesome, @Dartnalla, @Boolean Fiasco and everyone else who has helped to build up the collective knowledge around this plugin.
• Finally, best of luck to @Acren and his colleagues, who are working on Hyper-Jam down in Melbourne, which is built on the GAS.

​

​